Tom Masse, age 10, of West Warwick, Rhode Island, for his question:
How long is a solar year?
The year is based on the time it takes our dizzy old Earth to finish one orbit around the sun. But there are two ways to measure this yearly revolution. One method gives the sidereal year and the other method gives the solar year. Both jobs are done by experts and both answers are correct. But, of all things, the sidereal year turns out to be about 20 minutes longer than the solar year.
Perhaps you think that measuring the time it takes for the earth to orbit the sun is as simple as ABC. But pause a moment to wonder how you would go about it. Actually, the answer is in the sky. Driving in a car, things by the roadside seem to move past as you travel along. On a larger scale, as the earth spins around and orbits the sun, the heavenly bodies seem to pass by overhead.
We can measure the earth's traveling time by clocking the paths of the sun and the stars. For example, astronomers check the exact position of a star. As the earth continues on its orbit, the star's position changes. When the earth completes this orbit, the star has returned to its original position. This gives us the sidereal year ¬which is 365 days, six hours, nine minutes and 9.5 seconds.
Astronomers sometimes need the sidereal, or star time year. But we earthlings need the solar year, the yearly orbit that matches the seasonal path of the sun. Astronomers chart the sky in circles that match the lines of latitude and longitude down on the globe. For example, the celestial equator is directly above the earth's equator. They trace the sun's path against these man made lines over the sky.
However, there is a problem. During the year, the noonday sun marks a path called the ecliptic over the sky. But the earth's axis is tiled 23h degrees and the ecliptic is tilted to match it. So in the sky we have two great circles the celestial equator above the equator and the sun's ecliptic which is tilted across it.
The two great circles cross, or intersect at two places. When the sun reaches these points we get the equinoxes, the times when day and night are equal all over the world. One is the fall equinox, which comes in September. The other is the spring, or vernal equinox, which comes in March.
Actually the earth draws the sun's path as it circles around its orbit. So when the sun returns to the same place, we know that the earth has completed one orbit. This is the solar year and its length is 365 days, five hours, 48 minutes and 46 seconds. Astronomers measure the solar year from the moment the sun crosses the vernal equinox to the time it returns for the next vernal crossing.
As you see, the sidereal year is about 20 minutes and 23 seconds longer than the solar year. This is because of a tricky turn around called precession of the equinoxes. Sidereal star time helps astron¬omers. But we base our down to earth calendar on the solar year, which is measured from the seasonal path of the sun.