Neptune

Neptune

During the first half of the 19th century astronomers noticed that the actual position of Uranus was gradually falling behind its predicted position. The discrepancy indicated that a disturbing body in the form of an unseen planet was pulling Uranus “off course”, and two mathematicians, J. Couch Adams in England and Urbain J.J. Leverrier in France, independently calculated its most likely position. Their investigations ended in 1846 with the discovery of Neptune, found in a position close to the predicted places.

Neptune takes nearly 165 years to orbit the Sun, a period so long that the planet has not yet made one complete revolution since it was discovered. Its average distance from the Sun is 2,794 million miles, or about 30 times the corresponding distance for the Earth. In large telescopes is shows a definite disk, and measurements of this indicate an actual diameter of 27,800 miles, or slightly less than the Diameter of Uranus.

Neptune. Photo in public domain

Analysis of the sunlight reflected by Neptune shows the planet’s atmosphere is rich in methane, but since the temperature is thought to be as low as -270 degrees centigrade, any ammonia would be frozen solid. No belts can be seen, but there are two satellites – Triton, discovered by William Lassell in 1846, and Nereid, first seen on photographs taken by Kuiper in 1949. Triton has a diameter of about 2,300 miles and is therefore slighter larger than our moon. Nereid can be seen only on photographs taken with large telescopes, and is probably no more than about 200 miles across.