g apart as a year and a half,
and Mercury only about every four months. From this it will be further
gathered that transits of Mercury take place much oftener than transits
of Venus.

Until recent years _Transits of Venus_ were phenomena of great
importance to astronomers, for they furnished the best means then
available of calculating the distance of the sun from the earth. This
was arrived at through comparing the amount of apparent displacement in
the planet's path across the solar disc, when the transit was observed
from widely separated stations on the earth's surface. The last transit
of Venus took place in 1882, and there will not be another until the
year 2004.

_Transits of Mercury_, on the other hand, are not of much scientific
importance. They are of no interest as a popular spectacle; for the
dimensions of the planet are so small, that it can be seen only with the
aid of a telescope when it is in the act of crossing the sun's disc. The
last transit of Mercury took place on November 14, 1907, and there will
be another on November 6, 1914.

The first person known to have observed a transit of an inferior planet
was the celebrated French philosopher, Gassendi. This was the transit of
Mercury which took place on the 7th of December 1631.

The first time a transit of Venus was ever seen, so far as is known, was
on the 24th of November 1639. The observer was a certain Jeremiah
Horrox, curate of Hoole, near Preston, in Lancashire. The transit in
question commenced shortly before sunset, and his observations in
consequence were limited to only about half-an-hour. Horrox happened to
have a great friend, one William Crabtree, of Manchester, whom he had
advised by letter to be on the look out for the phenomenon. The weather
in Crabtree's neighbourhood was cloudy, with the result that he only got
a view of the transit for about ten minutes before the sun set.

That this transit was observed at all is due entirely to the remarkable
ability of Horrox. According to the calculations of the great Kepler, no
transit could take place that year (1639), as the planet would just pass
clear of the lower edge of the sun. Horrox, however, not being satisfied
with this, worked the question out for himself, and came to the
conclusion that the planet would _actually_ traverse the lower portion
of the sun's disc. The event, as we have seen, proved him to be quite in
the right. Horrox is said to have been a veritable prodigy of
ast