THE SOLAR SYSTEM
tion; it is exactly opposite to the Sun in the sky, and is best
placed for observation. The interval between one opposi-
tion and the next is known as the synodic period.
The movements of Mars are shown in the next two
diagrams. It is clear that oppositions do not occur every
year; the Earth has to ‘catch Mars up’, and the mean
synodic period is 780 days. Oppositions of Mars occur
in 1999, 2001 and 2003, but not in 1998 or 2000. As the
Earth ‘passes’ Mars, there is a period when the planet will
move against the stars in an east-to-west or retrograde
direction. The giant planets are so much further away, and
move so much more slowly, that they come to opposition
every year. Jupiter’s synodic period is 399 days, but that
of Neptune is only 367.5 days, so that it comes to opposi-
tion less than two days later every year.There should be no trouble in identifying Venus and
Jupiter, because they are always so brilliant; Mercury is
unlikely to be seen unless deliberately looked for, while
Uranus is on the fringe of naked-eye visibility, and
Neptune and Pluto are much fainter. Mars at its best can
actually outshine all the planets apart from Venus, but
when at its faintest it is little brighter than the Pole Star,
though its strong red colour will usually betray it. Saturn is
brighter than most of the stars, and because it takes almost
30 years to complete one journey round the Zodiac it can
be found without difficulty once initially identified.
Planets can pass behind the Moon, and be occulted.
The planets themselves may occult stars, and these events
are interesting to watch, but they do not happen very often,
and an occultation of a bright star by a planet is very rare. Saturn near the Moon.
Saturn almost occulted by
the Moon, photographed
through a 30-cm (12-inch)
reflector. Note the smallness
of Saturn!C Atl of Univ Phil'03stp 2/4/03 2:56 pm Page 63