Hit the dec!

By Phil Plait | December 21, 2007 2:30 pm

On December 22 at 06:08 UT (01:08 Eastern US time) the Sun will reach the point in the sky called the winter solstice. If you go out at the same time every day and note where the Sun is, it’s highest in the summer and lowest in the winter, with the difference being about 47 degrees (twice the Earth’s tilt). Once the Sun hits its lowest point in the winter (and no apologies to the southern hemispherite BABloggees), it starts to creep back upwards, and the days start to get longer. More or less.

Anyway, we measure positions on the sky using Right Ascension (or RA) and declination, which correspond to longitude and latitude on Earth. At the winter solstice, the Sun has its lowest declination (-23 or so degrees). Why am I telling you this? To explain the lame title of this post. Duh.

At the moment of the solstice, the Sun’s RA will be 18 hours, corresponding to 270 degrees. We measure RA by starting at the vernal equinox, and call that 0 hours. It’s been nine months since then, so the sun has moved 270 degrees around the sky (in reality, the Earth has moved 270 degrees around its orbit, but we’re standing on Earth, and we make the rules). So the coordinates of the Sun will be 18 hours RA and -23 degrees dec.

If you are attending any parties for the season, drop that into the conversation. You may think it’ll sound nerdy and dumb, but hey! Sometimes these things work out pretty well.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

Comments (15)

  1. I’ve often wondered how much northern winters’ temperatures are mitigated by Earth being 5 million kilometers closer to the Sun at this time of year than in the summer.

    Anyone know whether this seasonal variation causes a significant effect?

  2. Mc Atilla

    It seems like our forebears figured some of this out more than 5000 years ago! There’s a neolithic tomb in Ireland that is aligned with the rising sun on the Northern winter solstice so that it shines into the inner chamber.

    It will be webcast live if anyone is interested in seeing it, (though I expect the system will crash and/or it will be cloudy.)
    check:
    http://www.heritageireland.ie/en/Solstice2007/

    There is also a recording of this mornings event. (skip to 25 minutes into it if you don’t have the patience!)

    Amazing feat of science and engineering from before Stonehenge or the Pyramids.
    I think Noah was building his ark at around the same time that this tomb was being built ;-)

  3. nowoo

    On the opposite side of the sky, on Sunday around midnight the full moon will be at its highest declination for the year, at about 6 hours RA and almost +28 degrees dec. Go outside and notice how high up in the sky the full moon appears.

    Here in Vancouver the moon’s maximum altitude above the horizon will be 67.6 degrees – far higher than the sun’s maximum altitude of only 17.3 degrees for the next few days.

  4. LarrySDonald

    Having lived above the arctic circle and spent most of my life within driving distance, I’d be shocked if previous civilizations wouldn’t have noticed this. Sure, when day/night varies four-five hours, it’s not that obvious. When it varies between never getting past twilight (otherwise night) and twilight (otherwise day) or futher up “It’s dark 24/7″ or “The sun isn’t setting anymore now” even the less observant will consider the turning points more then trivia – it’s the halfway point of whichever you’re in (either half way through your favorite or only half left of your non-favorite).

    Good to see it though, I intended to look it up but the lazy prevail – it was mentioned here already.

  5. Isn’t that more like 23.45 degrees south? The tropic of Capricorn is about 30 km north of Alice Springs, where I have to go for work in a few weeks. I’ll talk to the boss to see if we can drive half an hour up the highway for some “no shadow” pics….

  6. One Eyed Jack

    For those who have been touched by His Noodly Appendage, Happy Saucetice!

    -OEJ

  7. Michael Lonergan

    I don’t know about using that as a pick up line?

    ” Hey Babe…At the moment of the solstice, the Sun’s RA will be 18 hours, corresponding to 270 degrees. We measure RA by starting at the vernal equinox, and call that 0 hours. It’s been nine months since then, so the sun has moved 270 degrees around the sky (in reality, the Earth has moved 270 degrees around its orbit, but we’re standing on Earth, and we make the rules). So the coordinates of the Sun will be 18 hours RA and -23 degrees dec.”

    Most of the bar babes I’m interested in don’t really have, how do I say this, the intellect of a pea. Although I must say, this line worked pretty well:

    “Babe, let’s go back to my place. I wanna show you my 8 inch Dobsonian.” (It worked until they actually found out what an 8 inch Dobsonian is.)

  8. Michael Lonergan: You’re looking for women in the wrong places.

    I hope everyone has a nice ecclesiastically rebranded celebration of Yule.

  9. The winter solstice was also the reason for the Roman holiday of Saturnalia, which was marked by gift giving, the singing of holiday songs, the decorating of halls with evergreen wreaths, and the visiting of family members.
    Hmmm…. If I didn’t know better, I’d say Christmas is based on an ancient pagan holiday.

  10. Steve Morrison

    Ian B Gibson:

    Check out pp. 53-54 of the BA’s book, which specifically cover this topic.

  11. Michael Lonergan

    Thomas, yes, I must search elsewhere.

    I’ll be celebrating the Axial Tilt this year. Maybe I’ll get a little tilted myself in honor of it. BTW, the BA’s post, The Real Reason For the Season had a very nice Greeting Card. I chose to use it as my Season’s Greetings card to my on-line friends this year. Many of whom are devout evangelical Christians.

  12. I noticed on my drive in to work at 5:00 this morning that the Moon was farther North than I ever recall seeing it. It was really quite a striking sight, and worth getting up ridiculously early tomorrow morning just to see!

  13. Michael Lonergan

    Harold: Uh oh. Then alienz we discovered on the Moon have powered up there hyperdimensional thermo turbo stardrive! Gazounds! They are moving the Moon! :)

    Actually, I was thinking that myself when I saw it the other night. It’s strange how we take something for granted every year, and then in a moment of observation, think, “Gee, I never noticed that before!” I’m the same with the Fall Harvest Moon.

  14. Lugosi writes:

    [[If I didn’t know better, I’d say Christmas is based on an ancient pagan holiday.]]

    I’m sure a lot of the holiday touches have pagan roots. But Christmas itself is, of course, a celebration of the birth of Christ. Christmas trees etc. are not essential. And to think that because pagan holidays included giving gifts, and so do Christian holidays, and therefore the Christian holidays stole from the pagan holidays, shows a mindset setting out to look for a predetermined conclusion and then, amazingly, finding what it wants to find. Forgive me if I’m not impressed.

  15. Robert Madewell

    Hey BA,
    Has anyone tried standing eggs on end at a solstice instead of an equinox?

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