Occasionally Mercury passes at inferior conjunction between us and
the disc of the sun, appearing like a black spot against the sun, and
thus makes what we call a transit. Because the planet is so small, his
transit across the sun cannot be seen with the naked eye; but it is
an interesting phenomenon to those who can view it with a telescope,
though, apparently, astronomers do not regard it as having any great
scientific importance. It is during a transit, however, that we watch
for confirmation of the theories concerning Mercury’s atmosphere,
which, if it were a reality, would show a diffused light about the
planet; and until this question is settled beyond any dispute it will
always come up at the time of a transit of Mercury. At nearly every
transit some observer sees these indications of an atmosphere; but
the better the telescope, the less they seem to be seen. Hence it
is probable that there is an illusion somewhere either of eye, or
instrument, or mind, and that the majority opinion, which accords to
Mercury practically no atmosphere, is about the correct one.
These transits occur at intervals of seven, thirteen, or forty-six
years, according to the position of the earth. They would occur every
time that Mercury passed inferior conjunction if the earth’s orbit
and that of Mercury were in exactly the same plane. But the orbit of
Mercury, we have seen, is tilted out of the plane of the ecliptic,
which marks our orbit, seven degrees, so that the only time the earth
and the planet are anywhere nearly in the same plane is when they are
at or near the points where their orbits cross each other.
The earth is near the two points where Mercury crosses the ecliptic
about May 8th and November 9th, so that transits can occur only near
these dates. Mercury passes these points four times every year, or once
in each revolution around the sun. But the earth is not always there
at the same time, and it is because of this that transits occur only
in periods of seven, thirteen, or forty-six years. They occur more
frequently in November than in May. The last transit was in November,
1907. The next will be on November 7, 1914, and there will not be
another in November until 1927, an interval of thirteen years. But at
the point where the May transits occur there will be one on May 7,
1924.