Sometimes icy lumps left over from the birth of the Solar
System visit our skies. We see them as comets. Although tiny, comets release
vast clouds of gas and dust as they approach the Sun and heat up. The clouds
form a bright head and long tails, often millions of kilometres long. When
Earth passes through the dust from past comets we see METEOR
showers.
One of the brightest comets of the 20th century, Hale-Bopp blazed in
the night sky for weeks during the spring of 1997. Its bright coma (head) hid a
nucleus about 30–40 km (20–30 miles) across. The effects of
sunlight and of the solar wind strung out the gas and dust released by the
comet into long tails.
Comets may head in towards the Sun from any direction. They have
highly elliptical (oval) orbits. Comets may take just a few years or thousands
to circle the Sun. Some seem to come from reservoirs of icy bodies in the
Kuiper Belt, or from further out in a region called the Oort Cloud. Unseen for
most of the time, comets become visible only when they approach the Sun.
BIOGRAPHY: EDMOND HALLEY English, 1656-1742
Halley was an astronomer and mathematician who became the second
Astronomer Royal. He is best known for discovering that some comets are regular
visitors to Earth’s skies. He correctly predicted that the comet he had
seen in 1682 would return again in 1758. It did, and was named Halley’s
comet after him.
On a clear night you may see little streaks of light in
the sky, which are often called falling or shooting stars. But these streaks
are properly called meteors. They are caused by little rocky specks plunging
through the atmosphere towards Earth. Friction with the air makes them so hot
that they burn up into dust. As much as 200 tonnes of meteor dust falls to
Earth every day.
When Earth crosses the orbit of a comet dozens of meteors per hour
may be seen. This is called a meteor shower. Showers are named after the part
of the sky they appear to come from. For example, the Leonid shower in
mid-November seems to come from the constellation Leo. It takes place when
Earth passes through dust from comet Tempel-Tuttle. Every 30 years or so it
puts on an exceptional display of hundreds and thousands of meteors per hour.
