t years. For instance, a transit
of Venus took place in 1761, and again in 1769. No further transits
occurred until those witnessed in 1874 and in 1882. Then, again, comes a
long interval, for another transit will not occur until 2004, but it
will be followed by another in 2012.

This arrangement of the transits in pairs admits of a very simple
explanation. It happens that the periodic time of Venus bears a
remarkable relation to the periodic time of the earth. The planet
accomplishes thirteen revolutions around the sun in very nearly the same
time that the earth requires for eight revolutions. If, therefore, Venus
and the earth were in line with the sun in 1874, then in eight years
more the earth will again be found in the same place; and so will Venus,
for it has just been able to accomplish thirteen revolutions. A transit
of Venus having occurred on the first occasion, a transit must also
occur on the second.

It is not, however, to be supposed that every eight years the planets
will again resume the same position with sufficient precision for a
regular eight-year transit interval. It is only approximately true that
thirteen revolutions of Venus are coincident with eight revolutions of
the earth. Each recurrence of conjunction takes place at a slightly
different position of the planets, so that when the two planets came
together again in the year 1890 the point of conjunction was so far
removed from the critical point that the line from the earth to Venus
did not intersect the sun, and thus, although Venus passed very near the
sun, yet no transit took place.

[Illustration: Fig. 45.--Venus on the Sun at the Transit of 1874.]

Fig. 45 represents the transit of Venus in 1874. It is taken from a
photograph obtained, during the occurrence, by M. Janssen. His telescope
was directed towards the sun during the eventful minutes while it
lasted, and thus an image of the sun was depicted on the photographic
plate placed in the telescope. The lighter circle represents the disc of
the sun. On that disc we see the round, sharp image of Venus, showing
the characteristic appearance of the planet during the progress of the
transit. The only other features to be noticed are a few of the solar
spots, rather dimly shown, and a network of lines which were marked on a
glass plate across the field of view of the telescope to facilitate
measurements.

The adjoining sketch (Fig. 46) exhibits the course which the planet
pursued in its