Busy day on the solstice this year [updated with APOD link]

The December solstice, shortest day of the year is tomorrow, December 21. At approximately 7:38 p.m. EST (2338 UTC), the Sun’s apparent motion in the sky will come to a standstill (Latin, solstitium).* That’s because it has finished its southward migration for the year, and from here until June old Sol will travel north. And as it does so, days in the northern hemisphere will get longer. The image below, from an excellent NASA website, shows the paths of the sun on the longest and shortest days of the year:

 The apparent path of the Sun across the sky.  In summer, the Sun's path is longest, and so are the days. In winter, the Sun's path is shortest, and so are the days. Image from http://www-istp.gsfc.nasa.gov/stargaze/Ssky.htm

The apparent path of the Sun across the sky. In summer, the Sun's path is longest, and so are the days. In winter, the Sun's path is shortest, and so are the days. Image from http://www-istp.gsfc.nasa.gov/stargaze/Ssky.htm

However, the solstice is an event that, while observable on relatively long time scales, is not so interesting to the observer at any given moment. But early tomorrow morning there will be a much more immediately observable event: a total lunar eclipse. The moon is scheduled to be entirely within the earth’s shadow (see image below) between 2:41 and 3:53 a.m. EST: a full 72 minutes of muted glory!

Lunar eclipse geometry, by Fred Espenak

During totality, the southern half of the moon will be much darker than the northern half, as Fred Espenak explains:

From the eclipse diagrams shown earlier, it is clear that the southern (bottom) edge of the Moon will dip much deeper into the Earth’s shadow than will the northern (top) edge. Since Earth’s umbral shadow is darker in the center than at the edge, the Moon’s appearance will likely change dramatically with time as the total phase progresses. A large variation in shadow brightness can be expected and observers are encouraged to estimate the Danjon value at different times during totality (Danjon Brightness Scale). Note that it may also be necessary to assign different Danjon values to different portions of the Moon at different times.

This could be an excellent opportunity for budding astronomers and students to test their observing skills. Try recording your estimates of the Moon’s brightness every ten minutes during totality using the Danjon Scale. Compare your results with your companions and classmates and discover how the Moon’s appearance changes during the total eclipse. The brightness of the totally eclipsed Moon is very sensitive to the presence of volcanic dust in Earth’s atmosphere. As part of a continuing research project, Dr. Richard Keen has been using reports of lunar eclipse brightnesses to calculate a history of optical thicknesses of volcanic dust layers (see: What Will 2004’s Lunar Eclipses Look Like?). If you’d like to help Dr. Keen by making eclipse observations, you can contact him at Richard.Keen@colorado.edu.

Now, as you probably know, lunar eclipses only occur during a full moon. So at midtotality, 3:13 a.m., the last full moon of the year will occur, only 16 hours or so before the official moment of the solstice. When will there be another lunar eclipse during the December solstice? I have no idea.

But to show you how rare even the possibility of such an event is, the next full moon scheduled within 24 hours of the December solstice will take place in 2094. (The last one occurred in 1991.) I read somewhere online that there were only 7 full moons on the December solstice between 1900 and 1999, but I haven’t fact-checked that claim. And I should, because that actually seems like a pretty strong correlation, when you consider that there “should” be only 1 full moon on the solstice every 29.5 years, given that the moon takes 29.5 days to go from full to full. Either the claim is wrong, or my understanding of math is wrong, or both are wrong.

[UPDATE: For an AMAZING pictorial explanation of the event, check out today’s Astronomy Picture of the Day from NASA. The text there answers my speculation above: 456 years since the last solstice eclipse, but no idea when the next will be.]

Full moon and equinox

Two images of last night’s full moon appear below. One was taken a few minutes before the equinox; the other was taken a few seconds after it. Can you tell which is which? (Hint: the moment at which the equinox occurs has no bearing on the appearance of the moon.)

At the time these pictures were taken, the sun was on the opposite side of the earth. The moon was about 6 hours shy of full (which occurred officially at 5:18 a.m. EDT this morning), but close enough that it would take a trained eye to tell that it’s not quite all the way there.

If you look really, really closely at the south polar region, you can see some sharper relief there than elsewhere. And if you compare the two images long enough, you might be able to convince yourself that the second image shows a little bit less relief than the first, but it might just be a trick of the scale (I wasn’t able to get the images to the exact same scale in Photoshop) or of the focus (the second one is slightly “softer” than the first one).

If that’s enough to convince you that the second shot was taken some 15 minutes after the first, bravo! Me, I went by the time stamp in the image metadata.

