Today's the vernal equinox!

By Phil Plait | March 20, 2011 7:00 am

Today, at 23:21 UT (19:21 p.m. Eastern US time), the Sun’s odometer resets, and it once again finds itself at the celestial coordinates of 0h0m0s Right Ascension, 0°0m0s declination.

Or, in other words, it’s the vernal equinox!

A lot of folks will say this is the first day of spring. I think it makes more sense to call this the mid-point of spring — as do many countries — but I’m less inclined to argue about it as much as I used to. What the heck; it’s getting warmer in the northern hemisphere after quite a long and adventurous winter, and I went biking in the sunshine yesterday. It’s sure starting to feel like spring. Good enough for me!

In real terms, the equinox means a few things, too:

  • Day and night are about the same length (12 hours each)… although the Earth’s non-circular orbit and atmospheric distortion mess that up a bit.
  • The Sun rises pretty much due east and sets due west.
  • In the northern hemisphere the length of daylight is increasing the fastest. That sounds funny, but it’s not too hard to understand. In the northern hemisphere, just after the winter solstice, daytime starts getting longer. But the difference day-to-day is small; one day may only be a few seconds longer than the day before. At the equinox (today!) that difference can be several minutes from one day to the next. The amount each day is longer is itself getting bigger every day from the winter solstice to the vernal equinox. After today it slows down, and near the summer solstice in June each day will only be a few seconds longer than the one before it… and then the whole thing reverses. Speaking of which, reverse all that for the southern hemisphere. And again, the Earth’s elliptical orbit complicates things, but hey, close enough.
  • And finally, it means you can stand an egg on end… as you can do any day of the year. Here’s proof!

So, what are you going to do with your equinox?

CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (63)

  1. AJKamper

    Hmmph. Given that we still have several inches of snow on the ground here in central Minnesota, it barely feels like the first day of spring, much less the midpoint.

  2. Vern

    The same thing I do every equinox. Try and take over the world.

  3. Thomas Siefert

    I will spend my equinox counting to infinity… again…

  4. James

    Sorry Vern, not today I’m afraid. My own plan for world domination is nearly complete! Mwuah ha ha!

  5. KEA

    Yay! someone who agrees with me. The longest day of the year should be mid-summer not the first day of summer, and the shortest day should be mid-winter. And that means today is Mid-Spring. Happy Ostara! (and no, i’m not Wiccan, just Traditional) and btw: Whose “wise” idea was it to abandon traditional Seasons for the ones in current vogue? (i’m serious, i’d really like to know.)

  6. Trebuchet

    The only eggs I’m likely to be balancing soon will be lightly poached and balanced upon a slice of Canadian bacon, placed on an English muffin, and covered in hollandaise sauce. Mmmmm!

    What countries use the equinoxes and solstices as mid-season points? I’ve heard of starting the seasons on the first of March, June, September, and December. That makes more sense to me than either of the other options. There’s a thermal lag as the earth catches up to the length of daylight so declaring this to be the midpoint would make for a mighty cold spring in many places.

  7. DDReese

    You know, it goes without saying…

  8. Björn

    The combination of the Supermoon and the impending equinox gave me a severe case of stomach flu; or maybe it is radiation poisoning from the Fukushima plant fallout. Anyway, I’ll be consulting with my astrologer.

    Happy Equinox ;-)

  9. Brian

    > and I went biking in the sunshine yesterday.

    My blurry-eyed self read this as “I went bikini-ing in the sunshine yesterday.”

    Sunday morning coffee: not only a good idea, it’s the law.

  10. James H.

    Cycling in the warm weather!!!!!

  11. “In the northern hemisphere the length of daylight is increasing the fastest. That sounds funny, but it’s not too hard to understand.”
    Not hard to understand if you took (and learned!) Calculus, and recognized how the sine and its derivative relate to day-length throughout the year.

  12. Floyd

    Equinox, Schmequinox.

