It’s not the greatest photograph you’ll ever see of Venus, but it’s the first I’ve managed to take that clearly shows the crescent phase. The Sun had just set and the sky was still very bright. The crescent was just discernible through a pair of 7×50 binoculars. Against a darker sky the dazzling brightness of Venus makes it difficult to make out the phase. I then decided to have a go with my Canon 530 zoom. Again, I have previously been unable to capture the phase against a darker sky. The 530 is a rather basic camera, and the lack of a viewfinder made it difficult to locate Venus. Eventually, I succeeded and photographed Venus against the still bright sky. I took eight images, six of which were of reasonable quality.
On 24 August 2006, the 26th General Assembly of the International Astronomical Union in Prague came up with one of the most controversial rulings in the history of astronomy:
The IAU therefore resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
The three glaring questions arising were (1) what is meant by “cleared the neighbourhood around its orbit” since nearly all the recognised planets, including Jupiter and Earth, share their orbit with other bodies (2) what of planets orbiting stars other than the Sun, which by 2006 had already been detected in large numbers (3) why was a definition of a planet needed at all?
The last of these is the easiest to answer: the Pluto question. The status of Pluto, the smallest and outermost planet, had been questioned for some time. Pluto was discovered by American astronomer Clyde Tombaugh in 1930 after a search for an undiscovered planet that was supposedly affecting the orbit of Uranus. Similar considerations had led to the discovery of Neptune in 1846, but subsequent observations suggested that more than one planet was involved. The problem was that instead of another planet the size of Neptune or Uranus, Pluto appeared to be no larger than Earth. Even this turned out to be a gross overestimate, and by the 1980s Pluto had been determined to be considerably smaller than the Moon – far too small to have any significant affect on the orbit of Uranus. It was eventually discovered that Neptune was more massive than had been believed, and there was no need for a second planet. That Pluto was found close to where the supposed planet lay was a pure coincidence.
By the 1990s, it was becoming clear that Pluto was merely the largest member of a region known as the Kuiper Belt, extending from the orbit of Neptune at 30 au from the Sun to around 50 au from the Sun. The region is populated by icy objects left over from the formation of the Solar System. The first was identified in 1992, and other discoveries soon followed. Although most of these were far smaller than Pluto, after 2000 much larger objects began to turn up, including Haumea, Makemake, Varuna, Quaoar, Orcus, and Sedna. These new objects were all named for mythological creation deities of various traditions. Though they were still rather smaller than Pluto, all were comparable in size to Ceres or larger, and it seemed likely that even larger objects might be found.
Sure enough, on 5 January 2005, an object was discovered by a team at Mount Palomar on plates taken some 15 months earlier. The new body was given the provisional nickname ‘Xena’ after the TV character. On 29 July 2005, the Palomar team went public, by which time they suspected that Xena was slightly larger than Pluto. A few months after the announcement, Xena was found to have a moon, enabling its mass to be determined. Regardless of its diameter (it eventually turned out to be fractionally smaller than Pluto), Xena was 27 percent more massive than Pluto, bringing to a head the whole debate on the latter’s status as a planet. It was obvious that if Pluto was a planet, then a more massive object like Xena had to be also. Conversely, if Xena wasn’t a planet, then neither was Pluto.
Even before the discovery of Xena, the International Astronomical Union had set up a committee to consider possible definitions of a planet. On 16 August 2006, a draft proposal was published at the IAU’s Prague conference. It stated: “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.” This concise and readily testable definition of a planet would have recognised Xena as a planet and retained Pluto’s status. Under this proposal, the asteroid Ceres, originally recognised as a planet, stood to regain its planetary status. In addition, it was proposed to elevate Pluto’s major moon Charon to planetary rank. At 1207 km (750 miles) in diameter, Charon is very large in comparison to Pluto, with 11.6 percent its mass. The Pluto-Charon system would be considered a binary planet, the only such entity in the Solar System.
However, the 16 August proposal was rejected, and the proposal of 24 August adopted instead. Pluto, Ceres, and Xena were denied planetary status. The latter was given the official name Eris a few days later, and the moon was named Dysnomia. Eris is the Greek goddess of discord and Dysnomia means ‘lawless’, an oblique reference to Lucy Lawless, who played Xena in the TV series. All three were given the confusing designation of ‘dwarf planet’.
The most confusing aspect of the definition was the phrase meant by “cleared the neighbourhood around its orbit”. Jupiter shares its orbit with two clusters of asteroids known as the Trojans at its Lagrange points. The asteroids are named for participants in the Trojan War, but the term ‘trojan’ is now used for any asteroid in a similar relationship with other planets. Neptune, Uranus, Mars, and Earth also share their orbits with trojan asteroids.
The term ‘gravitationally dominant’ is often used to explain the concept. Trojan asteroids and the Kuiper Belt objects in orbital resonance with Neptune have all been marshalled into their current orbits by a gravitationally dominant planet. The term does not form part of the 2006 definition, but even if it did, a planet will still be partially defined in terms of its location, which raises a problem. In 1949, Kuiper estimated the diameter of Pluto to be 10,300 km (6,400 miles), only slightly smaller than Earth. If that figure had turned out to be correct, Pluto would still not be a planet on the new definition, despite being larger than Mars or Mercury.
While accepting that the IAU was under pressure to define a planet, the question must still be asked: do we actually need a formal definition for a word that once simply referred to bright star-like objects that, unlike the fixed stars, moved over a period of time? The word ‘continent’ lacks a formal definition, yet most would list the continents as Europe, Asia, Africa, North America, South America, Australia, and Antarctica. Similarly, Sir Patrick Moore came up with the common-sense definition that Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are planets and everything else isn’t. He was long of the opinion that Pluto was not a planet, but just as some would combine Europe and Asia into Eurasia, or the Americas into a single continent, we could add Pluto and Eris nee Xena if we wished.
