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December solstice

Thu, 21 Dec 2017 at11:16 EST(10 days away)
16:16 UTC

Dominic Ford, Editor
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21 December will be the shortest day of 2017 in the northern hemisphere, midwinter day.

This is the day of the year when the Sun's annual path through the constellations of the zodiac reaches its most southerly point in the sky, in the constellation of Capricornus at a declination of 23.5°S.

On this day, the Sun is above the horizon for the less time than on any other day of the year in the northern hemisphere. This is counted by astronomers to be the first day of winter.

Conversely, in the southern hemisphere, the Sun is above the horizon for longer than on any other day of the year, and astronomers define this day to be the first day of summer.

At the solstice, the Sun appears overhead at noon when observed from locations on the tropic of Capricorn, at a latitude 23.5°S.

Solstice geometry

Solstices occur because the axis of the Earth's spin – its polar axis – is tilted at an angle of 23.5° to the plane of its orbit around the Sun.

The direction of the Earth's spin axis remains fixed in space as it circles around the Sun, while the Earth's sight line to the Sun moves through the constellations of the zodiac. As a result, sometimes the Earth's north pole is tilted towards the Sun (in June), and sometimes it is tilted away from it (in December). This gives rise to the Earth's seasons:

The origin of the Earth's seasons

The date of the solstice

YearTime of solstice
201321 Dec 12:07 EST
201421 Dec 17:57 EST
201521 Dec 23:40 EST
201621 Dec 05:34 EST
201721 Dec 11:16 EST
201821 Dec 17:09 EST
201921 Dec 23:05 EST
202021 Dec 04:48 EST
202121 Dec 10:46 EST

The Earth orbits the Sun once every 365.242 days, and this is the time period over which the cycle of solstices and equinoxes, and consequently all the Earth's seasons, repeat from one year to the next.

In any year which is not a leap year, the solstices occur roughly 5 hours and 48 minutes – just under a quarter of a day – later from one year to the next.

This is why the seasons would drift later in the year if it was not for an additional day being inserted inserted into every fourth year on 29 February.

The chart below shows the day of the month when the December solstice falls in each year. The gradual drift of the four-year cycle earlier in the month is due to the equinoxes repeating 12 minutes less than a quarter of a day later each year.

In the Gregorian calendar, this is fixed by omitting leap years in three out of every four century years, e.g. 1700, 1800 and 1900, but not 2000.

The date of Christmas

Christmas borrows its date from ancient pagan midwinter festivals, even though in the modern calendar Christmas now falls a few days after astronomical midwinter.

This anomaly has come about because the system of leap days which are sometimes inserted into our calendar on February 29 was only refined to its present form by the Gregorian calendar reforms of the 16th century. Before this, the average length of each year did not quite match the period of time with which the seasons repeat – 365.2422 days – and so the seasons drifted through the year by a small amount each century. So, in the distant past, the winter solstice occurred a few days later than it does today.

The 2017 solstice

The exact position of the Sun when it reaches its most southerly declination in 2017 will be (J2000.0 coordinates):

Object Right Ascension Declination Constellation Angular Size
Sun 17h58m -23°26' Sagittarius 32'30"
The sky on 21 December 2017
Sunrise 07:24
Sunset 16:49
Twilight ends
18:25
Twilight begins
05:48

3-day old moon
Age of Moon
3 days

All times shown in EST.
Rise Culm. Set
Mercury 05:58 10:55 15:52
Venus 07:06 11:47 16:28
Moon 09:46 14:49 19:52
Mars 03:10 08:30 13:49
Jupiter 03:47 08:59 14:11
Saturn 07:23 12:07 16:52

Source

The circumstances of this event were computed using the DE405 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

Image credit

© NASA

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Latitude:
Longitude:
Timezone:

39.04°N
77.49°W
EST

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