Want An Extra Day Each Year? Read On...
+ Von Del Chamberlain +
January 14, 1998
Easy question: How many days are there in one year? Answer: 365.25, rounded to the nearest quarter of a day. Harder question: How many times does Earth rotate in one year? Answer: 366.25, rounded to the nearest quarter of a day. Earth rotates once more than the number of days in the year? How can this possibly be so? Lets see.
Most people think that the period of time it takes Earth to rotate once on its axis is what we call one day, but this is not precisely so. Indeed, during one complete year Earth really does rotate once more than the number of days we count in our calendar. The reason for this has to do with the fact that while it is rotating Earth is also orbiting the Sun, making one complete orbit each year. We keep time by the Sun in the sky and this requires Earth to rotate a little bit more than one time for each of our solar days. How much more? About 4 minutes each day, more accurately 3 minutes 56 seconds. Earth's rotation period measured in our solar time units is 23 hours 56 minutes and 4 seconds. Still don't really understand all of this? Lets explore further.
If we were to keep our time strictly by the rotation rate of Earth we would keep star time, called by astronomers "sidereal time." The word sidereal comes from the Latin word sideris, "a star," so sidereal means "pertaining to stars." Sidereal time, then, is star time. It is the kind of time astronomers use to operate their telescopes. One sidereal day is the period of time it takes a star to appear to move completely around in our sky and this is the true rotation period of Earth.
The reason sidereal time differs from Sun time is because our motion around the Sun each year makes the Sun appear to change its place, relative to stars, a little bit each day, completely around the sky once each year. Thus, if we watch both Sun and stars, we will see a daily difference in their motions across our sky and this will amount to one complete cycle each year. Lets look still deeper.
Suppose you could see stars in the daytime. Now suppose we begin our day at the time when the Sun reaches its highest point in our sky, mid-day. Lets pick a time when both Sun and some particular star, call it Star X, are both located directly south at mid-day. To put this more precisely, they would both be crossing the celestial meridian. The celestial meridian is an imaginary line that starts at the horizon directly south, runs up to the zenith, directly overhead, and continues downward to the horizon directly north. This is the line where all celestial objects reach their highest point in their daily paths across our sky. So, lets begin our imaginary first day when both Sun and Star X are crossing the celestial meridian. We would stand facing south, looking up along the meridian into the sky to see them at mid-day. As the day goes on they move downward toward the west, set and then rise again the next day.
When Earth has rotated once, Star X comes back to the celestial meridian again. But the Sun will not be at the meridian this time. It will be a little bit east of the meridian and will arrive at the meridian a bit later than Star X. Why?
During that one sidereal day, star day, Earth rotated once, but it also moved a bit in its orbit. If we could have been watching Earth from the Sun, we would have seen it move more than 1.5 million miles along its orbit during that one day. Observing the sky from Earth, we see the Sun appear to change its place due to our motion around it. If we could see the Sun and stars at the same time, we could watch the Sun move a bit each day against the distant star background. Since there are 360 degrees in a circle and 365 days in the year, the Sun would appear to move about one degree against that star background each day, and it would appear to move eastward, bringing it to its mid-day position in our sky about 4 sidereal minutes later each day. Thus, Star X reaches the celestial meridian after one rotation of Earth, but the Sun has moved about one degree to the east along its orbital path in the sky within the band of constellations called the zodiac. The Sun comes to the meridian about 4 minutes later than Star X.
This happens each day. By Sun time, every day Star X reaches the meridian 4 minutes earlier, thus separating, day by day, from the Sun. Any particular star, or group of stars, appears to rise earlier, cross the meridian earlier and set earlier each day. After one full year Star X has reached the meridian one full rotation of Earth earlier than the Sun. This puts them back together again, but it requires Earth to have rotated one extra time. The 4 minutes each day has accumulated into one full sidereal day.
This fact, that Earth annually rotates on its axis once more than the number of days in the year, is what drives the stars in annual review across our sky. Right now you can go out in the evening after dark and look around to enjoy the most brilliant and beautiful stars we ever see, and you can watch them as they come up about 4 minutes earlier each evening, causing them to steadily march westward at any particular time by our clocks. At 9:00 p.m. Majestic Orion, with the box of four stars outlining his body, is well up in the south. His belt, marked by three stars in a close row, points downward to the left leading to the brightest star of the night, Sirius in the constellation Canis Major. Above Sirius, in the southeast, is the brightest star of Canis Minor, Procyon. Still higher you will see Castor and Pollux, the Twins of Gemini. Above Gemini, moving up toward the zenith of the sky is bright Capella in the constellation Auriga. Below Capella, very high in the south, you will find Taurus the Bull with ruddy Aldebaran (Orion's belt points diagonally up to the right to Aldebaran) marking his brighter eye at one end of the "V"-shaped Hyades outlining the Bull's nose. The wonderful bunch of stars named the Pleiades, also in Taurus, is higher and a bit west of the Hyades. Be sure to take binoculars with you and use them to look at the Pleiades and the sword of Orion, directly beneath Orion's belt.
What a glorious batch of stars we have on cold January nights. They are worth bundling up and going into the yard to see. Watch, night by night, as they parade toward the west, ticking off the clockwork movement of Earth in its orbit, and remember that this promenade of stars gives you one more rotation of Earth--one more sidereal day--each year than you might have thought you had when you began reading this article.