A major concern was that the 16 August proposal opened up the way to planethood for objects the size of Ceres, of which there are many in the Kuiper Belt. Haumea, Makemake, Varuna, Quaoar, Gonggong (discovered in 2007), and Sedna are larger than Ceres, and Orcus is only slightly smaller. Nor is Ceres the lower limit: the asteroid Hygiea, the Saturnian moons Enceladus and Mimas, and the Uranian moon Miranda are all around half the diameter of Ceres, but they are spherical and under the 24 August ruling the moons would qualify as dwarf planets if they orbited the Sun.
However, matters are not entirely clear-cut. The asteroids Vesta and Pallas are larger than any of these four bodies, and the Neptunian moon Proteus is larger than Mimas – yet all are irregular in shape. The Ceres/Orcus-Mimas size range also includes 67 known trans-Neptunian objects and the moons Dysnomia (Eris I) and Vanth (Orcus I).
Currently, in addition to Pluto, Eris, and Ceres, Haumea and Makemake are officially recognised as dwarf planets. Varuna, Quaoar, Gonggong, Sedna, Orcus, and Hygiea are probably dwarf planets, as are at least some of the trans-Neptunian objects intermediate between Ceres/Orcus and Mimas. If we were to adopt the 16 August proposal, the Solar System would comprise at least 14, probably 20, and potentially as many as 88 planets (including Charon).
Could we have a Solar System with 88 planets? Generations of schoolchildren have been able to name the nine (now only eight) planets. Even seasoned astronomers would struggle to remember 88. But is that a reason? How many chemists could name all 118 currently known chemical elements; how many geographers could name all the 193 members of the United Nations?
I now intend to go even further than the 16 August proposal, which proposed to elevate Charon to planethood, even though it is only the twelfth largest moon in the Solar System. Two of these moons – Ganymede and Titan – are larger, albeit less massive, than Mercury. If we accept that location cannot be a determinant of whether something is or isn’t a planet, then being a moon should not disbar it. Indeed, planetary scientists often refer to Ganymede, Titan, and other large moons as planets. The seven large moons, in order of size, are Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton. In addition, there are eleven medium-sized spherical moons: Rhea, Tethys, Dione, Enceladus, and Mimas (Saturn), Ariel, Umbriel, Titania, Oberon, and Miranda (Uranus), and Charon (Pluto). This adds a further 17 planets to the roster, not counting Charon which has already been included.
At a conservative estimate, that would give us a fifty-planet Solar System. Again, just as chemical elements are grouped in accordance with the periodic table, so planets can be categorised by type. Traditionally, planets were divided between rocky Earth-type planets (Mercury, Venus, Earth, Mars) and gas giants (Jupiter, Saturn, Uranus, Neptune), but Uranus and Neptune are now classed as ice giants, composed predominantly of methane, ammonia, and water rather than hydrogen and helium. The ‘new’ planets are of many types, including:
Silicate (rocky) surface and mantle, metal core e.g. the Moon
Icy surface, silicate mantle, metal core e.g. Ganymede
Icy surface, icy mantle, silicate and metal core, e.g. Triton
Icy surface, icy mantle, silicate core, e.g. Titan
Icy surface, icy mantle, icy core e.g. Mimas
Some of the icy moons are believed to have deep subsurface oceans, including Europa, Ganymede, and Enceladus.
The IAU has no plans to revisit its definition of a planet, but nobody is under any obligation to abide by it. Pluto’s demotion was especially unpopular in Clyde Tombaugh’s home state of Illinois. The Illinois Senate passed a resolution decreeing that Pluto “be reestablished with full planetary status, and that March 13, 2009 be declared “Pluto Day” in the State of Illinois in honor of the date its discovery was announced in 1930″.
The shortest day of the year, as Sun reaches its minimum declination of -23.5 degrees. From now, and for the next six months, the days will get longer.
Built on the site of the Tussauds Cinema, which was destroyed during the Blitz, the London Planetarium was opened by HRH the Duke of Edinburgh on 19 March 1958. Public presentations began the next day. The Planetarium was an immediate hit with the public, and it considerably boosted attendances at the adjoining Madame Tussauds gallery.
The Planetarium’s 18 m (60 ft) dome seated an audience of 330 who viewed presentations from a Zeiss Universal Mk IV star projector. This mechanical and optical wonder remained in use for nearly half a century before being replaced by a digital system in 1995.
Sadly, by the beginning of the millennium, attendances were no longer sufficient to keep the Planetarium going as a separate visitor attraction. Astronomical presentations ceased in 2006 and Madame Tussauds repurposed the building for shows about celebrities. Now known as the Stardome, it still features ‘stars’ – just not those up in the sky.
This beautifully-produced brochure dates to around 1960 and was sold for the very reasonable sum of one shilling (about £1.00 at today’s prices). The text is uncredited, but in his 2003 autobiography Eighty not out the late Sir Patrick Moore claimed to be the author. Moore turned down the opportunity to become the first Director of the London Planetarium because he did not wish to move to London; the job went instead to astronomer and author Dr. Henry C. King.
The phenomenon is neatly demonstrated by Stellarium. The Sun is at the point where the ecliptic intersects with the celestial equator. Hence day and night are of equal length. Tomorrow and for the next six months, the Sun will be south of the equator and the nights will be longer than the days.
Residents of the Montparnasse Cemetery on the Left Bank of the Seine in Paris include such household names as Charles Baudelaire, Samuel Beckett, and Camille Saint-Saens. Not quite so well-known outside of scientific circles, but certainly no less revered, is the astronomer and mathematician Urbain Jean-Joseph Le Verrier.
