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Post by Dave.B on Jun 14, 2005 17:42:27 GMT -4
I thought I'd introduce myself before asking a quick question:
I've been lurking on this forum for a couple of years (shame the old one is not retrievable), and despite being an engineer myself I've always been slightly in awe of the knowledge displayed by the regulars here (I'm a Civil Engineer though, so as that's a rather irrelevant discipline*, you may be able to forgive me for that!). Anyway, I'm Dave, I'm 27, from the South West of England, and ever since my Dad handed over all his Apollo memoribilia to me at the the age of 5 or so I've been enthralled by the whole thing. I've always had a kind of morbid curiosity for the Hoax Theory, but after reading as much stuff as I possibly can over the last twenty years, I've only reinforced my view that these 12 men certainly did land and walk on the surface of the moon.
Anyway, my question, which is in no-way hoax related, for the orbital mechanics people: Is it just a very bizarre coincidence that the Moon orbits in such a way that it always presents the same face to us? Or is there a reason for it? If it is a coincidence - it's certainly a strange one, especially given that the Moon is, again coincidentally, exactly the right size and distance from us to eclipse the Sun perfectly.
Thanks in advance for any replies - and don't get me wrong, I'm not trying to imply anything here - it's just that my wife just asked the same question and I couldn't answer her!
*Not totally irrelevant, as I've recently been reading "Gateway to the Moon", I think by the Army Corps of Engineers, which I bought at KSC in 2001. It's a fascinating book.
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Post by PhantomWolf on Jun 14, 2005 18:09:51 GMT -4
Hi Dave,
Actually it's not really strange at all, most of the moons in the solar system do it.
The moon is something called Tidally Locked. This occurs because the gravity from Earth pulls at the moon and creates drag on it, thus slows it down in it's rotation, pulling the heavier side towards the Earth. Over time the rotation of the moon has been slowed down so much that now the heaviest face is simply always pointing towards the Earth.
We see it all over the place. Venus is coming close to being Tidally locked to the sun, and Mercury is, though it is a 2:3 ratio I believe rther then a 1:1
All of the major and close moons in the solar system are tidally locked to their planets with only 1 or 2 exceptions and Pluto and its Moon are tidally locked to ech other meaning that they both show the same face to each other all the time.
As to the moon being the right size, well it actually depends on where in the orbit the moon is at the time of a solar eclipse. Since the moon actually differs in it's distance, it can pass in such a way that it appears bigger, or in fact smaller, thus creating different eclipe conditions (I'm sure someone here can recall their names.) However the actual current positioning is really prettey much a nice coinsidence and as the moon is moving away frm us, then in the future it'll appear much smaller, preventing any total solar eclipes, while in the past it was bigger since it was closer.
Hope that this explains it a bit for you.
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Post by martin on Jun 14, 2005 18:11:49 GMT -4
Anyway, my question, which is in no-way hoax related, for the orbital mechanics people: Is it just a very bizarre coincidence that the Moon orbits in such a way that it always presents the same face to us? Or is there a reason for it? I think I can explain basic idea, but not details. The earth generates tidal effect on the moon just like in reverse, so if the moon is rotating at a differing speed, then its shape is changing continuously so it is longer in the dimension pointing toward earth than in the dimension perpendicularly. This continuous change of shape creates frictions inside the moon which slow down its rotation, until it is exactly matching period of revolution around earth. This is occuring for some other moons and even planets, and the rotation of earth is also slowing down, but it is not yet happened because earth has much greater mass than moon, so it takes longer to slow down the rotation of earth. If it is a coincidence - it's certainly a strange one, especially given that the Moon is, again coincidentally, exactly the right size and distance from us to eclipse the Sun perfectly. I think size and distance of the moon to create eclipse is by chance only, but maybe some others here know some thing, I do not... Martin
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Post by JayUtah on Jun 14, 2005 18:51:57 GMT -4
The near part of the moon wants to orbit faster, since it's in a lower orbit, than the far part of the moon. But since they're connected they must orbit at the same average speed. That means the near part wants to fall toward Earth (it's orbiting too slowly) and the far part wants to fly away into space (it's orbiting too fast). That puts the moon in tension. And at that scale the moon is elastic enough to deform under tension.
Now when you throw rotation into the mix, every so often you get a "new" near and far part. So the elastic deformation is something akin to rolling a tennis ball between your palms while pressing inward to deform it. The internal friction of that action is what eats up the rotational energy of the moon over time.
