"The moon is eclipsed through the interposition of the earth... Anaxagoras was the first to set out distinctly the facts about eclipses and illuminations."
-Euripides, in Hippolytus, 431 B.C.
As we all know, the Earth revolves around the Sun while it rotates on its axis. The rotation causes day-and-night every 24 hours, while the revolution causes our seasons and our calendar year.
You'll notice, in the image above, that the Summer Solstice -- June 21st -- is when the North Pole of the Earth is tilted most directly towards the Sun, and that the Winter Solstice -- December 21st -- is when it's tilted most directly away from the Sun.
But you'll also notice something else: the Earth moves around the Sun in an ellipse, not a perfect circle. And the Earth gets closest to the Sun just slightly after the winter solstice, in early January, and gets farthest from the Sun in early July. This makes for a number of interesting things.
First off, the Sun physically appears larger when we're closer to it, known as perihelion, and smaller when we're farthest from it, at aphelion.
But these two phenomena -- the non-circularity (eccentricity) of the Earth's orbit and it's tilt (obliquity) -- make for a fun project. If you take a picture of the Sun at the same time every day, what would you see?
Image credit: Tunc and Cenk Tezel.
You'd get a shape known as an analemma, which is a combination of the effects of our elliptical motion around the Sun and our tilt, as we rotate, on our own axis. The point at the very top corresponds to the Summer Solstice, when the Sun is highest in the sky, and the point at the bottom is the Winter Solstice, coming to you in a mere four days.
In the meantime, the Moon orbits the Earth. Once every four weeks or so, the Moon makes its own ellipse around the Earth. When the Moon is roughly between the Earth and the Sun (on the "near" side), we get a New Moon, and when it's on the opposite side of the Earth from the Sun (on the "far" side), we get a Full Moon. But you might expect, if the Earth is between the Moon and Sun, that's the Earth's shadow would block out the Moon.
And yet, we haven't had a Total Lunar Eclipse since February of 2008! Why not?
Because the Moon doesn't orbit in the same plane that the Earth orbits the Sun! The Moon's orbit is tilted, and it's only on those fortuitous occasions -- where the Full Moon passes through the plane of Earth's orbit -- that we get a lunar eclipse.
The Earth is always casting a shadow, thanks to the Sun, but it's very rare that the Moon even partially passes through it; it normally happens just once or twice a year.
But this year is extra special. You see, we're not only getting a Total Lunar Eclipse this year.
We're getting it on the Solstice! Starting at 10:32 PM, Pacific Standard Time (it's where I live) on the night of December 20th, the Moon will start to slip into the Earth's shadow, and will start to be blocked out. 68 minutes later, the eclipse will be total, and will remain so for 73 minutes, well into the early morning of December 21st. What will the Moon look like, with respect to Earth's shadow?
Pretty amazing. So go out and have a look at this once-in-a-lifetime treat on the longest night of the year! Where can you see it from?
It's best in North America, but look for it in Europe near Moonset and in Japan near Moonrise, too!
The last time we had a Total Lunar Eclipse on the Winter Solstice? 1554. No kidding. So enjoy an eclipse on the longest night of the year, for what's likely to be the only opportunity you'll ever have! Me? I'll be hoping for clear skies. It could happen...
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According to the weather predictions, I can expect rain and clouds throughout the eclipse. How frustrating!
Holy crap that's awesome!
Ethan, thanks for the heads up. I too live and love in Portland and am marking my calendar.
It does look like we have a forecast of rain showers that evening.
Are you planning on driving somewhere to observe this? Suggestions?
Side note: do you attend any local meetups, science in the pub etc.? If so, I like to buy you a beer sometime. :)
And for those of us in the Southern Hemisphere, we can reverse Ethan's nothern-centric 'winter' and 'summer' nomenclature for the solstices so that they are the 'right' way around :), and we too should get to see some of the totality after moonrise on a pleasant summer evening on the 21st.
Fantastic. Except I live in Ohio, and it will undoubtedly be cloudy.
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Dang, I live on the Oregon coast and we are projected to have 93% cloud cover. Thanks for the heads up!
If you live on the coast, drive inland into the desert. Do some serious moon/star gazing. If you live in Portland, no excuses, except maybe worky worky. Take the next day off, it's the holidays after all.
First the meteor shower and now this! I've found a ton of great viewing information on this site. http://www.spacedex.com/lunar-eclipse - Hope you all enjoy Monday's show!
Unfortunately, even if I drove all the way to Enterprise(just a few miles from Idaho, and where I was raised) it would still be cloudy. Bend, John Day... as well. Good idea otherwise.
Wiki gives 1638 as the last total lunar eclipse on the winter solstice.
http://en.wikipedia.org/wiki/List_of_17th_century_lunar_eclipses
Richard,
In 1638 (vs. 1554), the total lunar eclipse occurred on December 21st, while in 1554 it occurred on December 9th. The one in 1554 occurred on the Solstice, though, and the one in 1638 occurred many days after.
How's that?