The tomb credits him with the discovery of Neptune in September 1846, making him only the second person ever to discover a planet – and the first to do so by purely mathematical means, unaided by a telescope. But does Le Verrier deserve sole credit, or should it be shared with the British mathematician John Couch Adams? Indeed, should Adams be given sole credit? The debate started soon after the discovery was announced, and it has been going on ever since.
The son of a government official, Urbain Jean-Joseph Le Verrier was born in 1811 in Saint-Lô, Normandy and studied at the École Polytechnique in Paris. An able scholar, he pursued an academic career in the first instance as a chemist, but he made the switch to astronomy when a teaching position came up at the École Polytechnique. His strong mathematical expertise made him well qualified for the job. His work on the gravitational influence of Jupiter upon the orbits of certain comets earned him significant recognition. In January 1846, he was elected a member of the Académie des Sciences.
By this time, Le Verrier was working at the Paris Observatory. The previous year, François Arago, director of the Observatory, encouraged him to work on the perplexing anomalies with the orbit of Uranus. The Sun’s seventh planet is just about visible to the naked eye, but it was not until 1781 British astronomer William Herschel identified it as a planet. Suggestions that it might have been noted earlier, but dismissed as a star, proved to be correct. No fewer than 19 ‘precovery’ observations were found, stretching back to 1690, when John Flamsteed, the first Astronomer Royal, recorded it as a star and gave it the designation 34 Tauri. The problem was that the orbit as computed from these old observations did not agree with that actually observed after 1841. By Le Verrier’s time, Uranus had completed about three-quarters of an orbit around the Sun since its discovery, and a new orbit had been worked out by French astronomer Alexis Bouvard – but the problems had persisted. Up to 1822, the planet seemed to be moving faster than predicted by Newton’s Law of Gravity; but subsequently it was moving too slowly. Attempts to explain the discrepancy included a massive (and somehow unseen) satellite, an impact from a comet, or the existence of a resisting cosmic medium. It was even possible that the fault lay with the Law of Gravity itself.
In 1834, the Rev. Thomas Hussey contacted astronomer George Airy with the suggestion that the gravitational pull of an undiscovered planet was affecting the orbit of Uranus and that the observed orbital data might make it possible to locate the disturbing planet. Hussey was certainly on the right lines, but Airy did not believe that there was any hope of tracking down the planet with the data available, even assuming that it existed at all. Airy – who became Astronomer Royal in 1835 – was equally unforthcoming when Alexis Bouvard’s nephew Eugene contacted him with a similar proposition in 1837.
The problem was next taken up in 1841 by John Couch Adams, an undergraduate studying mathematics at the University of Cambridge. After completing his degree in 1843, he began working on the Uranus question in earnest, and by the end of that year he had a preliminary solution based on the assumption that the new planet obeyed the Titius-Bode Law, an empirical rule which states that the mean distance from the Sun in astronomical units a for planet m in order from the Sun is given by the numerical sequence a=0.4+0.3 x 2m. Although there was (and still is) no theoretical justification for the law, it had been used four decades earlier to successfully predict the existence of Ceres (which at the time was still recognised as a planet but in an episode foreshadowing the recent Pluto controversy was later downgraded to an asteroid). The result obtained by Adams was sufficiently encouraging to convince him that the unknown planet hypothesis was correct, and by September 1845 he had refined his calculations to the extent that he had an approximate position for the planet.
What he lacked was access to a telescope. Accordingly, he communicated with James Challis, director of the Cambridge Observatory, who suggested he contact George Airy. To this end, in October 1845, Adams twice turned up unannounced at the Royal Greenwich Observatory. On the first occasion, Airy was in France and on the second he was having dinner, and his butler refused to disturb him. Adams left Airy a synopsis of his calculations, to which Airy later raised a query concerning the radius vector (i.e. distance from the Sun at a given time) of Uranus, but for reasons unknown Adams failed to reply (it has often been suggested that Adams regarded the query as ‘trivial’, but some sources dispute this).
Meanwhile, as noted above, Le Verrier had been tasked with the Uranus problem by Arago at the Paris Observatory, and in November 1845 he published his first memoir on the subject. A second memoire followed in June 1846, and on 31 August of the same year he published a predicted position for the disturbing planet in a third paper. Word of the second memoire reached Airy, who wrote to Le Verrier posing the same radius vector question he had asked of Adams. Le Verrier replied promptly, and like Adams, requested Airy’s help in locating the planet.
Airy did not respond, and he also kept quiet about Adams’ work, which he was now inclined to take more seriously. On 9 July, he wrote to Challis at Cambridge, asking him to search for the predicted planet. The 12-inch Northumberland refractor at Cambridge, which Airy himself had designed, was one of the biggest telescopes of its day, and it was far superior to anything at Greenwich. Challis began observing on July 29, but he was hampered by a lack of star charts for the zone of interest, and he was therefore forced to undertake a laborious program of observation and chart the positions of all the stars within it. Essentially, his approach was the same as that used to discover Pluto in 1930: comparing star fields over a period of days in order to find a ‘star’ that moved from night to night. Clyde Tombaugh was able to take photographs of the star fields of interest and use a blink comparator to find the moving dot of Pluto, but in the 1840s astrophotography was still in its infancy.
Le Verrier meanwhile had sent his results to the Paris Observatory, and given that he had been working on Arago’s instructions, it might have been expected that the matter would have been given some urgency. But it was not; a brief search was abandoned early in August. On 18 September, Le Verrier wrote to Johann Galle, assistant director of the Berlin Observatory, asking him to look for the planet at the position he predicted. The letter reached Galle on the evening of 23 September, and after getting approval from his boss Johann Franz Encke (of Encke’s Comet fame), he started a search without further ado. Encke did not take part, possibly because 23 September was his birthday. One of Galle’s students, Heinrich d’Arrest, suggested the use of the new Carta Hora XXI (map for Hour 21, i.e. the portion of the sky between R.A. 21h 00m and 22h 00m), a high-resolution star chart that was so recent it had yet to be sent to the publishers.
