I dare say you would think it more interesting to talk about some big
and bright comets rather than about objects so faint as that of Encke.
It unfortunately happens that most of the fine comets pay our system
only a single visit. There is only one of the really splendid objects
of this kind that comes back to us with anything like regularity.
It was last seen in the year 1835, and I am glad to tell you that it
is coming again; it is expected about the year 1910. You may ask, How
can we feel sure that such a prediction as I have mentioned will turn
out correctly? The fact is that this comet has been watched for a great
many centuries. We find ancient records, some of them nearly 2000 years
old, of the appearance of grand comets, and several of these are found
to fit in with the supposition that there is a body which accomplishes
its journey in a period of about seventy-five or seventy-six years. Of
course there are thousands of other comets recorded in these old books
as well; but what I mean is that among the records many are found which
clearly indicate some successive returns of this particular body.
I will explain how the movements of this comet were discovered. There
was a great astronomer called Halley, who lived two hundred years
ago, and in the year 1682 he, like every one else, was looking with
admiration at a splendid comet with a magnificent tail which adorned
the sky in that year. At the observatories, of course, they diligently
set down the positions of the comet, which they ascertained by
carefully measuring it with telescopes. Halley first calculated the
highway which this comet followed through the heavens, and then he
looked at the list of old comets that had been seen before. He thus
found that in 1607--that was, seventy-five years earlier--a great
comet had also appeared, the path of which seemed much the same as
that which he found for the body that he had himself observed. This
was a remarkable fact, and it became still more significant when he
discovered that seventy-six years earlier--namely, in 1531--another
great comet had been recorded, which moved in a path also agreeing
with those of 1607 and 1682. It then occurred to Halley that possibly
these were not three different objects, but only different exhibitions
of one and the same, which moved round in the period of seventy-five or
seventy-six years.
There is a test which an astronomer can often apply in the proof
of his theory, and it is a very severe test. He will not only show
himself to be wrong if it fails, but he will also make himself somewhat
ridiculous. Halley ventured to submit his reputation to this ordeal. He
prophesied that the comet would appear again in another seventy-five
or seventy-six years. He knew that he would, of course, be dead long
before 1758 should arrive; but when he ventured to make the prediction,
he said that he hoped posterity would not refuse to admit that this
discovery had been made by an Englishman.
You can easily imagine that as 1758 drew near, great interest was
excited among astronomers to see if the prediction of Halley would be
fulfilled. We are accustomed in these days to find many astronomical
events foretold with the same sort of punctuality as we expect in
railway time-tables. The Nautical Almanac is full of such prophecies,
and we find them universally fulfilled. Even now, however, we are not
able to set forth our time-tables for comets with the same confidence
that we show when issuing them for the sun, the moon, or the stars.
How astonishing, then, must Halley’s prediction have seemed! Here was
a vast comet which had to make a voyage through space to the extent of
many hundreds of millions of miles. For three-quarters of a century
it would be utterly invisible in the greatest telescopes, and the only
way in which it could be perceived was by figures and calculations
which enabled the mind’s eye to follow the hidden body all around its
mysterious track. For fifty, or sixty, or seventy years nothing had
been seen of the comet, nor, indeed, was anything expected to be seen
of it; but as seventy-one, and seventy-two, and seventy-three years
had passed, it was felt that the wanderer, though still unseen, must
be rapidly drawing near. The problem was made more difficult for those
skilful mathematicians who essayed to calculate it by the fact that the
comet approached the thoroughfares where the planets circulate; and, of
course, the flimsy object would be pulled hither and thither out of its
path by the attractions of the weighty bodies. It was computed that the
influence of Saturn alone was sufficient to delay the comet for more
than three months, while it appeared that the attraction of Jupiter was
potent enough to retard the expected event for a year and a half more.
Was it not wonderful that mathematicians should be able to find out all
these facts from merely knowing the track which the comet was expected
to follow? Clairaut, who devoted himself to this problem, suggested
that there might also be some disturbances from other causes of which
he did not know, and that consequently the expected return of the comet
might be a month wrong either way. Great indeed was the admiration in
astronomical circles when, true to prediction, the comet blazed upon
the world within the limits of time Clairaut had specified.
The remarkable fulfilment of this prophecy entitles us to speak with
confidence about the past performances of this comet. Among all the
apparitions of Halley’s comet for the last two thousand years, perhaps
the most remarkable is that which took place in the year 1066. I am
sure you will all remember this date in your English history; it
was the year of the Conquest. In those days they did not understand
astronomy as we understand it now; they used to think of a comet as
a fearful portent of evil, sent to threaten some frightful calamity;
such as a pestilence, a war, a famine, or something else equally
disagreeable. Hence in the year of the Conquest the appearance of so
terrific an object in the sky was a very significant omen. Attention
was concentrated upon the spectacle, and a picture of Halley’s comet
as it appeared to the somewhat terrified imaginations of the people of
those days has been preserved. There is a celebrated tapestry at Bayeux
on which historical incidents are represented by beautifully worked
pictures. On this fabric we have a view of Halley’s comet in a quaint
and rather ludicrous aspect. You will read of this comet also in the
early pages of Tennyson’s “Harold.”