Happy autumnal equinox: here's a year of sunrises

By Phil Plait | September 23, 2011 9:28 am

At 09:05 UTC (05:05 Eastern US time) this morning, the position of the center of the Sun’s disk, moving south, crossed the celestial equator on the sky. For normal people, this means it was the moment of the autumnal equinox. Some people like to call this the first day of autumn, which is fine, but the point could be argued.

There are lots of ways of describing this event. Astronomically, it’s how I wrote about it above. On Earth, it means that the length of time of day and night are almost exactly equal (they’re off by a bit due to the elliptical shape of Earth’s orbit, plus the presence of our atmosphere throwing off the time of sunrise and sunset). It also means the line marking day and night — technically called the terminator — is perpendicular to the Earth’s equator. That sounds funny, but maybe an animation is worth a thousand words:

Isn’t that awesome? The animation is composed of hundreds of images taken by a geostationary satellite, positioned in a special orbit so that it orbits at the same rate the Earth spins. This makes it seemingly hover over a single point on the Earth (though it really is in orbit). Each image in the animation was taken near 06:00 a.m. local time. You can see the day/night terminator line — in this case, marking sunrise — cutting across the Earth.

In northern hemisphere summer, the Earth’s north pole is tipped toward the Sun, and the south pole away from it. As it orbits the Sun, the Earth’s tilt remains constant, so six months later the north pole is tipped away, and it’s winter (this page may help). When the video starts, it’s the autumnal equinox (September 2010), so the shadow line is straight up and down. As the video progresses, we go into northern winter (which is southern summer) and the shadow tilts from the lower left to the upper right. Note the south pole has 24 hours of daylight!

Then we move to the vernal (spring) equinox, and the shadow is upright… and as time goes on, entering northern summer, the terminator tilts the other way, exposing the north pole to 24 hours a day of sunlight.

All of this is described in detail on the NASA Earth Observatory page, where there are images and links to higher-res versions of the video. I know things like this can be confusing — I’ve been doing this for years, and I still need to sit back sometimes and picture it all carefully in my head — but in a sense I find that in itself pretty interesting. The Universe is in many ways like a clock, with parts and gears and cogs all interacting in a rhythmic and lovely pattern, cycles upon cycles, repeating over and again. It’s beautiful, both to think about and to see.

Credit: NASA Earth Observatory


Related posts:

- Happy first day of spring… on Mars!
- Today’s the vernal equinox!
- The Sun stands still today!
- Snowpocalypse 2011 from space!

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (24)

  1. pumpkinpie24

    Is it time to come up with new names for the solstices and equinoxes that don’t favor the northern hemisphere? Does it frustrate people in the southern hemisphere to call their first day of spring the autumnal equinox? Or do they call it the vernal equinox there?

    Seasons, temperatures, climates vary around the world so it doesn’t bother me that when it’s winter here it’s summer there and we call those time periods different names. But those four dates are set in the stars, so they should each have one name that is used around the world but isn’t biased toward one hemisphere. All I can come up with is using the month names, though. December/June Solstice; March/September Equinox. Any other suggestions?

  2. Pete Jackson

    Beautiful and wondrous footage, but it would be nice if it could be slowed down a bit. I looked for the Moon passing behind the Earth near last quarter, but could never see it either on the right side or the left side of the frame. Anybody have better luck? It would look very tiny, less than one-half degree across while the Earth will look about 20 degrees across from geostationary orbit.

  3. GrogInOhio

    Oh man… outstanding. Using that to educate my family!

  4. Osian

    @pumpkinpie24
    You are right that using ‘Spring’ and ‘Autumn’ are used to discuss worldwide events and favour the North – and actually the names are being replaced as you suggested: to quote Wikipedia:

    “March equinox and September equinox: a usage becoming the preferred standard by technical writers choosing to avoid Northern Hemisphere bias (implied by assuming that March is in the springtime and September is autumnal—true for those in the Northern Hemisphere but exactly opposite in the Southern Hemisphere).” http://en.wikipedia.org/wiki/Equinox

    Nice video though :)

  5. QuietDesperation

    Is it time to come up with new names for the solstices and equinoxes that don’t favor the northern hemisphere?

    Why? There is no life in the southern hemisphere.

  6. Pete Jackson

    @1pumpkinpie24: Mmm you sound delicious!

    A nomenclature that works on Earth or Mars is:

    Northern solstice
    Southern solstice
    Northerly equinox
    Southerly equinox

    So today is the date of the southerly equinox.

  7. Pete Jackson

    @5me: I was thinking of the ambiguities caused by such terms as ‘northerly wind’ meaning a wind coming from the north, whereas ‘northerly motion’ or ‘northerly track’ definitely means going to the north; so it may be more clear to refer to equinoxes by the less common but unambiguous terms ‘northing’ and ‘southing’. So my list would be:

    Northern Solstice
    Southern solstice
    Northing equinox
    Southing equinox

    So today is the date of the southing equinox.