The Julian calendar had gone out-of-sync with the seasonal calendar by about 11 days. So the one in 1554 was on the Solstice; the one in 1638 was not. http://en.wikipedia.org/wiki/Gregorian_calendar#Gregorian_reform
I am scheduled to deliver a baby girl via c-section on 12/21. We decided to name her Winter; but, only after we decided did we find out that our doctor was only available on 12/21 to do the surgery, that it was the Winter Solstice and now a Total Lunar Eclipse. My husband is also a science teacher. Some things are not scientifically explained and this is one of them. We are anxiously awaiting Winter's arrival on Tuesday :)
@Hank Faulkner
Unfortunately for me, I have more pressing matters on 12/21 than worky worky and absoswively cannot take it off. I got called for jury duty in Multnomah County and will be performing my civil service. :)
I guess I'll cross my fingers and hope for a poor forecast (unlikely at this time of year).
Ethan, are you planning on driving to view this, or will you be stuck in PDX?
rats...mostly cloudy here in minnesota, as well...
Don't give up yet- our western Oregon weather forecast can change! Great post- thanks.
Informative post and great use of graphical examples! I am so thankful we have a forecast for clear skies, since I'll have to stay up much later to watch. If anyone wants a "reminder" to ping them through social media for this, someone made a Facebook page for the Solstice Lunar Eclipse, LOL... talk about a sign of the times. =)
Hi Ethan,
I'd love to understand the 1554 vs 1638 Solstice dating better.
Since we're citing Wikipedia, I'll note that the 9 Dec 1554 eclipse is described as "partial"
http://en.wikipedia.org/wiki/List_of_16th_century_lunar_eclipses
I am curious as to where the high point of the moon's ellipse is with regards to this eclipse. The graphic shows the moon passing above the equator, but how close is it to 5 degree maximum that it could be? How close is it to the possible 28 degrees above the sun / earth plane? Ron
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Too bad this won't be visible on my location. :( I really wanna witness this one, might as well just tune in to live streams.
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Oh well, completely covered sky here at 0730 when it should have been visible/invisible here in Kent, UK. My 7-yo son was very disappointed... how long will it be until the next one?
B
On Tunc and Cenk Teze'l's awesome photo of the analemma, what is the star/planet in the right third of the sky? Surely that wasn't stationary the whole time...camera error? Particularly fortuitous glare from an airplane?
The ellipse is very great one, now the full ellipse is occur at 3.38am Malaysian time zone
May I ask, what is the origin of the graph image with the different components of the analemma graphed separately?
see your image URL below:
http://scienceblogs.com/startswithabang/files/2010/12/analemma-dwg-uk.g…
This image has been reused in one or maybe two other places on the internet (do a google image search) but contains an error. You fixed some of these errors with notes at the bottom of this post:
http://scienceblogs.com/startswithabang/2009/08/26/why-our-analemma-loo…
Unfortunately, the image on this post has prevailed. The effect of obliquity alone (the tilt of the earth) gives you the up and down (declination) and side to side motion (making sun vs. clock time fast or slow) because as declination is changing day to day it is taking away or adding a little bit to how far the apparent position of the sun has to move through the sky to get back to the midpoint. The daily path is actually like a very tight spiral moving up or down between the extreme solstice points over the course of the year. This results in the perfect figure 8 as the middle graph shows.
The effect of eccentricity (the elliptical shape of earth's orbit with the earth moving faster and slower along that orbit) skews things by making the figure 8 larger on one side than the other by altering ONLY the axis representing minutes fast or slow for the sun. The declination stays the same since that is due to the tilt of the earth (obliquity). This skewed figure 8 is slightly asymmetrical since the fastest and slowest points in the orbit and the times when we are tilted exactly towards or away or exactly sideways on (the solstices and equinoxes) do not occur at the same time.
If the minutes fast/slow effect of eccentricity is to be graphed alone on a graph with declination on the other axis as in the comparative graphic, then it would be a horizontal line with the value fluctuating between the two extremes every half year (going back and forth twice to represent the full orbit). You could label the dates along such an 'analemma' but things would be very hard to distinguish with all the overlap and it really isn't necessary since there is no declination and thus no reason to graph along that scale (other than to illustrate the point).
In your other post you have the diagrams showing all the effects separately by graphing the minutes fast or slow against the months of the year instead of against declination. The mistake is in taking those sine waves and twisting them back on themselves from such a graph but then making the vertical axis represent declination instead of days of the year. This results in the first graph of the comparison where you show a narrow vertical ellipse. Instead, if you did the same thing to both sine waves leaving the other axis as evenly spaced days of the year/months, the comparison would make more sense, but wouldn't necessarily represent anything concrete since you wouldn't see those shapes in the sky which is the whole point of taking the graph and twisting it into the analemma.
I don't mean to be critical and in fact these images separating the effects is what launched my investigation into this phenomena and I learned a lot along the way. My only suggestion would be to put a note close to the original image as you have graciously done to add clarification to your other post.