The day

Starting from high noon, after one rotation of the earth relative to the stars, the sun is not quite back to its high point. A "Solar day" is the time it takes for the sun to return to its high point. A "Sidereal day" is the time it takes for the stars to return to the same position. So, "solar day" and "sidereal day" are different lengths of time. We run our lives according to the solar day.

The length of an actual solar day varies throughout the year, because the earth's orbital velocity varies. The (fictitious) mean solar day is the average length of all (real) solar days in a year. An hour is 1/24 th of this mean solar day, a minute 1/60 th of an hour, and a second 1/60 th of a minute. (``Mean'' here means average, of course.)

Using the hour defined this way, the sidereal day is 23 hours and 56 minutes.

The mean solar times at any two places on earth (as long as they are not on a north-south line) are different. Standard time is established by dividing the earth into 24 zones, each running from pole to pole, each 15 degrees of longitude wide. The standard time within any one zone is the mean solar time at its center, and there is an abrupt change of one hour at its boundaries. The center of our time zone (eastern standard time) is about 250 miles west of here (about 3${\frac{1}{2}}$ degrees) so that when the clock says noon, the sun is actually 14 minutes beyond its high point. (Why 14? 3.5^o/15^o = 14/60 = 14/60 ths of a time zone, or 14 minutes.)