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Astronomy and Space articles by Martin George of the Launceston Planetarium
10 July 2011
Neptune: Once Around The Sun
One of my favourite stories in astronomy is that of the discovery of Neptune in 1846, about which I wrote a few
months ago (See The Sunday Tasmanian, 6 March 2011). On Tuesday this week, Neptune completes exactly one
revolution around the Sun since its discovery.
In March, we were celebrating the 200th anniversary of the birth of Urbain Le Verrier, whose mathematical ability
was up to the task of predicting the planet's position before its discovery. Unknown to Le Verrier, John Adams in
England was working on the same problem. Both Adams and Le Verrier had trouble getting astronomers in their
own countries to search for the planet, even though both (but especially le Verrier) had come up with good
predictions as to where in the sky the new planet would be seen.
Eventually, Le Verrier, in frustration, wrote to Johann Gottfried Galle at the Berlin observatory, asking him to search
for the new planet at a specific direction in the sky. Galle received Le Verrier's letter on 23 September 1846, and
received rather reluctant permission from the Observatory's director, Johann Encke, to spend time on the search.
On that Wednesday night, Galle commenced the search with student Heinrich d'Arrest, using a fine refractor
telescope. They were looking for an object that was expected to show a small disc, but to appear essentially starlike
owing to its great distance from the Sun.
The stars, of course, remain almost completely fixed in their positions, whereas the planets are seen to 'wander' as
they, like us, move around the Sun.
Galle and d'Arrest were lucky. They had access to a particular star chart that had been only recently produced, and
were able to compare the positions of the stars as seen through the telescope with those on the chart, looking for a
'star' not previously noted.
Galle, at the telescope, called out the positions of the stars, while d'Arrest checked them off on the star chart. At
around 15 minutes after midnight, Galle called out the position of a 'star', to which d'Arrest responded "That one is
not on the chart!"
D'Arrest's comment must surely go down as one of the most famous in scientific history. Galle was staring at the
new planet, and the discovery was verified the following evening by observing the object's movement against the
starry backdrop.
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On 25 September 1846, Galle wrote in French to Le Verrier, commencing with the words "Monsieur, La planète
dont vous avez signalé la position réelement existe (Sir, the planet whose position you have announced really
exists). I can imagine Le Verrier's delight at receiving such a letter !
Neptune, the fourth largest planet, takes nearly 165 years to orbit the Sun because of its great distance. On
average, it is 4,497 million kilometres from the Sun, compared with only 150 million for the Earth. Because of the
position of the Earth in July compared with September, Neptune will not be seen against exactly the same
background of stars this week as on the night of its discovery, but it's a time for celebration, and for reflection on the
wonderful and exciting work done over 160 years ago.
Article by Martin George, Launceston Planetarium, QVMAG.
Reproduced with permission of the Sunday Tasmanian newspaper