The Perihelion Effect

# Winter Solstice — from Eric Weisstein’s World of Astronomy

As the Earth travels around the Sun in its orbit, the north-south position of the Sun changes over the course of the year due to the changing orientation of the Earth’s tilted rotation axes with respect to the Sun. ThisQuickTime movie illustrates the tilt of the Earth’s equatorial plane relative to the Sun which is responsible for the seasons. The dates of maximum tilt of the Earth’s equator correspond to the summer solstice and winter solstice, and the dates of zero tilt to the vernal equinox and autumnal equinox.

In the northern hemisphere, the Winter solstice is day of the year (near December 22) when the Sun is farthest south. However, in the southern hemisphere, winter and summer solstices are exchanged so that the winter solstice is the day on which the Sun is farthest north. The winter solstice marks the first day of the season of winter. The declination of the Sun on the (northern) winter solstice is known as the tropic of capricorn (-23° 27′).

The winter solstice is the shortest day of the year, respectively, in the sense that the length of time elapsed between sunrise and sunset on this day is a minimum for the year. Of course, daylight saving time means that the first Sunday in April has 23 hours and the last Sunday in October has 25 hours, but these human meddlings with the calendar and do not correspond to the actual number of daylight hours. In Chicago, there are 9:20 hours of daylight on the winter solstice of December 22, 1999.

The above plots show how the date of the winter solstice shifts through the Gregorian calendar according to the insertion of leap years. The table below gives the universal time of the winter solstice. To convert to U. S. Eastern Standard Time, subtract 5 hours, so the winter solstice occurs on December 21, 1998 at 20:43 (8:43 p.m.) EST; December 22, 1999 at 02:32 a.m. EST; and December 21, 2000 at 08:23 a.m. EST.

Note that the times below were calculated using WinterSolstice[] in the Mathematica application package Scientific Astronomer, which is accurate to within only an hour or so, and in practice gives times that differ by up to 15 minutes from those computed by the U.S. Naval Observatory (which computes December 22, 1999 at 07:44 UT instead of 07:32 UT and December 21, 2000 at 13:37 UT instead of 13:23).

 Date UT Date UT Date UT 12-21-1980 16:52 12-22-1990 03:07 12-21-2000 13:23 12-21-1981 22:41 12-22-1991 08:56 12-21-2001 19:12 12-22-1982 04:31 12-21-1992 14:47 12-22-2002 01:01 12-22-1983 10:20 12-21-1993 20:36 12-22-2003 06:51 12-21-1984 16:10 12-22-1994 02:25 12-21-2004 12:41 12-21-1985 22:00 12-22-1995 08:14 12-21-2005 18:30 12-22-1986 03:49 12-21-1996 14:05 12-22-2006 00:20 12-22-1987 09:38 12-21-1997 19:54 12-22-2007 06:09 12-21-1988 15:29 12-22-1998 01:43 12-21-2008 11:59 12-21-1989 21:18 12-22-1999 07:32 12-21-2009 17:49

References

United States Government Printing Office. The Astronomical Almanac for the Year 1999. Washington, DC: Navy Dept., Naval Observatory, Nautical Almanac Office, p. A1, 1999.

United States Government Printing Office. The Astronomical Almanac for the Year 2000. Washington, DC: Navy Dept., Naval Observatory, Nautical Almanac Office, p. A1, 2000.

U. S. Naval Observatory Astronomical Applications Department. “Earth’s Seasons, Equinoxes, Solstices, Perihelion, and Aphelion 1992-2020.”http://aa.usno.navy.mil/data/docs/EarthSeasons.html.