We never know when a really bright comet may appear, and when
they do so they cause a great deal of interest, and in former times fear,
as comets were regarded as portents of doom. “When beggars die there
are no comets seen, the heavens themselves blaze forth the death of
princes”– William Shakespeare.
Over the centuries there have been many alarms raised about possible
comet collisions, and it is quite true that the impact of a nucleus just a few
miles across could have devastating effects, but in the recent past, this
has not happened.
As a comet warms in the Sun’s light the outgassing that follows
causes a trail of debris to be left along its orbit. If the Earth’s orbit
encounters this debris, the typically sand grain-sized particles crash
into Earth’s atmosphere where they vaporize, leaving a streak of light
known as a meteor.Meteors and Meteorites
The term shooting star refers to the brief streak of light that you
sometimes see as a small particle known as a meteoroid vaporizes
in our atmosphere. Known formally as meteors, they are nothing to
do with real stars of course, and occur much closer to home, typically
at heights between 45-70 miles up. Randomly occurring meteors are
known as sporadics while those which appear because the Earth movesThe pace of a lunar eclipse is in complete contrast to the frenetic activity
surrounding totality of a total solar eclipse, giving you ample time to go out
and enjoy the event.Comets
A comet is quite unlike a planet. It consists of a small icy nucleus,
surrounded by dust and gas. Some comets move round the Sun in short
periods, so we know when and where to expect them. Others come
from the depths of space and take us by surprise, paying us only one
visit before retreating to the distant parts of the Solar System. Much the
most famous of all is Halley’s Comet, named after the well known English
astronomer Edmond Halley, which he did not discover but whose orbit
he was the first to calculate. It has a period of 76 years and can become
a brilliant naked eye object. At its last return in 1986 it was badly placed
and not very conspicuous. We know a lot about it because unmanned
spacecraft visited it. One penetrated the heart of the comet until it was
put out of action by small particle – probably no larger than a grain of rice.
A comet has no light of its own and depends upon what it receives from
the Sun.
Short period comets go round the Sun in periods of a few years – in
the case of Encke’s Comet, 3.3 years – but not many of these are visible
with the naked eye. Every time a comet passes its perihelion or closest
approach to the Sun it loses a certain amount of material and this
wastage cannot go on indefinitely so by astronomical standards comets
are short lived. Several comets which used to return regularly to the Sun
have now vanished completely. Large comets have tails, which may be
of two types, a curved tail made up of dust, and a straight tail made up of
ionised gas, all robbed from the nucleus.[15] Comet C/2006 P1 McNaught photographed against a bright twilight sky in
January 2007.
[16] Donati's comet over St Paul's cathedral in London. 1858 From Agnes Giberne's book
Sun, Moon and Stars published in 1884.1516Astronomer Book