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One of the most remarkable celestial coincidences is that the Moon and Sun — as viewed from the Earth — have almost the same apparent size. Depending on its position in orbit, the Moon can appear either larger or smaller than the Sun, resulting in solar eclipses on Earth occurring in two varieties: total, when the Moon is close enough to appear larger than Sun and completely cover it, and annular, where a more distant Moon fails to completely cover the Sun’s photosphere, resulting in a “ring of fire”.
In the first week of July, 2004 three completely unrelated celestial phenomena occurred within days of one another: the full Moon, the passage of the 
Two near-apogean full Moons on either side of the Earth’s perihelion passage on January 2nd, 2005 provided opportunities to photograph the converse case: Moon near apogee and Earth near perihelion. I had originally aimed for the full Moon of December 26th, which is only a little more than 24 hours before lunar apogee and a week before the Earth reaches perihelion. Planning a photo like this six months in advance amounts to daring the sky to do its worst, and the last week in December the sky took the dare and delivered a steady menu of clouds, howling wind, and precipitation.
Nothing lasts forever. After some date in the distant future, there will be no more total solar eclipses—only annular eclipses where the Moon fails to completely cover the Sun. Our distant earthbound descendants will be forever deprived of one of nature’s greatest spectacles. Why? Because of the tides in the ocean.
Paleontologists have been able to estimate length of the day in the distant past through the effect of tides on the formation of beds of sandstone. In the Neoproterozoic era (620 million years ago, well before the Cambrian Explosion of metazoan life), Earth’s day was only 21.9±0.4 hours long, and over the entire period from then until the present, the Moon’s average rate of recession was 2.17±0.31 cm/year—a little more than half the current rate.[2]
Exercises for the Reader
[CoC][CoC][CoC] John gave us homework! I suppose it is back to school time…
If I, sometimes, skip down to the videos and don’t actually READ the entire post, can I earn full credit by maybe clapping erasers or cleaning the blackboard?
Certainly! Got me through high school.
It has always seemed to me that the days went by quicker when I was a kid. Now I know why. Thanks, John.
So… just what is the consensus view on the origin of the moon?
The most widely accepted theory is that the Moon was formed by an impact on the early Earth by a Mars-sized impactor which ejected a portion of the Earth’s crust which then condensed into the Moon. The primordial Moon would have been much closer to the Earth, but would have receded over time due to tidal recession, as discussed in the post.
Evidence for this is that the composition of the Moon is much the same as that of the Earth’s crust with few of the elements in the Earth’s core. If the Earth and Moon had condensed independently from the protoplanetary disc, you’d expect their compositions to be similar. Also, there is evidence of a heavy bombardment during the early history of the solar system as objects coalesced into planetary bodies.
It would be supererogatory to call this a consensus. We have so little hard data that other theories remain viable.
Always the Suck-Up, I put an apple on your desk in Simon T‘s fine RM:AC post.
Heh.
Well, I’m glad I wasn’t around to witness that! Also, which level of Ricochet membership is required to use the word “supererogatory”?
I would think that president Coolidge, the last to write his own speeches and able to read and write Greek and Latin as easily as English, would have endorsed the use of this word by members at the level bearing his name.
Okay, so much for the short view. When will the moon just say adieu cuz we don’t give it enough gravity to care? I suspect this could be deduced from the above information, but it’s been so long since I took math, I used to ride a stegosaurus home from class.
[I know. It was a special saddle. But they were really loyal. Anybody tried to mess with you, they got a face full of tail spikes]
Calculable, or just useless, since the sun will already have done its supernova flashy thing?
Here’s a an almost related question: if “this space thing” every takes off in a big way, do we need to be concerned about Earth mass increases or decreases from removing a large amount of material from Earth, or delivering large amounts materials to Earth from other planets, asteroids, the moon, etc?
This will probably not happen before the Earth and Moon are swallowed up by the Sun as it expands into a red giant. Once the Sun begins to expand, the oceans will be boiled off the Earth and tidal braking will be much reduced. Hence it is probable the Moon will still in be in an orbit (albeit more distant) when both are swallowed up by the Sun.
In a word, no. The mass of the Earth is 5.97×10^24 kilograms. The mass of all of the asteroids combined is about 1.7×10^21 kilograms, which is a hundred times smaller than the mass of the Earth and far less than the mass of the Moon (7.35^22 kg).
I was on the sea of Cortez to watch the solar eclipse in 1991, one of the longest. It was a quite an experience. We here in the US will be able to experience a rare and not to be repeated anytime soon total eclipse Aug 21, 2017. A quick search will show the times and path. I can assure that making plans to be in the path should be on your “bucket list”. I will make every effort to see this one, it will be the last that can be easily viewed in our lifetimes.
Here are details of this eclipse. Astronomers visualise solar eclipses with an orthographic map. In planning an eclipse expedition, the weather is a prime consideration. This page provides an early look at expected weather conditions along the eclipse track.
