Halley’s Comet
ATLAS OF THE UNIVERSE
Of all the comets in the sky
There’s none like Comet Halley.
We see it with the naked eye,
And periodically.N
obody seems to know who wrote this piece of doggerel,
but certainly Halley’s Comet is in a class of its own.
It has been seen at every return since that of 164 BC, and
the earliest record of it, from Chinese sources, may date
back as far as 1059 BC. Note that the interval between suc-
cessive perihelion passages is not always 76 years; like all
of its kind, Halley’s Comet is strongly affected by the
gravitational pulls of the planets.
Edmond Halley, later to become Astronomer Royal,
observed the return of 1682. He calculated the orbit, and
realized that it was strikingly similar to those of comets
previously seen in 1607 and 1531, so that he felt confident
in predicting a return for 1758. On Christmas night of that
year – long after Halley’s death – the comet was recovered
by the German amateur astronomer Palitzsch, and it came
to perihelion in March 1759, within the limits of error given
by Halley. This was the first predicted return of any comet;
previously it had been thought by most astronomers that
comets travelled in straight lines.
Halley’s Comet has a very elliptical orbit. At its closest
it is about 88 million kilometres (55 million miles) from
the Sun, within the orbit of Venus; at aphelion it recedes to
5250 million kilometres (3260 million miles), beyond the
orbit of Neptune and the Kuiper Belt. At its brightest
recorded return, that of AD837, it passed by the Earth at
only 6 million kilometres (3.75 million miles), andcontem-
porary reports tell us that its head was as brilliant as
Venus, with a tail stretching 90 degrees across the sky.
Another bright return was that of 1066, before the Battle
of Hastings; the comet caused great alarm among the▲ Halley’s Comet as seen
from Christchurch, New
Zealand,16 March 1986.
Photograph by Peter
Carrington.▼ Giotto, the British-
built spacecraft which
encountered Halley’s Comet
and subsequently went on
to an encounter with Comet
P/Grigg–Skjellerup. Halley’s Comettaken
during the 1910 return.
When far from the Sun,
a comet has no tail; the tail
starts to develop when the
comet draws inward and is
heated, so that the ices in its
nucleus begin to evaporate.
This series shows the tail
increasing to a maximum.D108-151 UNIVERSE UK 2003CB 7/4/03 5:18 pm Page 140