  8. Stuart Capewell

    Awe inspiring video. I’ve known since I was a child that this is what happens, but it’s the first time I’ve actually seen it!

    I think it would look even better if the image rotated each frame to keep the terminator vertical, which would keep the Sun off to the right if the frame in the same place.

    Sadly, I lack the skills to accomplish that.

  9. Christopher

    There is a little-noted aside to this; watch the cloud cover over Southern Africa. I grew up in this area, but no one I know outside of the region ever seems to notice how the clouds “follow” the sun North and South. This animation really shows it up.

  10. dcsohl

    Christopher@10: Funny, I was *just* reading about that the other day! That band of clouds is the “intertropical convergence zone” aka the nautical “doldrums”. According to Wikipedia, the ICTZ stays roughly along the equator at sea, but over land tends to veer north and south with the seasons, generally staying over the hottest part of the world. Great illustration of it!

  11. Utakata

    Hmm…is it me, or does this year’s autumnal equinox seem to be 2 days late? O.O

    …I know Phil explained this before about AE’s and SE’s not falling exactly on the 21st of their respected months. But two days? (And don’t blame the neutrinos when attemtping to explain it to me. I thick sometimes…but not that think. :) )

  12. Pete Jackson

    @12Utakata: http://www.hao.ucar.edu/education/archeoslides/appendix.php

    “Kepler′s second law of planetary motion states that the Sun-Earth radius vector sweeps equal areas in equal time intervals. This means that the Earth moves slightly faster along its orbit near perihelion than near aphelion. This, combined with the fact that the solstitial line SSWS coincides with neither the major or minor axis of the ellipse, means that the number of days between each solstice and following equinox, and each equinox and following solstice, is not constant; it takes 92.75 days to go from VE to SS, 93.625 days from SS to AE, 90.825 days from AE to WS, and 88.0 days from WS to VE.”

    So, blame the Earth’s elliptical orbit. It takes a full 5.6 days longer to go from summer solstice to autumnal equinox than it takes to go from winter solstice to vernal equinox. Was it more than vanity that caused Augustus Caesar to move the last day of February to the end of his own month, August?

  13. Peter Tibbles

    We folks in the southern hemisphere are used to the northern hemisphereist bias. We console ourselves with the knowledge that we know more about them than they do about us.

  14. Utakata

    Thanks for explaining that to me Mr. Jackson at 15. :)

  15. @1. pumpkinpie24 :

    “Seasons, temperatures, climates vary around the world so it doesn’t bother me that when it’s winter here it’s summer there and we call those time periods different names. But those four dates are set in the stars, so they should each have one name that is used around the world but isn’t biased toward one hemisphere. All I can come up with is using the month names, though. December/June Solstice; March/September Equinox. Any other suggestions?”

    The constellations* esp. those of the ecliptic plane along which our Sun appears to move (the “zodiacal” ones) are shared by both hemispheres* so maybe use them?

    Ie. the relevant constellation our Sun is located “in” at the time; eg. Ophiuchus! ( ;-) )

    Or more broadly in terms of prominent seasonal “skypost” constellations and their “signature” bright stars I’ve always thought :

    Summer = Orion (Betelgeuse / Rigel – or Sirius in nearby Canis Major)
    Autumn = Bootes (Arcturus or Spica)
    Winter = Scorpius (Antares)
    Spring = Pegasus (The nearby~ish Altair-Vega-Deneb Great Triangle asterism.)

    This will be the same equinox & solstice~wise for those of you upside down in the Northern hemisphere too although seasons are reversed for you.

    So it could make sense to talk of the Scorpius (or Antares) solstice, the Pegasus (or Altair) equinox and so on. :-)

    ——

    * Excluding constellations which are circumpolar to one hemisphere or the other eg. I can always see the Southern Cross but never Ursa Minor from my hometown.

  16. joeyg

    How come one half of the earth looks dark the entire time? Can someone explain this to me, I am a bad astronomer.

  17. Tom K

    “The Universe is in many ways like a clock, with parts and gears and cogs all interacting in a rhythmic and lovely pattern, cycles upon cycles, repeating over and again.”

    NO. The universe makes a *terrible* clock. Nothing goes evenly into anything else. There is not an even number of rotations of the Earth (days) in one revolution about the sun (year). The revolutions of the moon around the earth, likewise does not divide evenly in one Earth revolution about the sun. The Earth’ orbit is not perfectly circular, nor is the moon’s nor the other planets. It all wobbles and sloshes around, *never* repeating exactly.

    “Time is an illusion. Lunchtime doubly so.”
    -Douglas Adams

    The video is great though!

  18. Grant

    I sort of knew how this stuff all works, but this really helped me figure it out better. Thanks! :)

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