Galle took charge of the telescope and described the positions and magnitudes of the stars he could see, while d’Arrest checked them off against the chart. It did not take long to find an eighth-magnitude star that did not appear on the charts; and the object also showed a small disk. Encke was hastily dragged away from his birthday celebrations, and he agreed that the object had a resolved disk. A repeat observation the following night confirmed that it had moved in relation to the other stars, and that it was indeed the predicted planet. It was less than a degree away from the predicted position. Galle then wrote to Le Verrier confirming that his planet did indeed exist.
There was understandable enthusiasm in France, and the fact that the actual sighting had been made in Germany was conveniently forgotten. Le Verrier’s achievement was described by Arago as “…of the most magnificent triumphs of theoretical astronomy, one of the glories of the Académie and one of the most beautiful distinctions of our country.” Then came a nasty surprise for the French in the form of a letter from Sir John Herschel (son of William Herschel) to the Athenaeum Club, making reference to the work of Adams. Shortly afterwards, it emerged that Challis had recorded Neptune four times, with the last observation being made on 4 August. On one occasion, he had even noted that one of the ‘stars’ he had observed “seems to have a disk”. Had Challis compared his observations more thoroughly, he would certainly have made the discovery.
To the British, it was an embarrassingly missed opportunity; to the French it was Perfidious Albion up to its usual tricks. Arago made it clear that Adams had “…no right to figure in the history of the new planet, neither by a detailed citation, nor even by the slightest allusion”. Airy and Challis came in for considerable stick on both sides of the Channel. But neither Le Verrier nor Adams took any part in the rumpus. Adams was happy to acknowledge Le Verrier’s priority, and he did not join in with the criticism heaped on Airy and Challis. When he and Le Verrier finally met face to face, they are said to have struck up an immediate friendship and they remained on good terms for the rest of their lives.
Le Verrier suggested the name ‘Neptune’ for the new planet, but then proposed to have it named after himself. This was not entirely unreasonable, as at the time, Uranus was still widely referred to as ‘Herschel’ or ‘The Georgian Planet’ (after Herschel’s patron King George III). However, the name ‘Neptune’ soon became widely adopted, and at Adams’ suggestion the variant names for Uranus were finally dropped.
So, who really deserves the credit – and the blame?
The Royal Greenwich Observatory was a publicly-funded institution, the purpose of which was the production of accurate tables of star positions for navigators at sea. As Astronomer Royal – basically a senior civil servant – George Airy would not have believed it appropriate to interrupt the Observatory’s program to go hunting for a planet. In any case, there was at that time no suitable telescope at Greenwich: the 28-inch Great Equatorial Telescope (still the seventh largest refractor in the world) did not see first light until 1893. By that time, though, the ‘mission’ of the RGO had been expanded to include astrophysics and astronomical photography. Airy’s decision to ‘outsource’ the search for the new planet to Challis at Cambridge and the Northumberland refractor was entirely justifiable. Airy could perhaps be faulted for his initial scepticism at the possibility of locating Neptune through its effects on the orbit of Uranus, but he acted quickly enough when he realised that two independent researchers had arrived at very similar solutions.
As noted, James Challis was hampered by a lack of star charts for the region, and therefore faced an extremely laborious task. However, it is inescapable that he recorded Neptune on four occasions and failed to recognise it. Challis apparently worked in secret, keeping knowledge of the search from his fellow British astronomers. One can but speculate as to his motives for so doing, but had he recruited one of his students as an assistant (as had Galle), then it is highly likely that he would have made the discovery.
After the row over priority had died down, a consensus emerged that Le Verrier and Adams should be jointly credited as the discoverers of Neptune, although recently it has been suggested that Adams’ predictions were significantly less accurate than those of Le Verrier.
Although Neptune is too faint to be seen with the naked eye, the most basic telescope or even a good pair of binoculars will show it as a bluish eighth-magnitude star. ‘Precovery’ observations were made by Sir John Herschel in July 1830; the French astronomer Jérôme Lalande recorded it twice in May 1795; and the Scottish-born astronomer Johann von Lamont recorded it least three times between 1845 and 1846, with his last observation on 11 September coming just days before the actual discovery. But none of these observers thought it was anything other than a star.
The best-known precovery observation of Neptune was made by Galileo at the very dawn of the telescopic era, more than two centuries before its ‘official’ discovery. The conventional view is that Galileo – as others would do later – mistook Neptune for a star. Galileo’s drawings show that he observed Neptune on December 28, 1612, and again on January 27, 1613 when it was in conjunction with Jupiter; but on both occasions he mistook it for a fixed star because his telescope was insufficiently powerful to show it as a disk.
However, in July 2009, physicist David Jamieson suggested that Galileo was indeed aware that there was something unusual about Neptune. In one of his notebooks, he noted the movement of a background star (i.e. Neptune) on January 28; a dot in Neptune’s position drawn in a different ink indicates that he found it on an earlier sketch, drawn on the night of January 6, and suggesting that he had searched through his earlier observations. But he does not appear to have identified the moving object as a planet; he never published his observations; and as far as is known, he never attempted to observe it again.
Jamieson, D., 2009. Galileo’s miraculous year 1609 and the revolutionary telescope. Australian Physics, 46(3), pp. 72-76.
Kowal, C. & Drake, S., 1980. Galileo’s observations of Neptune. Nature, 25 September, Volume 287, pp. 311-313.
Krajnović, D., 2016. The contrivance of Neptune. Astronomy & Geophysics, October, 57(5), pp. 5.28-5.34.
Moore, P., 1993. New guide to the planets. London: Sidgwick & Jackson.
Smart, W., 1946. John Couch Adams and the Discovery of Neptune. Nature, 9 November, Volume 158, pp. 648-652.