    Last night the Moon looked about as large and bright as it ever gets in Albuquerque, though the sky got kind of cloudy later in the evening.

    It’s cloudy today, I don’t know If I’ll see the Moon tonight or not. I’ll check the Moon out again tonight, just for drill.

  13. AJKamper (#1):

    Hmmph. Given that we still have several inches of snow on the ground here in central Minnesota, it barely feels like the first day of spring, much less the midpoint.

    Even here in southern New York, we still have patches of snow, even though it was in the 70’s a couple of days ago. Though, admittedly, these are typically on north-facing cliffs and other places that don’t get much direct sunlight.

  14. Jonathan Lubin:

    Not hard to understand if you took (and learned!) Calculus, and recognized how the sine and its derivative relate to day-length throughout the year.

    It’s been many years since I took calculus, but I still remember some of it. (Like the derivative of the sine is the cosine. Which also means that the derivative of sine(x) is at its extreme when it crosses zero.)

    Once you get passed “the ‘d’s don’t cancel out in dy/dx”, everything else is easy. :-)

    Trebuchet:

    The only eggs I’m likely to be balancing soon will be lightly poached and balanced upon a slice of Canadian bacon, placed on an English muffin, and covered in hollandaise sauce. Mmmmm!

    Sounds yummy. I’ve made it myself several times. Just please tell me you’re not getting the cheap imitation available at the place with “golden arches”.

  15. It’s cold and there’s still plenty of snow (especially in the bush, and that’ll be there till mid-May or later!) but it’s Spring and that means a ritual celebration at my place and planting the first seeds of the year. ^_^

  16. Bahador

    Today is Persian New Year (Nowruz)!
    So we’re celebrating :)

  17. katwagner

    You all prolly heard this before but we have two seasons in the Idaho mountains: winter and July. Today’s weather is 70% chance of snow, tonite 80% and tomorrow 90%. Ahh, but at least the lane is clear of ice and when it’s 55 out, we’ll get our shorts on. Cabin fever’s a killer this time of year.

  18. Marina Stern

    I’ll be staring at my azaleas, trying to will them into bloom.

  19. I will be grading astronomy assignments, lest my ears absorb the endless cry of “what’s my grade?”

  20. Thats a fake, you clearly fed the chicken loads of iron rich food and then used a magnet to make the egg stand.

  21. Donovan

    Phil?

    There’s no such time as 19:21 p.m. There’s 19:21, and there’s 7:21 p.m.

    Although it would be nice to put the clock on the metric system.

  22. Ken

    “Today, at 23:21 UT (19:21 p.m. Eastern US time), the Sun’s odometer resets, and it once again finds itself at the celestial coordinates of 0 Right Ascension, 0 declination. ”

    This may be one of those cases where I know just enough to be stupid, but does the coordinate system literally reset every year? Presumably the Sun’s location at that moment is not 0 RA 0 dec in the 1950 epoch, or in any epoch but the 2011 one. Is there some special coordinate system that smoothly shifts with precession, so that the moment of 0 dec (i.e. passing over the equator) is defined as also being 0 RA?

  23. The Mutt

    On the Vernon Equinox, you can stand a can of Skoal on its edge. It’s true!

  24. Jeff Keogh

    Tsk. Nice bit of northern hemispherism in your title there, Phil.

    From where I’m sitting, the vernal equinox is on the 21st of September. I think, on an internationally read blog, it would be more accurate to merely say in your title that today is the equinox.

    There. Consider that nit picked. :)

  25. Today, for everyone at some point or another, is March 20th. That simplifies things, right?…..

  26. I thought the minutes and seconds in a Declination were supposed to be written as ” and “, not as m and s.

  27. Georg

    This day is celebrated in Germany as the highest holiday of bureaucrats :

    Hibernation ends,

    spring tiredness starts.