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Post by Dave.B on Jun 14, 2005 18:54:00 GMT -4
Thanks for the replies PhantomWolf and Martin. That explains things very well. Just got to explain it to the wife now!
Edited: Also, thanks Jay for explaining it in terms I can relate to (elastic/plastic deformations - perhaps Civil Engineering isn't as irrelevant as I first thought!).
I appreciate the replies.
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jun 14, 2005 23:47:54 GMT -4
Hello, Dave. Welcome to the forum. ... the Moon is, again coincidentally, exactly the right size and distance from us to eclipse the Sun perfectly. Actually the apparent sizes of the Sun and Moon vary depending upon where in their orbits the Earth and Moon are. This is because the orbits are ellipses, not perfect circles. The Sun's size from Earth varies from about 31.5' (arcminutes) to 32.5', while the Moon varies from about 29.5' to 33.5'. Thus, when the Moon is at its smallest, it will not completely cover the Sun. This type of eclipse is called an annular eclipse. Below is a picture of an annular eclipse I took from New Mexico on 10-May-1994: It may not look like much, but the black circle in the middle is the Moon and the bright ring is the Sun. The sky looks black because the picture was taken through a solar filter. This eclipse occurred when the Moon was near its smallest and the Sun near its largest, therefore the ring is about as thick as it gets. Six months latter another eclipse occurred, but this time the apparent sizes of the Sun and Moon produced a total solar eclipse. I was in Bolivia when I snapped the following photo on 3-November-1994: This time the Moon completely covered the Sun's photosphere revealing the corona. There is no filter used in this photo -- the sky appears black because it was darken by the eclipse. ... perhaps Civil Engineering isn't as irrelevant as I first thought! Hey, don't underestimate civil engineers. I'm one too.
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Post by PhantomWolf on Jun 15, 2005 2:28:28 GMT -4
cool photos Bob
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Bob B.
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Post by Bob B. on Jun 15, 2005 12:37:54 GMT -4
I was actually rather disappointed with the photos I took from Bolivia. There were slightly out of focus and I experienced some camera shake. Also, when the eclipse was over I felt I spend too much time fumbling with the camera and not enough time just taking in the sights. I've seen two eclipses since then and I didn't bother photographing either one -- I just enjoyed the experience Here's a picture taken by a friend of mine, Bill Nunnelee, from Aruba in 1998. Notice how much better his focus is than my picture. www.braeunig.us/eclipse/1998/pics/photo9a.jpg
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Post by Data Cable on Jun 15, 2005 13:14:05 GMT -4
That's probably the one I'm remembering. I was living in upstate NY at the time, and I seem to vaguely recall that the disc of the moon was way off center, producing an extreme crescent. I tried the "pinhole projection" trick, among others, but noticed that ground shadows had a weird edge to them, particularly through trees, when the thousands of irregular holes between the leaves produced fuzzy crescents of light in the shadow of the tree.
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Post by Dave.B on Jun 15, 2005 13:28:23 GMT -4
Bob, thanks for the explanation and the photos (although I did already know that orbital eccentricity affects the relative size of the sun & moon as viewed from Earth - I guess I was just over-simplifying as it wasn't my main point)
I too have been lucky enough to experience a total eclipse - August 11th 1999. I lived in Torquay at the time which was just within the zone of totality, so I could have seen it from my garden, although I chose to go to the coast. I didn't bother photographing it, as I'm no good at it, but just enjoyed the experience like Bob. What was most amazing was seeing the (rather pointless) camera flashes going off from all round Lyme Bay, from Torquay all the way round to Lime Regis or maybe even further.
You can almost see why the Ancients offerred up sacrifices to the Gods during these things!
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Post by Dave.B on Jun 15, 2005 14:11:41 GMT -4
Hey, don't underestimate civil engineers. I'm one too. BTW, Bob - who do you work for? I work for Halcrow. Not that big in the States yet, although we've just recently swallowed up a firm called Yolles. My speciality is highway design. Also, there are no distant stars in your eclipse pictures - you sure you didn't fake them?! ;D (Sorry, couldn't resist)
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Bob B.
