passage across the sun on the two occasions in 1874 and
1882. Our generation has had the good fortune to witness the two
occurrences indicated on this picture. The white circle denotes the disc
of the sun; the planet encroaches on the white surface, and at first is
like a bite out of the sun's margin. Gradually the black spot steals in
front of the sun, until, after nearly half an hour, the black disc is
entirely visible. Slowly the planet wends its way across, followed by
hundreds of telescopes from every accessible part of the globe whence
the phenomenon is visible, until at length, in the course of a few
hours, it emerges at the other side.

It will be useful to take a brief retrospect of the different transits
of Venus of which there is any historical record. They are not numerous.
Hundreds of such phenomena have occurred since man first came on the
earth. It was not until the approach of the year 1631 that attention
began to be directed to the matter, though the transit which undoubtedly
occurred in that year was not noticed by anyone. The success of Gassendi
in observing the transit of Mercury, to which we have referred in the
last chapter, led him to hope that he would be equally fortunate in
observing the transit of Venus, which Kepler had also foretold. Gassendi
looked at the sun on the 4th, 5th, and 6th December. He looked at it
again on the 7th, but he saw no sign of the planet. We now know the
reason. The transit of Venus took place during the night, between the
6th and the 7th, and must therefore have been invisible to European
observers.

Kepler had not noticed that another transit would occur in 1639. This
discovery was made by another astronomer, and it is the one with which
the history of the subject may be said to commence. It was the first
occasion on which the phenomenon was ever actually witnessed; nor was it
then seen by many. So far as is known, it was witnessed by only two
persons.

[Illustration: Fig. 46.--The Path of Venus across the Sun in the
Transits of 1874 and 1882.]

A young and ardent English astronomer, named Horrocks, had undertaken
some computations about the motions of Venus. He made the discovery that
the transit of Venus would be repeated in 1639, and he prepared to
verify the fact. The sun rose bright on the morning of the day--which
happened to be a Sunday. The clerical profession, which Horrocks
followed, here came into collision with his desires as an astronomer. He
tells