  28. José

    Phil seems to contradict himself.
    A few days ago, on his post “The smallest and largest planets dance in the west tonight” he said “spring is almost here!”. But now he says today should be “the mid-point of spring”!
    I think most days he thinks spring starts today, but today he remembers spring may have started a month and a half ago and gives a lecture to us “comon people” about the begining of spring.

  29. Martha

    @ #32, ahh someone stepped on your supermoon.

  30. Theron

    Well I tried to paint one of my outside walls. Turns out that one gallon was not, in fact, enough to cover said wall. My house continues to look a bit odd.

  31. Jeff

    Hi Phil that last link goes to a Mars Rover article, probably not what you meant?

  32. Uncle Al planted black columbine and 77 Montanoa leucantha ssp arborssccens seeds in flats. We’d like to get a couple or three of those sub-millimeter seeds to sprout.

  33. #26 Ken:
    Yes, the equatorial coordinate system ( i.e. right ascension and declination ) is indeed defined as you ask, and moves with precession.
    The northern hemisphere vernal equinox is defined as the point where the ecliptic ( the apparent path of the Sun ) crosses the celestial equator ( zero declination ) moving south to north. This point is also defined as zero right ascension; hours of RA are measured from it. So the coordinate system itself moves with precession; the equinox moves with respect to the stars.
    Or to put it another way, the RA and dec. of every celestial object change with time, due to precession. This is why star atlases are always published for a specified epoch, such as 1950.0 or 2000.0; stars and other objects are plotted at their correct RA and dec. at that particular time.

  34. QuietDesperation

    You all prolly heard this before but we have two seasons in the Idaho mountains: winter and July.

    Ah, Spring in Southern California!

    Wait… what’s all this wind and rain? WHAT?! Damn you, climate chaos. Damn yoooouuuu! (shakes fist)

  35. “So, what are you going to do with your equinox?”

    —-

    Spending it crafting (painting destructo-bots, building volcanoes, and sewing hairbows) and watching Doctor Who.

  36. Daniel J. Andrews

    I think it makes more sense to call this the mid-point of spring

    Rather southerly geocentric, dontcha think? Contrary to what you southerners may pick up on the news, there is abundant life north of the 43rd parallel (and a hearty lot we are) who are still staring at a bit of snow both on the ground and falling from the sky. Mid-point of spring? I “pfft” in your general direction. :)

  37. vgbassman

    I’ve alway thought that the solstices should be “mid-points” of their respective seasons. It just makes so much more sense! (especially here at 8000+ ft. elevation….by 12/21, we’ve already had over a month of winter)
    9/1, 12/1, 3/1, and 6/1 should be the “first days” of the seasons!

    Who do we petition?

  38. LoboLoco

    About the third point, that the day length is increasing fastest right at equinox- I picture this the following way (correct me if anything is wrong):
    The day length is a sine curve, with the two extremes at the solstices. How flat it is depends on your distance to the equator. Anyway, anywhere in the world it´s equinox today, so we reach a turning point of that sine curve. The gradient is most inclined today.

  39. Thameron

    @6 James

    Sorry Vern, not today I’m afraid. My own plan for world domination is nearly complete! Mwuah ha ha!

    You mean you haven’t even finished your PLAN? I’m halfway through my execution!

  40. Gotta love the “context sensitive” ads that are run here. Four out of the 5 ads are for cars, and 3 of those are for the Chevy Equinox.

  41. spaceboyzach

    I can’t wait until next year, when we will be piping youtube into directly our brains!

  42. Messier Tidy Upper

    @44. Thameron said :

    @6 James : Sorry Vern, not today I’m afraid. My own plan for world domination is nearly complete! Mwuah ha ha! You mean you haven’t even finished your PLAN? I’m halfway through my execution

    So .. you’re half dead now then? ;-)

    How were you being executed? Must be kind of a slow method; eg. drinking hemlock in the Socratic style?

    BTW. I thought the equinox was the 21st March (Autumn equinox here in Oz) except for leap years when its the 22nd no?

