The swift messenger of the gods; sign [Symbol], his caduceus.
DISTANCE FROM THE SUN, 35,750,000 MILES. DIAMETER, 2992 MILES.
ORBITAL REVOLUTION, 87.97 DAYS. ORBITAL VELOCITY, 1773 MILES PER
MINUTE. AXIAL REVOLUTION, 24H. 5M.
Mercury shines with a white light nearly as bright as Sirius; is
always near the horizon. When nearly between us and the sun, as
at D (Fig. 46, p. 113), its illuminated side nearly opposite to
us, we, looking from E, see only a thin crescent of its light.
When it is at its greatest angular distance from the sun, as A or
C, we see it illuminated like the half-moon. When it is beyond the
sun, as at E, we see its whole illuminated face like the full-moon.
The variation of its apparent size from the varying distance is
very striking. At its extreme distance from the earth it subtends
an angle of only five seconds; nearest to us, an angle of twelve
seconds. Its distance from the earth varies nearly as one to three,
and its apparent size in the inverse ratio.
[Page 139]
When Mercury comes between the earth and the sun, near the line
where the planes of their orbits cut each other by reason of their
inclination, the dark body of Mercury will be seen on the bright
surface of the sun. This is called a transit. If it goes across
the centre of the sun it may consume eight hours. It goes 100,000
miles an hour, and has 860,000 miles of disk to cross. The transit of
1818 occupied seven and a half hours. The transits for the remainder
of the century will occur:
November 7th 1881 | November 10th 1894
May 9th 1891 | November 4th 1901