The universe may be conceived for the purpose of understanding the
sun's path among the stars as if it were a huge ball, of which looking
from the earth's surface we see part of the inside with the stars
marked on it, as on the inside of a dome. This imaginary inside of a
ball is called the celestial sphere, and the ancients believed that
it actually existed, and also, in order to account for the varying
distances of the stars, supposed that there were several of them, one
inside the other, and each with a number of stars fixed to it. The
sun and earth may be conceived as smaller solid balls suspended
inside this large one. Then looking from the surface of the earth
we see the sun outlined against the inner surface of the imaginary
celestial sphere. And as the earth travels round the sun in its orbit,
the appearance to us is that the sun moves over the surface of the
celestial sphere. The following figure will make this clear. [212]
Thus when the earth is at A in its orbit the sun will appear to be at
M, and as the earth travels from A to B the sun will appear to move
from M to N on the line of the ecliptic. It will be seen that as the
earth in a year makes a complete circuit round the sun, the sun will
appear to have made a complete circuit among the stars, and have come
back to its original position. This apparent movement is annual, and
has nothing to do with the sun's apparent diurnal course over the sky,
which is caused by the earth's daily rotation on its axis. The sun's
annual path among the stars naturally cannot be observed during the
day. Professor Newcomb says: "But the fact of the motion will be made
very clear if, day after day, we watch some particular fixed star
in the west. We shall find that it sets earlier and earlier every
day; in other words, it is getting continually nearer and nearer the
sun. More exactly, since the real direction of the star is unchanged,
the sun seems to be approaching the star.
"If we could see the stars in the daytime all round the sun, the case
would be yet clearer. We should see that if the sun and a star were
together in the morning, the sun would, during the day, gradually
work past the star in an easterly direction. Between the rising
and setting it would move nearly its own diameter, relative to the
star. Next morning we should see that it had got quite away from the
star, being nearly two diameters distant from it. This motion would
continue month after month. At the end of the year the sun would have
made a complete circuit relative to the star, and we should see the
two once more together. This apparent motion of the sun in one year
round the celestial sphere was noticed by the ancients, who took
much trouble to map it out. They imagined a line passing round the
celestial sphere, which the sun always followed in its annual course,
and which was called the ecliptic. They noticed that the planets
followed nearly the same course as the sun among the stars. A belt
extending on each side of the ecliptic, and broad enough to contain
all the known planets, as well as the sun, was called the zodiac. It
was divided into twelve signs, each marked by a constellation. The sun
went through each sign in a month, and through all twelve signs in a
year. Thus arose the familiar signs of the zodiac, which bore the same
names as the constellations among which they are situated. This is not
the case at present, owing to the precession of the equinoxes." It
was by observing the paths of the sun and moon round the celestial
sphere along the zodiac that the ancients came to be able to measure
the solar and lunar months and years.