In the early hours you will see Jupiter shining brightly in the Southeast. Further towards the south, close to the boundary between the constellations of Capricorn and Aquarius, lies the planet Neptune. However, this planet is too faint to see with the unaided eye. A telescope will show it as a small, blue-green disc.
Neptune is the fourth-largest planet in the Solar System. It has a diameter of about 49,530 km, compared with our Earth’s 12,756, which means it has about 58.5 times the Earth’s volume. However its mass is only about 17 times larger than our planet’s. That means it has a mean density of 1.6 grams per cubic centimetre, compared with our world’s 5.5 grams per cubic centimetre. Our world is mainly rock, with a nickel-iron core. Neptune has to be like Jupiter, Saturn and Uranus; big balls made mainly of gas, with lumps of rock and ice in their cores. The thing that makes Neptune unique in astronomy is that it was the first planet to be discovered by computation rather than observation.
In 1821 Alexis Bouvard published tables of calculated positions of Uranus, the seventh planet out from the Sun. However, subsequent observations showed his predictions to be in error. Bouvard suggested the discrepancies were due to there being another, unknown planet out beyond Uranus. Adams in England and Le Verrier in France independently calculated the mass and position of the unknown world. One other thing they had in common was that both of them had a lot of trouble persuading someone to actually point a telescope at that position and make a serious attempt to see if there was actually anything there. Finally, in 1846 someone did check, and there it was. There was the usual squabble about who would name the new planet. Le Verrier proposed “Neptune”, and then decided that the new planet should be named after him. Luckily, everyone agreed that Neptune would be a good name for that distant, blue-green world.
Neptune rotates once on its axis every 16 hours. Points on Neptune’s equator would be moving at about 9700 km/h compared with 1660 km/hr for points on Earth’s equator. Neptune’s rapid rotation drives huge, long-lived and violent storms.
We celebrate our birthday each time the Earth completes a full orbit around the Sun since our last birthday. Neptune’s discovery was announced in 1846. The planet takes 165 years to complete each trip around the Sun, so as far as Neptune is concerned, measuring things in Neptunian years, the first anniversary of its discovery comes up in the year 1846 + 165 = 2011.
Like many famous discoveries in science, things could have been very different. When Galileo was turning his telescope on the sky, he actually observed Neptune. He recorded it in 1612 and again in 1613. However, he did not do enough observations over a long-enough period to see the planet moving against the starry background, and marked it down as a star. On the other hand, Galileo has a lot of great discoveries to his name. Maybe it was nice to leave some for others.
Mercury lies low in the Northwest after sunset. Saturn is higher in the Southwest, very close to the star Porrima. Jupiter and Mars lie in the east before dawn. Saturn is high in the south. The Moon will be Full on the 14th.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC.