Winter Solstice

By Phil Plait | December 20, 2005 11:19 pm

Today, at 18:35 Universal Time (10:35 a.m. Pacific), the Sun will reach its lowest point in its travels around the sky. If you go out every day from now until summer and observe the Sun at the same time every day, it’ll appear a little bit higher in the sky each time.*

The map above shows this. I used some planetarium software to map the Sun’s position on the sky at three dates, but the same time of day: 10:35 a.m. Pacific time for November 21, 2005 (right), December 21, 2005 (center), and January 21, 2006 (left). The curved path is the apparent path the Sun takes in the sky as the Earth orbits it once per year. This is an illusion; the Earth is doing the moving. It’s like seeing trees appear to pass you when you’re in a car, when really it’s the car that’s moving. But you can see that today the Sun is at its lowest position.

This day is called the Winter Solstice. Some people reckon it as the first day of winter (you’ll no doubt hear that on the news today), but I think the solstice marks the season’s midpoint.

You can see in that picture that a month from now, the Sun will be substantially higher in the sky. It’ll keep getting higher until June 21, then it’ll start to dip again. By next December 22, it’ll be at its lowest point once again, and the dance will start anew.

Happy Solstice!



And the cause is not due to the Earth’s distance from the Sun! It’s actually due far more to the tilt of the Earth’s axis. In fact, the Earth will be nearest the Sun on January 4th at 15:00 Universal time. Just so’s you know.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

Comments (45)

  1. Alice

    Thank you!!! I agree with you that we are in the middle of the season, it is the only way that really makes sense.

  2. Antipodean

    Well according to our seasons in australia, we are now on the 21st day of summer, happy christmas!

  3. BioBrat

    What about us down in the southern hemisphere? Doesn’t this mean that the sun is at it’s highest point? Just because it’s one way there doesn’t mean it’s that way every where.

  4. M. L. Green

    I think orbital mechanics precludes your conclusion BioBrat. If it’s the shortest day “up here”, it must be the longest day “down there”….and it doesn’t really matter exactly where you are….all this fun converges on any single position on the equator. (And not the geographical equator, BTW, but the sun-oriented (damn, what’s that called again?) equator as it intersects our planet)

    Bad Astronomy, correct me if I err.

  5. Blake Stacey

    Isn’t this the one day of the year when the Sun’s morphogenetic field aligns in phase so that you can stand an egg on its end? (wink)

  6. Lost

    Blake Stacey said: “Isn’t this the one day of the year when the Sun’s morphogenetic field aligns in phase so that you can stand an egg on its end?”

    I’ve carried out a few experiments and discovered that today is the day you can stand eggs on their sides.

  7. horseshoe (Brisbane, Australia)

    This is southern-hemisphere discrimination!

    http://www.flourish.org/upsidedownmap/mcarthur-large.jpg

    I tried to convince my supervisor at work that we should all get free ice cream and chilled beer on the summer solstice at the exact moment of solar noon… I wait in hope :P

    …although for you foreigners down under in the United States… that would be hot chocolate and chicken soup on the winter solstice at the exact moment of solar midnight… ahem

    Of course, the solstices aren’t always the hottest/coldest days of the year, and almost certainly won’t be… but there is free beer at stake… so who cares?

  8. A “season” is more of a climatic concept than an astronomical one. The coldest weather is still ahead of us.

    OTOH, the Lunar New Year is about a month away and that is sometimes called the “Spring festival” in China.

  9. Astro359

    Phil,

    I believe the actuall day for solstice is 12-21-05. Your article implies it is 12-20-05.

    Sources:
    http://scienceworld.wolfram.com/astronomy/WinterSolstice.html
    http://aa.usno.navy.mil/data/docs/EarthSeasons.html

    Just thought you might want to make it clear.

    Good Solstice to you.

    Astro

  10. Roy Batty

    BA, there’s an extra ‘h’ in your linky to Universal Time.

  11. Great point, Phil!

    I totally agree with you when it comes about the seasons definitions, although I had not thought much about this before. But, yes, if we are just talking about definitions, and we are, then why not consider some “symmetry, beauty and common (good) sense”?…

    And in my opinion this is, it has to be an astronomical definition, not a weather-like, geographical dependent one. Take the place where I am, now, Lisbon, Portugal. We have been experiencing a serious, dramatic drought for more than a year, although we have had some rain last October and November, but this has been a sunny, mild December with no signs of considerable rain and I´m tempted to conclude that we are already approaching the end of “our Winter”!… not the beggining, not the mid-point, but the end of it, just like it happened last year! People are still enjoying the sunshine in open-air cafes by the sea (and we are not in California or Australia), when I remember I used to stay indoors weeks in a row in December due to the cold and heavy rain (snow sometimes) we used to have some decades ago.