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Post by Bob B. on Jun 15, 2005 16:03:03 GMT -4
That's probably the one I'm remembering. I was living in upstate NY at the time, and I seem to vaguely recall that the disc of the moon was way off center, producing an extreme crescent. I tried the "pinhole projection" trick, among others, but noticed that ground shadows had a weird edge to them, particularly through trees, when the thousands of irregular holes between the leaves produced fuzzy crescents of light in the shadow of the tree. Yeah, that was a biggie. The path cut right across the middle of the country from El Paso to Lake Erie. Too bad it wasn't total. There are a lot of weird effects during eclipses. The shadow thing you talk about happens because the size of the light source becomes smaller as the Moon covers up the Sun. A smaller light source means there is less of a penumbra to the shadows; therefore the edges become less fuzzy and more distinct. It really becomes noticeable just before totality when the Sun is almost a point source. The edges of the shadows become very sharp -- even the hairs on your arm cast sharp shadows.
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Bob B.
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Post by Bob B. on Jun 15, 2005 16:38:35 GMT -4
Bob, thanks for the explanation and the photos (although I did already know that orbital eccentricity affects the relative size of the sun & moon as viewed from Earth - I guess I was just over-simplifying as it wasn't my main point) I didn’t mean to insult your intelligence. Those of us on this forum will often provide more explanation then may appear necessary for the benefit of those who might be lurking and not as familiar with the subject. I too have been lucky enough to experience a total eclipse - August 11th 1999. I lived in Torquay at the time which was just within the zone of totality, so I could have seen it from my garden, although I chose to go to the coast. That one was the last of my three eclipses. I observed it from a cruise ship on the Black Sea. BTW, Bob - who do you work for? I work for a general contractor, Ulliman Schutte Construction, so I’ve never actually done any design work. We build water and wastewater treatment plants; mainly in parts of the midwest, mid-atlantic, and southeasten states. Also, there are no distant stars in your eclipse pictures - you sure you didn't fake them?! ;D (Sorry, couldn't resist) Sorry, no pictures of stars, but I did get some planets. I had two cameras with me in Bolivia – one attached to a telescope and the other on a tripod with a 28mm lens. The following picture is one of my wide-angle shots. The eclipsed Sun is the brightest object and to the upper right you can easily see Venus. At about the 5 o’clock position, and approximately twice as far from the Sun as Venus, is Jupiter. Near the upper left corner is Mercury. Both Jupiter and Mercury are rather faint. www.braeunig.us/eclipse/1994/pics/rab_01.jpg
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Post by Dave.B on Jun 15, 2005 19:33:35 GMT -4
Bob, Absolutely no offense taken, I can assure you - just finding my feet here, so for some reason feeling a bit over-cautious. I'll readily admit to stuff I don't know (for example - I couldn't work out Jay's orbital mechanics problem that he proposed to "Unknown" last week). I must admit to working it through after he gave the answer, and it seemed to click.
Anyway, it took me a while but I can see both Mercury and Jupiter in your photo. I'm just getting to grips with my telescope - I had Venus the other night with a very low powered lens. The disc was very apparent, but it moves too fast to be able to keep a good view on it. The moon is just impossible to look at - I can focus on it, but can't get used to the upside-down motion of the telescope. Once I get used to tracking these objects as they move, my aim is to see Saturn and its rings (which I'm told is possible). Once I've seen it, I might turn my ambitions towards photographing it. One step at a time. After 8 years of looking through an EDM you would think I'd be used to it!
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Bob B.
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Post by Bob B. on Jun 15, 2005 20:42:39 GMT -4
Anyway, it took me a while but I can see both Mercury and Jupiter in your photo. At the time of the eclipse Jupiter was a very easy naked-eye object, but I couldn't see Mercury even though I looked where I knew it should be. It wasn't until I got my pictures back that was able to identify it. What telescope and mount do you have? I have an 8-inch Newtonian reflector on a German equatorial mount. An equatorial mount makes it pretty easy to track objects, but I know several people who have Dobsonian mounts (alt-az) and love them. I tried one once and was lost. I guess it all depends on what a person is accustomed to (nothing can replace practice). I've rarely gotten my telescope out since I moved back to Ohio in 1998 because I don't have a good dark-sky observing site. Seeing Saturn for the first time through a telescope is almost always a delight. I don't know anyone who hasn't been trilled by it. Saturn will soon disappear behind the Sun, so you better hurry unless you don't mind waiting for your chance to see it. Coming up soon is a very close conjunction of planets that you may want to put on your observing agenda. From June 22nd through the 29th, Venus, Saturn, and Mercury will all be visible within a 5-degree circle. On June 25th and 26th the three planets will lie within a circle just two degrees across and will form a nearly straight line with Pollux and Castor. For a few hours on June 27th, Venus and Mercury will be only 0.1-degree apart.
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