[3] A thin crescent Moon showing earthshine and the planet Mercury lie close to the
Pleiades open cluster.
[4] The planets Venus and Jupiter close together in a winter evening sky.Superficially, planets look like stars, and some are very brilliant –
particularly Venus. It has been said that while stars twinkle, planets
do not. A star appears as a point of light while a planet is a disc. All the
same, when a planet is low in the sky and its light is coming through
a thick layer of our atmosphere there is a definite twinkling effect.
Apart from Mercury and Venus, all the planets are tended by
secondary bodies known as satellites. Earth has one satellite, our
familiar Moon, but other planets have more; Jupiter, the largest planet,
has four main moons and a whole host of smaller ones. Also in the
Solar System we find various minor bodies, notably the asteroids. They
are small, usually less than 100 miles across, and most orbit the Sun
between the paths of Mars and Jupiter. Right at the edge of the main
Solar System, beyond the planet Neptune, we have another ring of small
bodies, known as the Kuiper Belt, in honour of the Dutch astronomer
Gerard Kuiper, one of the first to recognize their existence and to study it.
Comets are quite different from planets; they move round the Sun,
but most do so in long, highly eccentric orbits so their periods may
amount to many centuries. A typical comet consists of a nucleus of ice
and rocky particles which becomes surrounded by layers of gas and
dust as the nucleus warms. There are many periodical comets which
orbit the Sun in periods of a few years, but really bright comets are
rare visitors. Some have become so bright they even cast shadows.
Note, of course, that planets and comets shine only by reflected
sunlight; they have no light which they themselves generate.
Most people have seen shooting stars or meteors. A meteor
appears as a streak in the sky which may last for a few seconds before
disappearing. It has absolutely nothing to do with a real star. A meteor
occurs when a tiny particle known as a meteoroid, typically a few
millimetres to centimetres in size, dashes into the Earth’s atmosphere
and vaporizes, leaving only fine dust. Some meteoroids collect in swarms,
and every time we pass through such a swarm meteors are plentiful.
Now and then the Earth is hit by a much more massive body which lands
intact and is then known as a meteorite. There is no connection between
a meteor and meteorite. Meteors are cometary debris, but meteorites
are made up of stone, iron or both. In the past there have been really
major impacts. About 65 million years ago it is widely believed a huge
meteorite landed in the Yucatan Peninsula and caused such devastation
that the dinosaurs died out in the resulting conditions. This theory is by no
means proven, but it is well supported and plausible. What has happened
in the past can happen again, and we will be hit by a major meteorite.
We hope it will not be in a densely populated area. If a meteorite say half
Introduction
3 4a mile across landed in the middle of London the death toll would be
catastrophic. Fortunately the chance is slight.On to the Stars
Beyond Neptune and the Kuiper belt, it’s believed there’s a huge region of
cometary objects known as the Oort Cloud. Then there’s a vast gap before
we come to the nearest star. The first star maps were drawn up many
centuries ago, notably by the Greeks, and the stars were divided into
“constellations”, many of which had mythological names. One of the best
known constellations is Ursa Major, the Great Bear. Astronomically we
use the Latin names, for Latin is still the universal language even though
no one speaks it. The seven main stars of Ursa Major are spread out in
the pattern we often call the Plough or (in America) the Big Dipper.
All the main stars are many tens of light years away and, although
they are moving through space at high speeds, their individual or proper
motions are so small they can only be noticed over periods of lifetimes.
Come back to the Earth in 10 million years time, and the pattern of the
Great Bear will be distorted because two of the main stars are moving in
the opposite direction to the other five. Note that a constellation is purely a
named pattern of stars and has no real significance because the stars are
at very different distances away from us.
The constellation Orion, the Hunter, dominates the sky in winter
evenings in the northern hemisphere, and the two main stars, known
as Betelgeux and Rigel, are particularly brilliant. They are placed in the
same constellation but there is no connection between the two. Rigel is
much the more remote. From another vantage point many light years
from Earth, they may appear to lie on opposite sides of the sky. The main
constellations were drawn in 1651 and were given mainly mythological
names. Other constellations have been added since and given modern