When the total eclipse of the sun occurred in 1878, [Page 138] astronomers were determined that the question of the existence of an intra-mercurial planet should be settled.  Maps of all the stars in the region of the sun were carefully studied, sections of the sky about the sun were assigned to different observers, who should attend to nothing but to look for a possible planet.  It is now conceded that Professor Watson, of Ann Arbor, actually saw the sought-for body.

VULCAN.

The god of fire; its sign [Symbol], his hammer.

DISTANCE FROM THE SUN, 13,000,000 MILES.  ORBITAL REVOLUTION, ABOUT 20 DAYS.

MERCURY.

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

VENUS.

Goddess of beauty; its sign [Symbol], a mirror.

DISTANCE FROM THE SUN, 66,750,000 MILES.  DIAMETER, 7660 MILES.  ORBITAL VELOCITY, 1296 MILES PER MINUTE.  AXIAL REVOLUTION, 23H. 21M.  ORBITAL REVOLUTION, 224.7 DAYS.