    So, if we stick to weather reasoning for this definitions, sooner or later
    we gonna have to “change” something … because weather is changing!

    Now, the earth is still going around the sun, tilted by the same angle, year after year … until one day :)

    Merry Christmas and Happy New Year, BA.

  12. As an astronomer– an extragalactic astronomer, even– I generally like to think of planets as nothing more than test masses that sample the gravitational field of the Sun.

    However… with seasons, it’s clear that what we experience in terms of temperature and climate depends on more than just the energy input from the Sun. The oceans (in particular) and the landmass forms a nice thermal mass that buffers the temperature chages, leading to a time lag between the energy input rate and the effective average air temperature at latitudes where most of those who write the history books live. (If you’re on the equator, greatest energy input is actually on the equinoxes, but it never varies *too* much from that, what with cos 23.5 being about 0.92, and length-of-day effects not contributing at all.)

    All that geology and meterology stuff is much too complicated and fiddily for me, so I’ll stick with the empiricism and say that, hey, it’s kinda Winter from mid-late November until early March. That’s when it’s cold, anyway. :)

    Unless I’m travelling to Chile to observe.

    -Rob

  13. So will we see all those “druids” and hippys clustering around Stonehenge?

    I remember watching a programme that said that Stonehenge was built to celebrate the Winter Solstice, not the summer one.

  14. Bah. Pesky outhern hemispherians. ;-)

  15. sophia8

    Yes, there will be modern druids at Stonehenge today. (Don’t know about “hippys” – what do they look like?)
    And yes, they do know that Stonehenge is aligned to the Midwinter Solstice sunset (as well as the Midsummer solstice surise), and would like to observe that instead of the today’s sunrise. But English Heritage, the body that owns the site, won’t let anyone into the circle after 4pm today. So sunrise its gotta be.

  16. I agree with the BA on the season’s thing. Why does winter START when the days begin to get longer? To me, that should at the very least begin the trend towards spring. (Northern or Southern hemisphere). While it will take a while for the longer days to effect the temperature, it is still headed towards warming up and spring. Who do we speak to to change this?

  17. hale_bopp

    Okay, I suspect some of you know this already, but although today is the shortest day, the day of earliest sunset was a few weeks ago. Where I live, in early December, the official sunset was at 5:18pm, and now it is 5:23pm. The days are shorter as sunrise moved from 7:07am to 7:21 am (latest sunrise occurs even later at 7:25am around the first of the year…I can’t tell the exact date since its listed at 7:25 am for several days in a row).

    So, have fun with these figures! I am sure there are plenty of people here who know what’s going on :)

    Rob

  18. Jon Niehof

    Bah, it’s winter when the studded tires go on (so it’s been winter for about three weeks here).

  19. Oops– sorry about the typos, folks. I corrected them. :-)

  20. Sure, sure. We all know it was intentional, nefarious astronomical disinformation. :P

  21. Bah, it’s winter when the Long Johns go on (so it’s been winter for about three weeks here).

  22. David

    All I know is today is one of my favorite day of the season! It means that summer is on the way. My son and I are big time baseball fans (he plays and I watch), and now that the days are getting longer (for us on the top half), we get to spend more time out side playing catch!

  23. Tim G

    Tilt the Earth 60 degrees with respect to the ecliptic. Now, move to Singapore (about 1 degree north latitude).

    You’d probably have very distinct seasons but they’d be half as long as before. The warmest periods would be just after the equinoxes. I’d call those periods “summer”. The coldest would be just after the solstices. I’d call those “winter”.

  24. Tim G

    Actually, don’t move to Singapore. I large tilt would mean a more even distribution of heat over the globe. Averaged over the year, the equatorial regions would cool down and the Polar Regions would heat up. The Antarctic ice sheet may melt and flood Singapore.

  25. Ruth

    I totally agree about the seasons and four times a year I have to avoid weather forecasts and the news because of it. Midwinter, it’s called MIDwinter.

    I think a lot of the problem is having a season called Fall instead of Autumn. If you look at Autumn as the season of harvest then August/September/October makes perfect sense. If you spend all your day inside with electric lights and think that Fall is just about when the leaves fall off the trees then of course you’ll get all mixed up.