When the Greek mathematician Eratosthenes estimated the circumference of the Earth around 240 BC, it was common knowledge that the planet was spherical. But prior to the sixth century BC, belief in a flat earth was common. Early Egyptian, Mesopotamian, Hebrew, and Homeric Greek cosmologies all viewed the Earth as flat. The Hebrew cosmology viewed the Earth as a disk supported by pillars and surrounded by a primal Ocean. Below it is an underworld known as She’ol and above it, again supported by pillars, is the Firmament of Heaven, a solid dome separating the Sun, Moon, stars, and planets from the Ocean of Heaven. It is not clear how or when the shift to the modern view of a spherical Earth came about. The earliest references to a spherical Earth come from ancient Greek sources, but there is no account of how the discovery was made.
Is it possible that under suitable conditions, the Earth’s curvature could be seen with the naked eye? Strictly speaking, what we mean here is a curved horizon rather than the curvature of the Earth. It is, nonetheless, an artefact of a spherical Earth. If Bronze Age people were able to see a curved horizon, it might have given them a strong hint that they were living on a sphere, not a plate.
Almost half a century ago, I watched in wonder as live TV pictures showed Neil Armstrong take his ‘giant leap for mankind’. The blurry pictures appeared to show the curvature of the Moon’s surface clearly visible in the background. It is also apparent in the colour picture Armstrong took as Buzz Aldrin exited the lunar module to join him on the lunar surface.
The Moon is of course much smaller than the Earth at just over a quarter of the diameter. Consequently, the lunar curvature is almost four times as pronounced as that of earth; moreover, it appears so prominent in the photographs as to suggest that Terrestrial curvature should at least be perceptible at ground level.
But first, let’s take a closer look at the picture of Aldrin climbing down the ladder of the lunar module:
Does it really show the lunar curvature? The answer is ‘no’. The horizon is tilted due to the camera angle; it’s not actually curved. But the horizon is very close – as one would expect on a small planet, and this combines with the camera angle to give an illusion of a curved horizon. For our observer of average height, the distance to the horizon is 2.4 km (1.5 miles) as opposed to 4.7 km (2.9 miles). This is of course an effect of the curvature, but it’s not the same as seeing a curved horizon.
It is often claimed that the curvature of the Earth can be seen from an aircraft, a mountain, or even a tall building. The idea is that if you sight the horizon looking out to sea over a level straight edge such a ruler from approximately a metre, you should be able to see a convex meniscus. Unfortunately, you need to be very high up for this method to work. Standing on a clifftop looking out to sea you will see nothing (I’ve tried). In fact, even from an airliner at 35,000 feet it is hard to see (although it is clearly visible at 60,000 feet from high-altitude aircraft such as Concorde). There are many photographs purporting to show a clear curvature from ordinary airliners, but the ‘curvature’ in these cases will almost certainly be due to barrel distortion of the lens used to take the picture. We can thus rule out any possibility that Bronze Age people could have seen the Earth’s curvature for themselves.
The classic example of a proof that the Earth must be round is the appearance or disappearance of ships over the horizon. As a ship sails away from the land, it will gradually disappear; first the hull, then the superstructure, and finally (for a sailing ship), the masts and sails. Similarly, the upper parts of an incoming vessel will be seen before the hull comes into view. The effect can clearly be seen with a telescope or a pair of binoculars and the internet abounds with photographs and videos taken with high-zoom cameras.
Bronze Age traders were voyaging across the Mediterranean two millennia before the fifth century BC. While ships often kept close to land, prevailing winds meant that it was far easier for Ancient Minoan ships to sail directly from Crete to Egypt, only coasting on the return voyage. The question, then is, why didn’t the Minoans notice the gradual disappearance of their ships below the horizon?
The problem is, there were no telescopes, binoculars, or high-zoom cameras in the Bronze Age. Also, Bronze Age ships were far smaller than modern freighters or liners. If the evidence of the fourteenth century BC Uluburun shipwreck is anything to go by, a typical merchant vessel of that time was only around 15 – 16 m (50 ft) in length, about the size of a present-day Moody 54 sailing yacht. Even the triremes that came into use as warships in the late sixth century BC were much smaller than a present-day coastal freighter.
Would even a keen-eyed observer have been able to see that the hull of an outward-bound Bronze Age ship vanished while its sails remained visible? Even if they could, would they have been able to do so with enough consistency to realise that it was a distinct phenomenon and not just an artefact of sea, weather, or lighting conditions?
Let us assume that an individual of average height (eye level 5ft 7 in or 1.7 m) is sited on the shore, watching a ship of comparable size to the Uluburun merchant vessel standing out to sea:
Length of hull = 15 m approx.;
Height of hull above waterline = 2 m approx.;
Hoist of mainsail = 15 m approx.
Using a computer program that takes eye height and target distance, and calculates target hidden height, we find that the hull will have just disappeared as the ship reaches 10 km (6.2 miles) from the shore. At that distance, the sail will subtend 15/10,000 = 1.5 e-3 rad = 5 arcmin. This is about the same apparent size as a five pence coin or a US or Canadian dime viewed from 12 metres (39 ft). For a second observer, sited on a clifftop at a height of 10 m (32 ft), the whole of the ship will still be visible, but the hull will subtend an angle of just 2/10,000 = 2 e-4 rad = 0.7 arcmin above the waterline. This is less than the apparent diameter of Venus, and I think that it would be quite difficult to distinguish the hull from the sea even under favourable conditions. In conclusion, I am by no means convinced that it ever occurred to Bronze Age people that ships were doing anything other than vanishing into the distance.
