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Page 32
But the watchers on the Acropolis do not get turned over so as to
see the moon at the same time every night. [Page 110] We turn down
our eastern horizon, but we do not find fair Luna at the same moment
we did the night before. We are obliged to roll on for some thirty
to fifty minutes longer before we find the moon. It must be going in
the same direction, and it takes us longer to get round to it than
if if it were always in the same spot; so we notice a star near the
moon one night--it is 13� west of the moon the next night. The moon
is going around the earth from west to east, and if it goes 13� in
one day, it will take a little more than twenty-seven days to go the
entire circle of 360�.
[Illustration: Fig. 42.--Showing the Sun's Movement among the Stars.]
[Page 111]
In our outlook we soon observe that we do not by our revolution
come to see the same stars rise at the same hour every night. Orion
and the Pleiades, our familiar friends in the winter heavens, are
gone from the summer sky. Have they fled, or are we turned from
them? This is easily understood from Fig. 42.
When the observer on the earth at A looks into the midnight sky
he sees the stars at E; but as the earth passes on to B, he sees
those stars at E three minutes sooner every night; and at midnight
the stars at F are over his head. Thus in a year, by going around
the sun, we have every star of the celestial dome in our midnight
sky. We see also how the sun appears among the successive
constellations. When we are at A, we see the sun among the stars
at G; but as we move toward B, the sun appears to move toward H.
If we had observed the sun rise on the 20th of August, 1876, we
should have seen it rise a little before Regulus, and a little
south of it, in such a relation as circle 1 is to the star in Fig.
43. By sunset the earth had moved enough to make the sun appear
to be at circle 2, and by the next morning at circle 3, at which
time Regulus would rise before the sun. Thus the earth's motion
seems to make the sun traverse a regular circle among the stars
once a year: but it is not the sun that moves.
[Illustration: Fig. 43.]
There are certain stars that have such irregular, uncertain, vagarious
ways that they were called vagabonds, or planets, by the early
astronomers. Here is the path of Jupiter in the year 1866 (Fig.
44). These bodies go forward for awhile, then stop, start aside,
then retrograde, [Page 112] and go on again. Some are never seen far
from the sun, and others in all parts of the ecliptic.
[Illustration: Fig. 44.]
First see them as they stand to-day, as in Fig. 45. The observer
stands on the earth at A. It has rolled over so far that he cannot
see the sun; it has set. But Venus is still in sight; Jupiter is
45� behind Venus, and Saturn is seen 90� farther east. When A has
rolled a little farther, if he is awake, he will see Mars before
he sees the sun; or, in common language, Venus will set after,
and Mars rise before the sun. All these bodies at near and far
distances seem set in the starry dome, as the different stars seem
in Fig. 42, p. 110.
[Illustration: Fig. 45. Showing Position of Planets.]
The mysterious movements of advance and retreat are rendered
intelligible by Fig. 46. The planet Mercury is at A, and, seen from
the earth, B is located at _a_, [Page 113] on the background of the
stars it seems to be among. It remains apparently stationary at _a_
for some time, because approaching the earth in nearly a straight
line. Passing D to C, it appears to retrograde among the stars to
_c_; remains apparently stationary for some time, then, in passing
from C to E and A, appears to pass back among the stars to _a_. The
progress of the earth, meanwhile, although it greatly retards the
apparent motion from A to C, greatly hastens it from C to A.
[Illustration: Fig. 46.--Apparent Movements of an Inferior Planet.]
It is also apparent that Mercury and Venus, seen from the earth,
can never appear far from the sun. They must be just behind the
sun as evening stars, or just before it as heralds of the morning.
Venus is never more than 47� from the sun, and Mercury never more
than 30�; indeed, it keeps so near the sun that very few people
have ever seen the brilliant sparkler. Observe how much larger the
planet appears near the earth in conjunction at D than in opposition
at E. Observe also what phases it must present, and how transits
sometimes take place.
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