Jan Reeler, age 12, of Decatur, Georgia, for her question:
How does the solar year differ from the civil year?
There are a number of different years for different purposes. Some are longer and some shorter than the calendar year we use to clock our passing birthdays. But all of them are related. The civil year and its assorted annual systems are based exactly on the solar year.
Since the earth's year is based on our planet's solar orbit, you might expect every yearly period to be called a solar year. But a planetary orbit is a celestial operation, and such things are complicated parts of an immense heavenly hoedown. The earth's orbit is slightly oval and when closest to the sun it orbits a trifle faster. Its north polar axis weaves around in a slow circle, changing our view of the sun's position in relation to the background of fixed stars. Meantime the moon is weaving an orbit around us in its own way and its own time.
These and other heavenly motions cause many slight contradictions in the length of the earth year. The simplest measurement is called the "solar year." It is the time it takes for the earth to return to the same spot on its orbit, as this is measured from one spring equinox to the next.
On this spring date at the end of March, day and night are equal all around the globe. The sun is crossing the equator, midway on its journey between the two tropics. So we can see why the solar year also is called the "tropical year" or the "equinsx:i"1 year." The length of this solar year period is 365 days and five hours and 48 minutes and 46 seconds. It is the useful clock on which we base our calendar.
The major part of our modern lives is run by our calendar. We conduct our business by the calendar year, attend school classes, open our law courts and carry on countless other items of civil affairs. As you know, our civil calendar consists of three years of 365 days each and one year of 366 days, and we repeat this cycle over and over again. With a few other minor adjustments at the end of some centuries, this calendar pattern keeps us in exact tune with the solar timetable, although no civil year is exactly equal to the solar year of 365 daysr5 hours, 48 minutes, and 46 seconds.
Astronomers use another celestial time unit to calculate the positions of the fixed stars. this star year is called the sidereal year. The northern polar axis takes almost 26,000 years to complete its great circular swing around the skies. Because of this, the fixed stars lag behind by one 26,OOOth part of each solar year. The sidereal year is about 20 minutes and 23 1/2 seconds longer than the solar, alias the civil year.
The eclipse year is based on the tricky orbit of the moon. It is measured by the moon's return to eclipse position and the moon has a mind of its own. Its plans coincide with the earth's motions only in a general way. The eclipse year is shorter than the solar year by 19 days and almost 15 hours. What's more, it is part of a larger lunar cycle that repeats itself every 18 years solar years, that is.