How, then, might sixth century Greek scholars have deduced that they were living on the surface of a sphere? There are several clues that can be gleaned from the night skies. Long before the sixth century BC, astronomers were aware that the stars at night all appear to revolve in a clockwise direction around a fixed point. That fixed point is the celestial North Pole, which is the point in the sky that would be directly overhead if you were standing at the geographical North Pole. But for observers sited elsewhere in the Northern Hemisphere, the celestial North Pole appears in the sky due north and at an altitude above the horizon corresponding their latitude: thus from London, it is to be found at about 51 ° 30’ above the horizon, but from Athens, only 38 °.
The celestial North Pole is currently marked by the moderately bright star Polaris, but in classical antiquity there was no naked eye star close to the spot. Instead, navigators used the entire constellation of Ursa Minor for navigation purposes. It would nevertheless have been apparent from travellers’ tales that the further south you went, the lower in the sky this constellation would appear. Furthermore, constellations that are circumpolar (never setting, or as Homer put it, “never bathing in Ocean’s stream”) as seen from Greece would at times be out of view. But other, more southerly constellations would rise higher in the night sky, and the Southern Cross, which disappeared from the Mediterranean skies around 1700 BC, would come into view. Such reports could only be explained if the Earth was spherical.
Another clue comes from the Moon, whose phases might have been recorded as long ago as 35,000 years. When the Moon sets just after sunset, it is seen as a crescent with the illuminated side facing the western horizon; a half Moon (either waxing or waning) is always to be found 90 degrees away from the Sun; a full Moon always rises at sunset; finally, when the Moon rises just before sunrise, it is seen as a crescent with the illuminated side facing the eastern horizon. The phases of the Moon can easily be simulated by illuminating a ball with a torch in a darkened room and observing it from different angles. The Ancient Greeks could have carried out the same exercise, substituting the torch for an oil lamp or candle. A spherical Moon doesn’t necessarily imply a spherical Earth, but it is inconsistent with a flat earth.
Lunar eclipses provide a confirmatory clue. It would long have been known that a lunar eclipse can only happen when the Moon is full, and that a full Moon happens when the Moon is on the opposite side of the sky to the Sun and its entire Earth-facing surface is illuminated. A lunar eclipse must therefore be caused by Earth getting in the way of the Sun and casting a shadow across the surface of the Moon.
This shadow – a shadow of Earth – always appears circular. While a flat plate could cast a circular shadow, it would not always do so. The shadow would depend on the angle of the plate with respect to the Sun, and it would typically appear elliptical. But, regardless of the where the Moon is in the sky when an eclipse occurs, the Earth’s shadow is circular. This can only be explained by a spherical Earth, which casts a circular shadow from all angles.
During the sixth century BC, it is likely that Greek scholars pulled these three strands of evidence together and concluded that the Earth is spherical. It is not clear who made the breakthrough, even assuming it was just one person. It is commonly suggested that Pythagoras (c. 570 – c. 495 BC) or at any rate the Pythagorean school first put forward the idea of a spherical Earth.
The pre-requisites for discovering that the Earth is spherical would have been:
1. A literate society with writing technology capable of keeping records of astronomical phenomena.
2. A society in which trade and other long-distance interactions occur, where travellers have opportunities to observe night skies in distant lands.
3. An intellectual climate in which rational investigations of astronomical phenomena are likely to occur.
These conditions were certainly met in the Archaic and Classical periods of Ancient Greece, but what about the earlier civilisations that flourished during the Late Bronze Age in the Mediterranean and Ancient Near East? These civilisations interacted in a ‘Club of Great Powers’ from around 1500 to 1100 BC. Their combined realms spanned 20 degrees of longitude from the Hittite capital of Hattusa at 40° N to Nubia at 20° N; there would have been significant differences between the night skies of Anatolia and the southern reaches of the Nile. The keeping of astronomical records goes back to around 1600 BC in Mesopotamia, and astronomy was also important to the Ancient Egyptians for calendrical and astrological purposes. The first two conditions were certainly met. The Ancient Egyptians and Mesopotamians could surely have deduced that the Earth is spherical. Yet apparently they did not. As noted above, the cosmology of these societies was based upon a flat Earth.
One possibility is that belief in a flat Earth might be an intrinsic feature of the neural architecture of the human brain. South African cognitive archaeologist David Lewis-Williams has noted that a central tenet of many religions is the existence of a three-tiered cosmos with realms located ‘above’ and ‘below’ that of our every-day experience. The Abrahamic tradition of Heaven and Hell are only one example of such a cosmology. Lewis-Williams suggests that the widespread belief in the existence of these other realms arises from visions and hallucinations experienced in altered states of consciousness as may be induced by meditation, psychotropic substances, and various ritual practices. All human brains are wired up the same way, and so all will experience broadly the same visions and hallucinations. The specifics of how they are interpreted varies from culture to culture, but they share the same basic aspects of a Heaven, Earth, and Underworld.
Could this have hampered attempts to interpret celestial phenomena that could not be explained by the standard flat Earth model? Possibly it took the rationalism of the sixth century BC pre-Socratic philosophers to break an ancient, hard-wired mindset, and to allow a truer (albeit still far from complete) view of the cosmos to emerge.
In 1838, an eccentric British inventor named Samuel Birley Rowbotham attempted to demonstrate that the Earth is flat, using a long stretch of water known as the Bedford Level. Rowbotham’s views were based on the Hebrew cosmology of the Old Testament, which depicts a three-tiered cosmos with the heavens above, the earth in the middle, and the underworld below.
In support of his Flat Earth model, Rowbotham recruited the Bedford Level, a 9.7 km (6 mile) section of the Old Bedford River. The latter is an artificial river formed by a partial diversion of the River Great Ouse in the Fens, Cambridgeshire, intended to reduce flooding of the fenlands. It is named after Francis Russell, Fourth Earl of Bedford, who began the project in 1630 although it was not completed in his lifetime. The Bedford Level is an uninterrupted stretch which runs in a straight line to the north-east of the village of Welney.
