In our previous lesson we showed how sunset is the time of day when we can determine the position of the sun relative to the stars (see previous lesson). Now let us observe how the position of the sun relative to the stars changes as the days go by.
The animation at right shows how the sun's position changes day by day. (For this animation I have moved the sun slightly above the horizon but kept the sky dark so we can still see the stars.) Note how each day at sunset the constellation Capricorn (the goat) gets lower and lower until it is completely below the horizon. The sun appears to travel upwards along the green line through Capricorn towards Aquarius (the water bearer). The green line, representing the path the sun travels along, is known as the ecliptic.
If I allowed the animation to continue past January 10 we would see the sun continue it's journey upwards along the green line through the constellation Aquarius. (The corresponding animation file would be too big for some to download, so I stopped at January 10. However, you may download and install a trial version of the program Starry Night Backyard which generated these images from www.starrynight.com. The instructions tell you how to set the program so it will follow the sun's annual journey through the stars. 2006 Note: Starry Night Backyard has been retired. They have a similar program now.)
If I allowed the animation to continue for the rest of the year we would observe the sun travel through all 12 constellations of the zodiac. These 12 constellations are: Capricorn, Aquarius, Pisces, Aries, Taurus, Gemeni, Cancer, Leo, Virgo, Libra, Scorpio, and Sagittarius. At the end of the year the sun completes its journey by arriving back at Capricorn where it started, ready to begin anew its annual journey through the 12 constellations.
Note that the ecliptic is a closed circuit around the celestial sphere. (We shall encounter the word "circuit" again later on.)
By observing which constellation the sun is in you can tell what time of year it is.
This is important if you are a farmer. You need to know when to plant the crops and when
to reap the harvest. You need to be familiar with the seasons of the year. This is why
astronomy was always important in ancient civilizations.
To the right we have a side view of the sun passing through the constellation Capricorn the goat. (In this side view I have switched to using icons to represent the Zodiac constellations.) Above the earth we have the visible night sky; below the earth we have the abyss of whatever is under the world that we can not see. Recall again that the sun rises in the East, and sets in the West. In this animation the sun is over there on the West side setting in the West. Note how each day at sunset Capricorn the goat gets lower and lower until it is completely below the horizon. The sun appears to travel through Capricorn as the days go by. On December 22 (the Winter Solstice) the sun is just starting to go through Capricorn. By January 10 the sun has made its way through most of Capricorn and we see that the zodiac constellation of Aquarius the water bearer is next in line.
(The other members of the zodiac would form a circle around the earth in this animation. I'm not showing them all right now to keep things simple.)
Compare this animation with the one above.
(All dates are for “Biblical Times”, about 2000 years ago. In our present
21'st century you have to wait until January 18 to see what they saw on December 22 due
to the earth’s precession. See Lesson on Earth's