122 ASTRONOMY. 
transit begins. Arrived at c, we have the first internal contact of the 
limbs of Mercury and the sun, or the so-called internal immersion ; Mer- 
cury has now entered altogether on the disk of the sun. When the planet 
reaches b, the second internal contact, or the so-called internal emersion, 
takes place; at this moment the planet begins to leave the sun. Finally, 
the arrival of Mercury at a, brings about the second external contact, or the 
external emersion, the transit ceasing at this moment. 
In reality, or when observed with the terrestrial telescope, these four 
phases follow in a direction from left to right; in other words, Mercury 
occupies in succession the points a, 6, c,d. The case is precisely the same 
in a transit of Venus. The method of obtaining the sun’s parallax, and 
hence the mean distance of the earth from the sun, from such observations 
instituted at different places on the surface of the earth, cannot be here 
intelligibly exhibited and explained, as geometrical considerations combined 
with trigonometrical calculations are absolutely necessary. 
The Total Eclipse of the Sun, June 4th, 1788. 
41. Fig. 56, pl. 14, furnishes a representation of the extent and course of 
the moon’s shadow over the earth’s surface during the existence of a total 
eclipse of the sun. Tor our illustration we have taken the total eclipse of 
the sun, which was cbserved on the 4th of June, 1'788, in the eastern hemi- 
sphere of the earth. The figure represents, in the first place, a broad curved 
line or zone of intense black. This zone covers all places at which the 
total eclipse was seen. ‘To all places in its very central line, the eclipse 
was annular and of longest duration, while the localities on its external 
border saw a total eclipse only for a moment. Parallel to this zone, and to 
the north and south of it, lines are drawn with these indications, 9, 6, and 
3 digits obscuration, with corresponding shading. They include all those 
places where the eclipse affected 9, 6, and 8 digits of the sun’s disk respec- 
tively ; to the north of the zone of total obscuration, the sun being eclipsed 
on its inferior, and to the south of it on its superior limb, and this in pro- 
portion to the proximity of the place to the central zone. The upper and 
lower arcs, GH and JK, cut all those places which saw, for a moment only, 
the beginning or the end of the entire eclipse. The arcs uniting the ends 
of GH and JK, cut the places which—the westerly at sunrise, the easterly 
at sunset—perceived just half the eclipse (or 6 digits). The other curved 
lines cut all those places where the beginning, the middle, or the end was 
perceived at sunrise, or the beginning, the middle, or the end at sunset. A 
glance at the chart consequently shows that this eclipse was visible in the 
whole of Europe and Asia, except Kamtschatka, the greater part of Sibe- 
ria. and the island Celebes, as also in north and middle Africa. 
42. The explanation of the origin and progress of eclipses of the sun and 
moon has already been given in section 22. To project any other great 
solar eclipse upon a map, as has been done in fig. 56, it will be first of all 
necessary to call in the assistance of a terrestrial globe, and likewise to 
122 
