Solstices, Equinoxes and Leap Years
Ready or not, summer solstice comes early this year--eighteen hours ahead of last year. In 1980 it occurs at Universal Greenwich meridian time 5:47 am on June 21. Since it takes almost half a day for the sun to get from the Greenwich meridian around to our sky, solstice comes at 8:47 pm on June 20, Alaska daylight savings time (10:47 pm Yukon Time).
Summer solstice occurs at that instant when the sun attains its maximum excursion above the earth's equator. Winter solstice is when the sun is farthest south on or about December 21. The instant when the sun moves back north across the equator, on or about March 21, is the vernal (spring) equinox. On this day the shadow of the top of a pole or other object will march eastward in a straight line across the ground, just as it will on the autumnal equinox, about September 22.
Each year the time of a solstice or an equinox comes about five hours and forty-nine minutes later than the one the year before, except on leap years when it comes about eighteen hours and twelve minutes earlier. The main reason for the jerky annual pattern is the use of the leap year to partially correct for the year being about 365-1/4 instead of a whole number of days long.
Since the year is not exactly 365-1/4 days long, yet another adjustment is required once in a while. The method adopted in the Gregorian calendar, which we use today, is to not always have leap years every four years. Years divisible by 100 are not leap years unless they are also divisible by 400. Hence the years 1800, 1900 and 2100 have no February 29, but the years 2000 and 2400 do. (Thanks to Edee Rohde of Fairbanks for suggesting this article).