Astronomer Book
The planets go round the Sun, rather than the Earth,
and are very much further away than the Moon. The
closet planet, Venus, is always at least a hundred
times as far as the Moon. To observe the moons of
Jupiter is an inspiring experience. To view and image
Saturn and its rings is perhaps the most breathtaking
experience in the whole of astronomy.THE PLANETS
they condensed out of what we call the solar nebula, a cloud of dust and
gas surrounding the youthful Sun. The fact that they are so different in mass
means they evolved in different ways. Obviously, tremendous attention has
been paid to the search for possible life. So far there has been no success
at all, and we have to admit that at the moment the only life we know in the
entire Universe is here on Earth.
Other stars have their own planetary systems. These must be dealt with
separately, because they are so far away that we cannot see any detail.
All the Solar System planets are fascinating telescopic objects, and even
a small instrument will give superb views. Moreover, the planets, unlike
the Moon, are always changing and there is always something new to see.
Neither does one know quite what to expect, and every planet can spring
remarkable surprises on the unwary observer.The Solar System is divided into two well-marked parts. First we have four
small solid rocky planets: Mercury, Venus, Earth and Mars. Beyond Mars
there is the wide gap filled by tens of thousands of dwarf worlds making up
the main belt asteroids. Of these only one, Ceres, is as much as 500 miles
across and only one, Vesta, is ever visible with the naked eye. The rest are
much smaller and many are just pieces of rock. Just how they were formed
is still a matter of debate; in all probability a planet is unable to form in this
part of the Solar System because of the destructive gravitational force of
giant Jupiter, more massive than the rest of the planets combined.
Beyond the asteroid belt we come to the four giant planets: two gas
giants, Jupiter and Saturn, and two ice giants, Uranus and Neptune. All are
much more massive than Earth and they are made up in a different way.
We are also dealing with much greater distances. The further from the Sun,
the larger the orbit and the slower the planet moves; Neptune, for example,
takes 165 years to complete one journey round the Sun. All the giants have
quick rotation periods, less than 10 hours in the case of Jupiter. There is still
discussion about the way in which the planets were formed, but certainly[1] Trails of Venus and Mercury over
Portsmouth, England.[2] Mercury seen from the
MESSENGER spacecraft (NASA).1