I stayed up late to get these shots, though, because of what was happening on the other side of the earth at the time: the center of the sun’s disk was crossing an imaginary plane projected up from the earth’s equator. In other words,the equinoctial point was being passed. You might notice that, in the northern hemisphere, the sun will now rise slightly to the south of east, more and more each day. And it will set slightly to the south of west, more and more each day. Until right around December 20 at 11:44 EST, when the center of the sun’s disk will reach about 23.5° S, appear to pause for a little while, and then head back north. This will be the solstice, a word which comes to us from the Latin solstitium, which, loosely translated, means “the sun stands still.”

The sun won’t actually be pausing in its motion, of course, because it isn’t moving. (At least, its apparent motion across our sky is not at all correlated with the direction of its travel through space.) We are the ones moving, and we will have swung around to the point in our orbit where we perceive the sun as reversing its course.

I was motivated to take these pictures because it’s fairly unusual that the full moon falls this close to equinox; the odds of that happening are pretty long. After all, the equinox can happen on any of the 29.5 days of lunation, and there are only two equinoxes a year. Throw in the solstices, and that’s still only four times a year that the moon has a shot at being full at a particularly exciting moment. I’m sure there’s a way to calculate the closest full moon to an equinoctial or solstitial moment, but I don’t know how to do it. Anyone out there know how?

And, as usual with these full moon posts, here are all the full moons of 2010:

June solstice, or, the longest day

Today is the June solstice. That means that today, at 7:28 a.m. EDT, the Sun’s apparent motion in the sky will stop moving north, and start heading south. This is because we are at that point in our orbit around the Sun where the following image takes place:

Earth's rotation axis relative to our orbital plane

Earth's rotation axis relative to our orbital plane (image from NASA, http://www-istp.gsfc.nasa.gov/stargaze/Sseason.htm)

As you can see, the northern hemisphere is receiving the majority of the Sun’s rays today. The further south you go, the shorter your day length will be, as this comparison between Boston and Boca Raton illustrates:

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As you can see, the sun rises earlier and sets later the farther north you go. Twilight lasts longer as well, extending Boston’s day an extra 45 minutes over Boca’s.

But, if you’ve been paying close attention to day length, you’ll have noticed that here in Boca, the longest day was actually June 16th; since then, even though the sun has been setting a minute or two later, it’s also been rising a minute or two later.

December solstice, or the shortest day

The December solstice, shortest day of the year is today, December 21. At approximately 12:47 p.m. EST (1747 UTC), the Sun’s apparent motion in the sky will come to a standstill (Latin, solstitium).* That’s because it has finished its southward migration for the year, and from here until June old Sol will travel north. And as it does so, days in the northern hemisphere will get longer. The image below, from an excellent NASA website, shows the paths of the sun on the longest and shortest days of the year:

 The apparent path of the Sun across the sky.  In summer, the Sun's path is longest, and so are the days. In winter, the Sun's path is shortest, and so are the days. Image from http://www-istp.gsfc.nasa.gov/stargaze/Ssky.htm

The apparent path of the Sun across the sky. In summer, the Sun's path is longest, and so are the days. In winter, the Sun's path is shortest, and so are the days. Image from http://www-istp.gsfc.nasa.gov/stargaze/Ssky.htm

Down here in South Florida, the contrast between the longest and shortest days of the year isn’t really all that great–almost exactly three hours. Travel north to Boston, though, and you get a very different picture: twice as much “extra” daylight. The table below shows the five “daylight” markers from the USNO for December 21, 2009, and the next solstice, June 21, 2010, in Boca Raton and Boston. (December times are EST; June times are EDT):

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Boca gets an extra hour of sunlight in winter compared to Boston; what we gain in December, though, we pay back with interest in June: Boston’s longest day is nearly two hours longer than Boca’s. If you’re interested in other astronomical events for this past year, or next year, click the links.

Another thing you can infer from the data in the table: Boca is well to the west of Boston. Solar transit is much earlier in beantown than South Beach. (Yes, Boca has a South Beach; it’s just not That South Beach.)

The solstice marks the beginning of the return of the sun; many cultures celebrated its arrival with pageantry and celebration. Our current celebration here in the western hemisphere involved bringing a fresh-cut or live pine tree into the house, decorating and lighting it lavishly, and hoping that one’s house doesn’t catch fire. Those of us who really want to risk a flaming catastrophe put much of our annual earned income, represented by colorfully wrapped boxes, near the potential conflagration, in hopes of heightening the pleasure of giving and receiving by the relief at having, once again, escaped with our lives. Is it any wonder that this event takes place near the longest night of the year?