Public Lectures for the Session 2001-2002 held in Room E7 of the Renold Building, UMIST.
Delivered to the Manchester Astronomical Society

17th January 2002

''The Outer Planets'''

Dr Andy Hollis
Director of the Asteroids and Remote Planets Section, BAA.

Presidential Address

Dr Hollis said that observing the outer planets had more similarities with observing astreroids than with the nearer planets, they essentially appear star-like. After introducing his subject with reference to the classical planets, he confined his talk to Uranus, the first planet to be discovered telescopically in historical times by Herschel in 1781; Neptune located mathematically by Adams and LeVerrier in 1846 using Uranus' orbital perturbations; and Pluto, found after extensive photographic searches by Tombough in 1930.

Both Uranus and Neptune are easily visible in binoculars and a good way to observe them is to consider them as variable stars. This is a useful area of study if carried out systematically and regularly as both planets show variability, probably due to the coming and going of dark and light surface features in their dense atmospheres. The small angular sizes of these planets do not lend themselves to detailed amateur telescopic scrutiny and only vague markings are occasionally visible. Uranus shows a distinctly greenish disk while Neptune is blue. Both have satellites that can be recorded with CCD cameras. Pluto can also be detected in this manner but is very difficult to see visually in all but large amateur telescopes.

It is still debatable whether Pluto is a true planet or a member of a growing number of Kuiper Belt Objects, trans Neptunian objects the first of which, Chiron, was discovered in the early 1990's. Tombough's original plates are still used to discover TNOs and about 80 are now known. Dr. Hollis felt, however, that as both Pluto and its large moon, Charon are considerably bigger than any of the TNOs it was arguably safe to consider Pluto as a true planet. It will be necessary to wait for the proposed Pluto-Kuiper Express spacecraft to visit the planet before a final categorisation can be made.

Synopsis by Kevin J. Kilburn (Secretary)

Maintained by Michael Oates
Page modified 28 October, 2006