What Time is It?

Not long ago, a visitor to our observatory asked why we needed three clocks. She could see that the top clock was keeping Eastern Standard Time (EST), but the two lower clocks were out of step. The middle clock was set to Greenwich Mean Time (GMT) which is five hours later than EST. GMT is the standard reference time for most astronomical events. The bottom clock was the trickiest to explain. It keeps something called Local Sidereal [Star] Time (LST). It seemed to bear no relation to the other two clocks. But why should there be any difference local sidereal and local standard times?

The answer lies in the fact that the Earth makes one complete 360° rotation in 23 hours, 56 minutes and 4.1 seconds (one sidereal day). The fixed stars to make one revolution per sidereal day. This is not the case for the Sun. As the Earth turns once it also moves forward in its orbit by 360/365¼ degrees. [The extra ¼ day is the allowance for leap years.] For the Sun to make one full revolution, the Earth has to turn 360.99°. This extra 0.99 degrees requires an extra 3 minutes and 55.9 seconds, giving us our full 24 hour standard day. These few minutes per day may not seem like much, but they are the reason why the Sun seems to move through the heavens. So each day the sidereal clock gains almost 4 minutes per day on the standard clock. Small wonder that astronomers need special sidereal clocks which a bit faster than most standard clocks. Trying to keep track of the stars without a special clock to help you means a lot of extra calculations.

Until the last two or three hundred years, the Sun was about the best time keeper available to anyone. However, it really isn't a very accurate timepiece. On a carefully constructed sundial you will see a series of corrections based on the date. On such a sundial, you can keep time to about a minute if you apply the adjustment, but you will be off as much as a quarter hour if you don't. Why is this so?

While it appears that the Sun is going around the Earth, the opposite is the actual case. The Earth moves in a slightly flattened circular shape called an ellipse. The Sun isn't at the exact center of this ellipse but sits off to one side. This means that the Earth is closer to the Sun at some times of the year and is farther away at other times of the year. When the Earth moves closer to the Sun, the Sun's gravity becomes more intense causing the Earth to move more rapidly. As the Earth moves to the far end of its orbit, the Sun's gravity is less and the Earth slows down. The difference is enough to amount to that quarter hour sundial error.

The Spring sky is above us now. We will have to say good-bye to Orion just after Sunset, and welcome Leo, Boötes and Virgo, my favorite Spring constellations. Those who stay up until midnight will get a preview of the stars of the Summer Triangle on the Eastern horizon.

If you happen to be up around 3 AM on May 4th, you may see the Eta Aquarids meteors. Don't bother to set an alarm to get up, expecting something extraordinary. There won't be anything approaching a fireworks show, just a meteor every now and then.

This Spring brings a cessation of viewing the planets in the evening sky. There simply aren't any planets to be seen at this time. They are either clustered near the Sun and lost in its glare or they rise long after midnight. Venus will pass near Jupiter and Saturn this month, getting as close as the diameter of the Moon to each of the gas giants. You will have to be a very early riser to catch these events since they will occur only an hour or so before Sunrise. If you are especially lucky, you may glimpse the dusky pink glow of Mercury just before Dawn. Mercury is always tricky to locate with the unaided eye, because it never strays far from the Sun.

Leslie Coleman
Leslie Coleman
Entry Date:
Apr 1, 1998
Published Under:
Leslie Coleman's Columns
Subscribe to Leslie Coleman's Columns RSS Feed