Rowbotham’s idea was to set up a telescope 20 cm (8 inches) above the surface of the water at one end of the Bedford Level and use this to watch while a boat was rowed towards Welney Bridge at the other end, six miles away. The boat had a flag on three-foot (0.91 m) mast. If the Earth really was curved, the top of the mast would disappear long before the boat reached Welney Bridge. Rowbotham relied on an approximation used by surveyors: the drop expressed in inches is 8 times the square of the distance in miles. After one mile the drop is eight inches; after two it is 32 inches; and after three it is 72 inches or six feet.
This approximation holds good for the distances involved, but it fails to take account of the height above the ground of the observer. A more accurate calculation shows that the portion of the boat hidden from Rowbotham’s view would be zero after a mile, 8 inches (0.2 m) after two miles, and 2 ft 8 inches (0.81 m) after three miles. Nevertheless, only the top of the mast would be visible at three miles, and long before it reached Welney Bridge the whole of the boat would have disappeared. Instead, it remained completely visible throughout its journey, which Rowbotham took as proof that the Earth is flat.
Based on his observations, Rowbotham published a pamphlet entitled Zetetic Astronomy, which he expanded into the book Earth Not a Globe in 1865. He suggested that the Earth is a flat disc with its centre at the North Pole, and that it is surrounded by a wall of ice. The Sun and Moon were located 3,000 miles (4,800 km) above Earth, and the stars and planets were a further 100 miles (160 km) away.
But Rowbotham had made an elementary mistake. He had failed to realise that the effects atmospheric refraction close to the water can cancel out the effect of curvature, making the Earth’s surface appear flat or even concave. The same effect can make the Sun appear above the horizon shortly before it actually rises, or after it has set. The phenomenon was well known in Rowbotham’s day, and it was routinely allowed for by sailors navigating at sea, and by surveyors.
Rowbotham’s views understandably attracted very little attention until 1870, when a supporter by the name of John Hampden (unrelated to the Civil War-era politician) wagered £500 that that nobody could repeat the experiment and show that the Earth was other than flat. The challenge was taken up by none other than Alfred Russel Wallace, co-proposer of the theory of evolution by natural selection. Wallace’s day job was a surveyor, and he would have known how to avoid the pitfalls that had affected the original experiment albeit he was apparently unaware of Rowbotham’s efforts. £500 was a considerable sum of money at the time (over £50,000 at today’s rates) and Wallace doubtless saw it as an opportunity to make some easy money. With the benefit of hindsight, perhaps he shouldn’t have.
Wallace noted that the parapet of Welney Bridge was 4 m (13 ft. 3 in.) above the water but that the Old Bedford Bridge, at the other end of the Bedford Level, was somewhat higher. To this bridge he affixed a calico sheet with a black stripe, which he positioned so that the lower edge of the stripe was also 4 m (13 ft. 3 in.) above the water. The centre of the stripe would be as high as the line of site as a telescope he set up Welney Bridge. Midway between the two bridges he set up a long pole with two red discs attached, the upper one having its centre the same height above the water as the centre of the black band and of the telescope, while the lower disc was four feet below it. The telescope, upper disc, and black band were thus all the same height above the water; the greater height than that of the earlier experiment would reduce the effects of atmospheric refraction. If the Earth was flat, the upper disc as viewed through the telescope would appear level with the black band. If on the other hand the Earth was round the upper disk would appear above the black band.
To ensure fair play, John Henry Walsh, editor of The Field magazine, was appointed as an independent referee. Walsh was a good choice, as he was not personally acquainted with either Hampden or Wallace. He also had prior experience deciding wagers. But shortly after funds were lodged with Walsh to guarantee the wager, Hampden demanded a referee of his own. Wallace was agreeable, but Hampden chose fellow flat earth enthusiast William Carpenter. Furthermore, Walsh was unable to remain at the Bedford Level for the whole duration of the experiments and a surgeon named Martin Wales Bedell Coulcher stood in for him as Wallace’s referee.
As might be expected, the experiments showed the upper disk apparently raised above the Old Bedford Bridge marker. Coulcher sketched the view through the telescope, and Carpenter signed it to affirm that this was what both men had seen.
Then things started to go awry. Carpenter declared that the result did not prove that the Earth was round because the telescope had not been levelled and did not have cross-hairs. Wallace obtained a spirit level and a smaller telescope equipped with cross-hairs, and then performed the experiment again. The outcome, of course, was exactly the same as before. Both referees then sketched what was seen in the second telescope, and Coulcher signed the sketches. But now Carpenter refused to accept the result because the distant marker appeared below the middle one as far as the middle one did below the cross-hair, which he claimed proved that the three were in a straight line, and that consequently the earth was flat.
Hampden eventually agreed to allow Walsh to review the results and make a decision. Both sides submitted reports, and after considering these Walsh ruled in Wallace’s favour and published the reports and his conclusions in The Field. Ignoring Hampden’s protests, he awarded the £500 wager to Wallace.
At this point, things got decidedly ugly. Hampden went to court to get his money back and won because at this stage it was still in the hands of Walsh and had not yet been paid to Wallace. He published a pamphlet alleging that Wallace had cheated and not content with that, he wrote Wallace’s wife Annie threatening to kill her husband. The death threats landed Hampden in jail for three months. Early in 1871, Wallace sued for defamation and was awarded £600. But Hampden transferred all his assets to his son-in-law and claimed he could not pay. Wallace was left having to pay the court costs. The row continued for another twenty years, during which Hampden was twice imprisoned for defamation. It was only brought to and end by Hampden’s death in 1891.
