APOD: Earth at Equinox (2014 Sep 22)

[APOD Robot]

Earth at Equinox

Explanation: Earth is at equinox. Over the next 24 hours, day and night have nearly equal duration all over planet Earth. Technically, equinox transpires at 2:29 am Universal Time tomorrow, but this occurs today in North and South America. This September equinox signal that winter is approaching in the northern hemisphere, and summer is approaching in the south. At equinox, the dividing line between the sunlit half of Earth and the nighttime half of Earth temporarily passes through Earth's north and south spin poles. This dividing line is shown in clear detail in the featured video, taken by the Russian meteorological satellite Elektro-L during last year's September equinox. The Elektro-L satellite is in geostationary orbit over one spot on Earth's equator and always points directly toward the Earth. The featured video shows a time lapse for an entire day surrounding the equinox, with a new image taken every 30 minutes. Cloud motions are visible as well as the reflection of the Sun are visible as the equinox day progressed. The next Earth equinox is scheduled for March.

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[jimsaruff]

Does the moon also go thru a higher and lower seasonal ascension in the skies as the sun?

[Boomer12k]

Me thinks portends Autumn....at least for Northern Hemisphere-rs...

Wonderful view.
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[Boomer12k]

Does the moon also go thru a higher and lower seasonal ascension in the skies as the sun?

I have noticed that The Moon is "lower" across the sky at times...but don't know if it is "SEASONAL"....

:---[===] *

[tneuschatz]

I see from the comment that the next equinox has been scheduled - by whom?

[anon]

This movie proves the Earth is fixed and everything is revolving around it... some people might say.

[RedFishBlueFish]

Very nice - though it goes by a bit fast: As does life, one supposes.

And, yes, the "scheduling" of the next equinox was as amusing to me as to some others.

All in all, a good start to the day.

Thanks

[Indigo_Sunrise]

Does the moon also go thru a higher and lower seasonal ascension in the skies as the sun?

Google "Analemma"



And Whew! I too, thought the video was super fast!

[HellCat]

instead of "This September equinox signal that winter..."

did you mean

"This September's equinox is a signal that winter ..."

PS - I'm also glad that the next one has been scheduled.

[starsurfer]

I see from the comment that the next equinox has been scheduled - by whom?

By Sandgirl! cue dramatic music

[copperjet]

Did anyone else notice that sprite-like bright white cloud that sprang up over the northern end of Sumatra at 1 second and appeared to move west and south until it disappeared into a larger cloud at about 2 seconds? Was that a reflection of sunlight on a small cloud or something? It seemed to move faster than other clouds, I wonder if it was a higher in the atmosphere?

I love starting my day with APOD! Thanks.

[BMAONE23]

Did anyone else notice that sprite-like bright white cloud that sprang up over the northern end of Sumatra at 1 second and appeared to move west and south until it disappeared into a larger cloud at about 2 seconds? Was that a reflection of sunlight on a small cloud or something? It seemed to move faster than other clouds, I wonder if it was a higher in the atmosphere?

I love starting my day with APOD! Thanks.

That is the reflection of the Sun on the Ocean. Watch again and you will notice a general round brightning of the Cloud Cover along the same path westward toward Africa. It is centered on the area of Light shining on Earth. Place close attention and you will see it glinting off Lake Victoria near the end of the file

[Nitpicker]

Does the moon also go thru a higher and lower seasonal ascension in the skies as the sun?

http://en.wikipedia.org/wiki/Orbit_of_t ... nclination

The mean inclination of the lunar orbit to the ecliptic plane is 5.145°. The rotation axis of the Moon is also not perpendicular to its orbital plane, so the lunar equator is not in the plane of its orbit, but is inclined to it by a constant value of 6.688° (this is the obliquity). One might be tempted to think that, as a result of the precession of the Moon's orbital plane, the angle between the lunar equator and the ecliptic would vary between the sum (11.833°) and difference (1.543°) of these two angles. However, as was discovered by Jacques Cassini in 1722, the rotation axis of the Moon precesses with the same rate as its orbital plane, but is 180° out of phase (see Cassini's Laws). Thus, although the rotation axis of the Moon is not fixed with respect to the stars, the angle between the ecliptic and the lunar equator is always 1.543°.

So, this means that seasons, based on changing Sun angles, are virtually non-existent on the Moon. But perhaps that wasn't your question. Daily observation of the Moon from Earth, will reveal the Moon to vary its height in the sky over a monthly cycle, quite similar in magnitude to the variation in the Sun's height in the sky over a yearly cycle. The reason the magnitudes are similar is that the "mean inclination of the lunar orbit to the ecliptic plane is [only] 5.145°."

[Nitpicker]

The next Earth equinox is scheduled for March.

I see from the comment that the next equinox has been scheduled - by whom?

Humans have doubtless been able to predict (and hence schedule) things like equinoxes and solstices since ancient times, at least to the nearest month (or lunation). However, left to our own devices, the vast majority of humans would not be able to predict (and hence schedule) them as precisely as this:
2014 Southward Equinox: 2014-09-23 02:29 UT (in just over two hours from my typing this),
2014 Southern Solstice: 2014-12-21 23:03 UT,
2015 Northward Equinox: 2015-03-20 22:45 UT,
2015 Northern Solstice: 2015-06-21 16:38 UT.

For that level of precision, we can thank science and scientists. (Of course, they are still just predictions. An unforeseen massive object could still hurtle through the inner Solar System, say, to perturb our orbit and put these predictions in error. Let's just hope it doesn't happen. Fingers crossed.)

[alter-ego]

Does the moon also go thru a higher and lower seasonal ascension in the skies as the sun?

I have noticed that The Moon is "lower" across the sky at times...but don't know if it is "SEASONAL"....

:---[===] *

The variation in lunar declination is periodic monthly, and on the Moons precession cycle time = 18.6 years. It's position is therefore not tied to our seasons. Over the 18.6 year period, the declination extrema varies by ≈ ±28.5° (23.4°+5.1°) to ≈ ±18.3° (23.4°-5.1°).
I like visuals when it comes to this kind of thing:

Lunar Declination Over 18.6 Years (2006.0 - 2024.6)

Maximum Lunar Declination Variation Over 1 Year (2006)

Minimum Lunar Declination Variation Over 1 Year (2015)

This is also meant to add to Nit's comments.

[Nitpicker]

I always appreciate your graphs alter-ego, thanks.

One of the things that first got me interested in astronomy as a not inconsiderable hobby -- only a couple of years ago and before I ever bought a telescope -- was the relationship between the Earth's orbit and the way we define units of time. For instance, I'd never before much pondered why the time it takes for Earth to orbit the Sun is not exactly the same for every orbit. Nor had I pondered the fact that it takes about 7.5 days longer for the Sun to pass from Northward to Southward Equinox, than from Southward to Northward, meaning we in the Southern Hemisphere get a shorter astronomical Summer (albeit it with a slightly closer Sun). And I'd certainly never realised that, from year to year, the length of each astronomical season can vary with a difference of as much as 20 minutes (possibly more) and that these differences were largely caused by the continually varying gravitational forces exerted on the Earth by Jupiter and Saturn. Once I'd realised that, I was hooked, and I went out and bought myself a telescope ... and then I started noticing how interesting the Moon is ... and then all these stars and other stuff ... and then I started taking photos ... and now I'm a completely tragic case.

[starsurfer]

and now I'm a completely tragic case.

Welcome to the club!