Is 21st December the shortest day of the year?

Question

Somebody told me that it varies from year to year, but I don’t see how that can be!

Answer 1

Hi Country!

It does vary from year to year. There's nothing mystical about it. Almost all the the variation is simply the effect of leap years and time zones.

1. This year, the solstice fell at 1:08 Eastern Time this morning, December 22nd here in New York (= 6:08 GMT). (Unlike what several writers said, it definitely is not the 21st this year, unless you're in Denver [corrected; thank you to Scientia] or on the west coast.)

2. Next year, it comes on December 21st, however, at 7:04 a.m. (Eastern Time) Why the change of date? Because we add a day to the calendar on February 29th, so all such astronomical events fall a day earlier starting in March 2008.

3. In 2009, the winter solstice comes on December 21st, but at 12:47 p.m. (Eastern Time). Why five and three-quarter hours later? Because the earth takes 365 days 5 3/4 hours to complete a year. After the 365 days, the earth must spin an extra 5 3/4 hours to complete the year, so next year's solstice will come about 5 3/4 hours after the time of this years.

4. If the solstice happens at 1:08 a.m. Eastern Time on December 22nd, this corresponds with 10:08 p.m. on December 21st in Los Angeles. If it happens at 12:47 p.m. (Eastern) December 21 in 2009, this corresponds with 4:47 a.m. December 22nd in Sydney, due to the effects of time zones and Daylight Saving Time.

So you can see, it has nothing to do with mysterious forces or aberrations in the earth's motion. It's just the fact that the year is 5 3/4 hours longer than 365 days, plus the effect of leap years and time zones.

Answer 2

A solstice is either of the two events of the year when the sun is at its greatest distance from the equator plane. The name is derived from Latin sol (sun) and sistere (stand still), because at the solstice, the Sun stands still in declination, that is, it reaches a maximum or a minimum. The term solstice can also be used in a narrower sense of being the instance in time that such a passage happens. The solstices then, together with the equinoxes, are related to the seasons. In some languages they are considered to start or separate the seasons; in others they are considered to be center points (in English, for example, the period around the June solstice is known as midsummer, and Midsummer's Day is the 24 June — now two or three days after the solstice). The two solstices can be distinguished by different pairs of names, depending on which feature one wants to stress. Summer solstice and winter solstice. These names can be used when one wants to relate the solstices to the seasons. The seasons of the northern hemisphere and southern hemisphere are opposites (the summer solstice of one hemisphere is the winter solstice of the other) so these names can be ambiguous. June solstice and December solstice. An alternative to the previous set, but without the ambiguity for which hemisphere they are intended. Still not universal, however, as not all people on Earth use a solar based calendar where the solstices occur every year in the same month (as they do not in the Jewish calendar, for example), and the names are also not useful for other planets (Mars, for example), even though they do have seasons. First point of Cancer and first point of Capricorn. Alternative names for the previous set. One disadvantage is that due to the precession of the equinoxes these astrological signs where these solstices are located, do not correspond any longer with the actual constellations. Taurus solstice and Sagittarius solstice. Names that indicate in which constellations the two equinoxes are currently located. These terms are not widely used, the more so because until December 1989 the first solstice was in Gemini according to official IAU boundaries. Northern solstice and southern solstice, indicationg the direction of the sun's movement. These names are neutral and unambiguous.

Answer 3

It can vary from year to year, because the length of the year is a fractional number of days. The year isn't 365 days long, it's closer to 365 1/4. And that makes the difference.

Once a year, at the end of December, the Sun reaches its southernmost point in the sky, the winter solstice. This happens at one precise instant in time. This year, the solstice happens on December 22 at 6:08 GMT. Then in 2008, we'll have a leap year, and the solstice will occur on December 21 at 12:04 GMT. In 2009, it will drift forward another 6 hours (almost) to December 21 at 17:47 GMT.

And so it goes: the solstice drifts forward in time about 6 hours every year, until every 4 years we add a leap day and bring it back to early on the 21st again.

Answer 4

Correct answer given Dec 22 at 1:08 means "Logic's Logic" (fanfare)
The beginning of each season, the equinoxes and solstices, vary within a three day range surrounding the 21st of the month (Mar, Jun, Sep, Dec). The confusion comes from some calendar publishers reflect the dates and times in standard time while others use the UCT or Greenwich Mean Time at zero longitude. There will be a time coming up when spring will begin at 5:18 am UCT on Mar 20th. That is probably the earliest as it is just 18 minutes after midnight or the end of Mar 19th.

Answer 5

The day of the December solstice would be either December 21 or 22 depending upon your location on the planet. This year, the division lies between the Central and Mountain time zones of the Americas.

The exact time and day is determined by where your time zone is in relation to the Greenwich time zone.

The time zone used is "Greenwich Mean Time" (GMT) or "Universal Time" (UT). GMT or UT is time kept on the Greenwich meridian in London, England at longitude zero.

2007 - 22nd December 06:08 GMT / UT
2008 - 21st December 12:04 GMT / UT
2009 - 21st December 17:47 GMT / UT
2010 - 21st December 23:38 GMT / UT

You will need to adjust your time zone accordingly.

For example, if you live in the Eastern time zone, the December solstice would occur at 1:08 a.m. Eastern Standard Time on Saturday, December 22. If However you live in Mountain time zone, December solstice would occur on December 21 at 11:08 p.m.

The exact time and date of winter solstice changes from year to year. On leap years solstice occurs about eighteen hours and ten minutes earlier than it did the previous year. On other years solstice occurs five hours and forty-nine minutes later than the year before.

Changes are caused by aberations in Earth's rotation caused by it's core movements, the fact that a year is not exactly 365 days long, precession of the axis, and the fact that Earth's orbit is not round but elliptical and the degree of ellipse changes.

The December solstice day can be from December 20 - 23.

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anne is correct except that Chicago would lie within the Central time zone and would experience the December solstice at 12:08 am on the 22nd not the 21st.

Answer 6

True, it does vary. Here is a copy from Spaceweather.com

HAPPY SOLSTICE: This morning, Dec. 22nd at 6:08 am UT, the sun reached its lowest declination of the year: -23.5 degrees. Astronomers call this the winter solstice; it marks the first day of winter and the longest night of the year in the northern hemisphere. In the southern hemisphere, where things are reversed, it marks the beginning of summer and the longest day. Either way, happy solstice!

By the way; Aero-Eng has an impressive background in aerospace and astronomy. Any answer you get from him, I'd be willing to bank on. As for me - the cut and paste I posted is from the source sited below. They KNOW what they are talking about. You really should check the site out. It's amazingely interesting.

Answer 7

NO NO! Not this year that is. The Winter Solstice took place at 1:08 AM EST December 22nd which makes December 22nd the shortest day in the year 2007.

Answer 8

It is usually the shortest day of the year. It can vary depending on which side of midnight the winter solstice actually occurs.

ADDED: Thanks for the kind words Tercir - obviously, I make a lot of mistakes, too.

Answer 9

It varies slightly in the NORTH, Dec 20-22

Answer 10

Yes, 21 is the shortest day of the year.