The whole affair left Wallace significantly out of pocket. He had not been a wealthy man before the wager not least of all because his financial acumen fell rather short of his abilities as a scientist. As a result, he struggled financially until 1881, when he was awarded a £200 p.a. pension thanks to the lobbying of Charles Darwin. Wallace was also criticised for becoming involved with a wager to ‘decide’ an issue which had long been an established scientific fact. All in all, it was a thoroughly unpleasant episode in the life of one of the greatest scientists and thinkers of the nineteenth century.
In 1901, Henry Yule Oldham, a lecturer in geography at King’s College, Cambridge, repeated the Bedford Level experiment using three poles fixed at equal height above the water level, which he viewed with a theodolite and a plate camera. Oldham’s version of the Bedford Level experiment was regarded as definitive because of the photographic evidence. He presented his results in a lecture at the British Association for the Advancement of Science, and they were taught in schools for many years until the first photographs taken from space became available in the late 1940s.
As is often the way with what the late Sir Patrick Moore once described as ‘independent thinkers’, the Flat Earthers were not about to give up. In 1904 Lady Elizabeth Anne Blount hired photographer Edgar Clifton to use a camera equipped with telephoto lens to take a picture from Welney Bridge of a large white sheet touching the surface of river six miles (9.7 km) away, at the position Rowbotham had conducted his original experiment. The camera was mounted two feet above the water, and Clifton was able to obtain a picture of the target, which should have not have been visible. The experiment, of course, suffered from the same vulnerability to atmospheric refraction as had the original. Needless to say, this did not stop Lady Blount from publishing the pictures far and wide.
To this day, Flat Earth enthusiasts continue to champion the Bedford Level experiments as having proved that the Earth is flat. A search on Youtube will reveal an attempt in 2016 to recreate the Rowbotham’s original experiment using a laser pointer mounted on the rear deck of a kayak. Problems faced by the researchers included the discovery that the kayak deck sloped upwards and ‘best judgment’ had to be applied to place the laser ‘as level as possible’. The video has received 1,600 ‘likes’ against 805 ‘dislikes’, but many of the 2,404 comments posted are unfavourable.
It would be a worthwhile project – possibly for a science ‘outreach’ group – to repeat the Bedford Level experiment with modern equipment. In the meantime, anybody who seriously believes that the Earth is flat is urged to take a look at this spectacular video of the Turning Torso building in Malmo, viewed from successively greater distances on the Danish side of the Øresund strait between Denmark and Sweden.
“They all laughed at Christopher Columbus
When he said the world was round”
George and Ira Gershwin were responsible for some of the most memorable songs of the last century, but this particular line highlights a common misconception. In 1492, when Columbus’ small fleet sailed from Palos de la Frontera in Spain, it had been the best part of two millennia since any serious scholar had believed that the world was flat.
The ancient Greeks were not only aware that the Earth was spherical, but around 240 BC they made an estimate of the circumference and obtained a surprisingly accurate result. Eratosthenes of Cyrene (276 – 194 BC) was a Greek polymath who held the post of Chief Librarian at the Library of Alexandria. He had heard that in Syrene (now Aswan), the noontime Sun on the day of the summer solstice lit up a well, casting no shadow on the side and implying that it was directly overhead. At Alexandria, however, the Sun did not quite reach the zenith and therefore did cast a shadow. By measuring the length of the shadow cast by a vertical rod of known height, the angular distance of the Sun from the zenith in Alexandria at noon could be determined. Of course, Eratosthenes couldn’t simply look at his watch to see when it was noon, so he would have relied on the shadow being at its minimum length at noon.
Eratosthenes found that the angular distance was a fiftieth of a whole circle (i.e. 50/360 = 7.2 degrees) and that the distance from Alexandria to Syrene was therefore a fiftieth of the circumference of the Earth. He then used a value of 5,000 stadia for the distance between the two cities (clearly a rough estimate) to obtain a value of 250,000 stadia for the Earth’s circumference.
Unfortunately, we don’t know the exact value of the stadion Eratosthenes used. The stadion (from which we get the word ‘stadium’) was defined as 600 Greek feet, but different values of the foot were used in different parts of the Greek world and Eratosthenes’ stadion is thought to have been anywhere between 150 and 158 meters (492 and 519 feet), making the distance from Alexandria to Aswan 750 -790 km (466 – 490 miles). We thus obtain a value of 37,500 – 39,500 km (23,300 – 24,500 miles) for the Earth’s circumference, which is very close to the accepted value of 40,075 km (24,901 miles). The actual distance from Alexandria to Aswan is 843 km (524 miles), giving a value of 42,150 km (26,200 miles).
Eratosthenes made the following assumptions: firstly, the Sun is so distant that rays of light reaching Alexandria and Syrene are effectively parallel; secondly that Syrene is located on the Tropic of Cancer (the latitude where the Sun is directly overhead on the summer solstice); and thirdly that Alexandria lies on the same meridian as Syrene.
Eratosthenes’ first assumption was correct, but the other two were not entirely accurate. The Tropic of Cancer is currently located at latitude 23°26′12.7″ N, but in Eratosthenes’ day, it was lay at approximately latitude 23°43′ N. Syrene lies at 24°05′ N 32°54 E, 22 minutes of an arc north of the Tropic of Cancer as it then was; and three degrees further east than Alexandria, which lies at 31°12′ N 29°55′ E.
Essentially, Eratosthenes’s experiment entailed simultaneous measurements the sun’s altitude at two separate locations on the same meridian. The experiment was simplified by choosing the solstitial sun at noon, and a second location that he either believed or approximated to be due south and at the latitude of the Tropic of Cancer. Had both been the case, he would have obtained an answer of 41,600 km (25,850 miles) based on a second location 7°29′ due south of Alexandria. Today we could easily replicate the experiment with two observers equipped with clinometers at John O’ Groats and Weston Super Mare, which are very close to sharing a common meridian and are 810 km (503 miles) apart. The experiment demonstrates how fundamental data can sometimes be obtained from a subtle but easy to measure phenomenon.