    Also seasons~wise, for me anyhow, it’s simple :

    Autumn = March, April, May

    Winter = June, July, August

    Spring = September, October, November

    &

    Summer = Glau ;-)

  43. katwagner

    Whoa. I can tell by a bunch of these posts here that I’m not the lone ranger (the onliest one with Cabin Fever). OOO! I hear thundersnow!

  44. Messier Tidy Upper

    NB. Seasons start on the first of the month thus 1st March is the start of Autumn (Fall),1st June = first day of winter, 1st September = “official” beginning of Spring & 1st December = the start of Summer.

    Or at least that’s how I think of things seasons~wise & I’m probably not alone in this.

    Of course, seasons depend on latitude and while some Northern hemispherers get seasons thereverse of mine (eg. their summer = my winter) other folks get very different seasons again. (eg. The Wet & the Dry up in the Tropical North of my nation. Even the Light & the Dark in the Antartic /Arctic?)

    Oddly enough, I simultaneously think of the Solstices as being mid-summer /winter respectively which doesn’t quite add up but a-n-y-h-o-w! ;-)

  45. Messier Tidy Upper

    @ 49. katwagner :

    Whoa. I can tell by a bunch of these posts here that I’m not the lone ranger (the onliest one with Cabin Fever

    Cabin fever? Ooooohhh! :

    http://www.youtube.com/watch?v=YoJWIXj1VvM

    That’s nasty! ;-)

    I won’t be drinking the water – and staying well clear of dogs – if I were you! ;-)

  46. Pilotlars

    @47: The actual dates/times of equinoxes (equini?) change over time. Our arbitrary system for naming days of the month and all that don’t always coincide with the stars, so to speak. Normally the exact time is on 2 days, depending on which side of the date-line one is on.

  47. baberg

    Just wanted to make a quick comment – perhaps the best analogy you could’ve used for the “days lengthening the fastest” would be to think of a Sine curve – the equinox is at the origin of Sine where it is increasing the fastest and the solstices are the limits of 1 and -1. It might not match up exactly but the analogy would work, right?

  48. Thameron

    @47. Messier Tidy Upper Says:

    More than half, but whose counting? As to method – The grateful people that I have saved from their drab, humdrum existence are killing me with kindness (and soft cushions) naturally.

  49. Here in southern New York, on the first “full” day of Spring, the current weather is…

    snow.

  50. The Mutt

    Here in Memphis it is supposed to be in the low 80s today.

    Memphis has only two seasons: Cold & Flu or Mosquito.

  51. Nancy Ruth

    all of my resources show the vernal equinox on March 20 ……. That was yesterday! Are they wrong? just wondering…..I’m a teacher and I like to be accurate when we discuss such things in class. thanks!

  52. alex lehar

    wow, I love it! another controversy quashed!

  53. Jeff

    I show the students a 3 dimensional celestial globe with ecliptic and moveable sun on it. This easily shows the motions of the sun from day to day and effects like daylight increase, it is very easy to grasp and fun to do this demonstration.

  54. Buzz Parsec

    To the people claiming that yesterday marked the middle of spring and not the start… Try telling that to the 2 inches of slush that fell from the sky today. Seasons are a climactic notion, not an astronomical one. Climate lags solar heat input by at least 1.5 months, so it is completely appropriate to say that spring begins on the vernal equinox, and summer begins on the solstice.

    If I sound a little cranky about this, it’s because my feet are cold and wet and I’m totally justified in feeling this way. So There!

  55. Adam English

    My desktop…OF SCIENCE!

  56. Sheila

    For YEARS I would get up in the wee hours of the morning just to do the egg balancing act. Only a few years ago did I find out it wasn’t true. There went the one magical thing I could do!

  57. If you really want to bust up the egg balancing meme, have an egg-balancing gathering at summer/winter solstice — about as far as you can get from equinox.

    … and things like that will get the attention of journalists who are having a boring day at work.

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