    Happy Solstice all.

  26. Babbler

    I remember seeing on supper time news announcing the “beginning” of winter. I thought “what the…” as the previous Friday snowstorm brought about 47 cm of snow.

  27. CR

    “Midwinter” (and “midsummer”) has my vote.

  28. Antipodean

    If everybody could just accept a standard system where the seasons either start on the first of the month of the solstice/equinox or six weeks before them, the world would be a better place.

    Same goes for if Astrology was outlawed everywhere

  29. Tim G, Well… what is it? Are you going to tilt the earth or not? Please notify us in advance since I sometimes travel through Singapore and I don’t want my feet wet. :-)

  30. Antipodean

    And i don’t want Melbourne flooded

  31. Antipodean

    This (the Solstice) is the point at which the sun hits what is labeled on maps as the tropic of Capricorn, correct?

  32. Regarding the computer generated pic you showed on the BA site, here’s a pic that shows the actual position of the sun at sunrise on the summer solstice, the equinoxes, and the winter soltice:
    http://www.star.ucl.ac.uk/%7Eapod/apod/ap051221.html
    Perhaps you’ve seen it, but for those who haven’t, it really conveys a great visual for where the sun appears because of the tilt of the Earth in its revolving aroung the Sun.

  33. Tim G

    I get shivers whenever some broadcaster says something like, “Winter is still a few days away and yet we have snow…�. How can anyone be so pedantic?

    However, if we need a precise time frame for winter, I wouldn’t classify the solstice as the midpoint, either. I associate winter more with cold temperatures than with lack of daylight. Therefore, for most areas north of the Tropic of Cancer, winter should be defined as December, January and February, in my opinion. It’s certainly more arbitrary than using the solstice as a reference point, but it’s much more convenient.

  34. Steve Cooperman

    Well, I agree that mathematically and even esthetically, Dec. 21 should probably be “mid-winter”. But that’s not usually how the season is defined.

    And physically, defining the start of winter on that day makes sense. Although the amount of daylight will increase, the radiation from the Earth is still so great (even with global warming) that the extra insolation won’t start balancing it until February-March. So the days will keep getting colder.

    The same thing works during a typical day. The thermal inertia of the atmosphere is such that the warmest time of the day is about 2 pm: AFTER midday.

    It’s so different on Mars . . . midday IS the warmest time of the day because the atmosphere has such a low thermal inertia.

    Cheers!

    — Steve >>>>

  35. TRACY

    Silly people, don’t you know that Lawrence, Kansas is the center of the earth? On ‘google earth’ it is the default zoom location.

    I’ve always thought that the calender year should either start or end with either the winter or summer solstice.

  36. Note that most western societies already have some sort of festival on what you call the “cross quarter” days: Groundhog Day (Feb 2) is the halfway point between the December solstice and the equinox, May day (May 1) is halfway between the equinox and the June solstice, and, of course Halloween (Oct 31) / All Saints’ Day (Nov 1) / All Souls’ Day (Nov 2) split the difference between the September equinox and the December solstice. It seems only August 2 doesn’t have a generally recognized “tweener” holiday.

  37. I forgot to add in the previous post that there’s a wonderful site on Earth/Solar relative motion: http://www.analemma.com. This has both mathematical and graphical explanations for the sun’s path through the sky along with some great animations. It also shows what the Earth looks like from the sun throughout the year.

    If you’re a solar system traveller, there’s a page showing the analemma’s of the other planets, too.

  38. I see others have already pointed it out: Phil Plait is a hemisphericist! Here in the south, it is now midsummer.

    Bad astronomer! BAD astronomer!

    No, but seriously: great blog. Keep up the good work. ;-)

  39. winter solstice julian

    i had read on wikipedia that the julian calendar was created to align with the tropical year. would this not mean that january 1st was the winter solstice of 45 BC?

  40. There so much misunderstanding about the Winter Solstice; does make you wonder why the calenders aren’t sorted out once and for all. This ones makes no sense! I know the creative journal, Fallyrag, is coming out with a special edition this year examing the problem.. maybe make some changes!

    Much love,

    Lije

  41. Len Hartka

    I have 15 calendars in my office.
    Two of them have 12/21 as the first day of Winter.
    Three have 12/22 as the first day of winter.
    The two happen to be ‘Military’ related since they came from Memorial funds. for WWII, and the Korean War.
    The other don’t say.

    Any ideas why ?

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