Was it tried in a comment? (If you want to flip a disk, ask a dime. :-))
Off topic.
There is now a bug in the system that puts the date stamp of the most recent edit and puts your post at the top of the Member Feed. I did wonder when they tried to fix the “Draft Bug” if they would test for this. Of course only a rookie programmer would make this mistake. I am beginning to think the developer is in Junior High but I am optimistic.
Mobility is the key. If it’s partly cloudy you may be able to dash for a hole in the clouds and rescue an otherwise missed opportunity. Around the time of solar eclipses, beware of drivers looking out the window at the Sun instead of oncoming traffic.
Yes, mobility is the key. Back in the early 1990’s after checking the forecasts for that “hole in the clouds” I drove to Columbia, MO with a #14 arc welders filter to see an annular eclipse of the sun. I avoided the inattentive drivers problem by leaving Lincoln the day before the eclipse. The night before I asked the clerks at my hotel if they knew of a place where amateur astronomers would be setting up their telescopes to allow people to share their view of the eclipse. By good fortune I found that the hotel was two blocks from the Columbia public library, where the local astronomers club was planning to hold just such an exhibit. The next day I found the place quite easily. There were several people setting up reflector telescopes with a white screen mounted on the side a few inches from the eyepiece. (cont).
(Continued). For those who haven’t seen an eclipse the image of the sun is projected through an eyepiece to the screens which are mounted at a 90 degree angle from the telescope. It works kind of like a movie projector, so the entire audience can see the eclipse in real time (and of course is not subjected to having their eyes burned out by looking directly at the sun through a large telescope). I asked the amateur nearest to me what time the eclipse would begin, and made my standard eclipse joke of expressing hope that the eclipse would begin on time!
One strange thing always happens at an eclipse while I am looking at one of the telescope screens: About 30 seconds or so before the eclipse begins something goes “click” in my head and I know the eclipse is about to begin. I can point directly to the exact spot on the screen where the moon will begin eating up the sun. My guess is that I am sensitive to some part of the infrared spectrum. Sure enough the moon made contact exactly where I thought it would. (continued).
Which song would go best with this Post?
This one?
Or this one?
I thought I’d leave You’re So Far Away out of the mix. It’s a mawkish sort of song.
(Continued). That day began as a hot day, but it became much cooler. When the moon occludes even a small portion of the sun’s surface, it intercepts a lot of heat and light. People began asking to borrow the arc welders filter. From that time on I spent most of my time passing that filter around. Eclipses are only fun if you share them. The telescopes were great, but there is nothing like seeing an eclipse with your own eyes. The sunlight began to change. It was still broad daylight, but the daylight had a “silvery” character to it. Eventually I saw what I had come to see. For about 7 minutes there was a big burning basketball hoop in the sky (of course people were still borrowing my filter). Then A TV camera crew asked to borrow the filter. They put it in front of their camera before pointing the camera at the sun. (I’m afraid I never saw their broadcast, and I don’t know whether they even used the footage.) All in all it was one of the best days in my life.
Excellent post . . .
Having watched a Great Courses series . . I’d heard this account of tidal friction and the moon’s drifting away.
It’s all so fascinating. We actually went to Aruba with our astronomer planetary scientist son to see a total eclipse of the sun in ’98. Very stirring.
Michael, the annular eclipse you saw was just a partial here on the west coast. It was late in the day and as usual the fog started to move in. The fog provided the “filter” for this shot out side our shop in Carmel.
There are several ways to view an eclipse. An easy way, which most people can do, is the projection method using regular binoculars and a white paper. Point the binoculars at the sun projecting it on the paper. It can be focused by moving the paper back and forth. No, it will not burn the paper or harm the binoculars. When the event is near I”m sure there will be many “eclipse viewers” on the market which will consist of a mylar film mounted in a cardboard holder. Also available are metalized filters that mount on telescopes, they are usually used to view sunspots. Just make sure any thing you plan to use doesn’t have a hole in it!
Solar filters are fixed to the objective end of the telescope, so that they reduce the intensity of light before it enters the telescope. Be sure the filter is securely fixed to the front of the telescope—if it were to fall off while you were observing, your vision would be destroyed before the blink reflex could activate. Also, if your telescope has a viewfinder, make sure its objective lens cap is in place (or if it doesn’t have a lens cap, block the objective by wrapping it in aluminium foil. Otherwise, you could inadvertently set your hair on fire (or, in my case, singe the scalp) by sunlight concentrated through the finder while you’re observing the partial phase through the telescope.
After the last bit of the photosphere is covered (the end of the initial “diamond ring” phase), you’ll want to remove the filter to observe or photograph the corona. But you must be careful to start a timer which will warn you of the end of totality so you can replace the filter before the photosphere is again exposed.
All of the eclipse photography linked above was made using an Orion metal-on-glass solar filter.
I’ll try again
See post above…