THEORETICAL ASTRONOMY. 111 
eccentricity of the earth’s orbit, partly of the varying velocity with which 
the earth moves around the sun. 
The Transit of Mercury and Venus across the Disk of the Sun. 
29. When the inferior planets, Mercury and Venus, during their revo- 
lution around the sun, come into inferior conjunction, and at the same time 
into or not far from the imaginary straight line drawn through the centres 
of the sun and earth, they will, if examimed through a telescope, be seen as 
dark spots passing over the sun’s disk (pl. 14, fig. 55). These transits of 
Mercury and Venus belong to the rarer celestial phenomena, since they 
evidently can only take place when inferior conjunction occurs near one of 
the nodes of their orbits. It is plain that with regard to the origin and pro- 
gress of these transits, the conditions are precisely the same as in eclipses of 
the sun, which latter might be called with equal propriety, transits of the moon 
over the sun’s disk. Astronomy teaches that Venus at her inferior con- 
junction must be within 1° 49’ of one of her nodes, and Mercury within 3° 
28’, for a transit to occur to any observer on the earth’s surface. These 
two limits, then, determine the periods of these occurrences, which for 
Venus are 8 and 1133 years, and for Mercury 6, 7, 10, and 13 years. 
Thus, transits of Venus happened on June 5, 1761, and June 8, 1769; the 
next will take place December 9, 1874, and December 6, 1882, June 7, 
2004, and June 5, 2012. Pl. 9, fig. 6, exhibits the 13 transits of Mercury 
during the present century, with its direction each time over the sun’s disk. 
It is to be remembered that Mercury as well as Venus will enter on the 
left (eastern) limb of the sun, and emerge on the right (western) limb, for the 
reason that at the time of their inferior conjunction these planets are retro- 
grading. The reason of the transits of Venus occurring in the beginning 
of June and December, and those of Mercury only at the beginning of May 
and November, lies in the fact, that at these times the earth is in the line 
of nodes of each planet respectively. On account of the small apparent 
diameters of Venus and Mercury, these phenomena could not be detected 
before the discovery of the telescope. The first transit of Mercury was 
observed by Gassendi at Paris, November 7, 1631; the first transit of 
Venus. by Horrox, at Hoole, in England, December, 1639. 
The observation of the transit of Venus is to the astronomer of vast im- 
portance, as it is almost the only certain way of obtaining the sun’s parallax. 
and hence of finding the mean distance of the sun from the earth. Halley 
first recognised this fact, and recommended these transits to the observations 
of astronomers. His suggestion was followed out when the next transits 
occurred in 1761 and 1769, and at these times observations were made in 
many places. The transit of 1769 was observed in the South Seas, in 
California, as also in the northern regions of Asia. Venus at the time of 
her inferior conjunction is very near the earth, and consequently seen from 
different places on the earth’s surface, will be referred to very different 
points on the sun’s disk, so that first of all the parallax of both Venus and 
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