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Grumpy Martian

Movement of The Moon through the Heavens.

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I have been interested in astronomy for decades. I understand quite a lot about the movement of celestial objects.

But I am ashamed to admit that I donot fully understand the Moons orbit around The Earth. I know that we  only see one face with daylight on the Moon. Does this mean that there are times when the back side of The Moon faces The Earth but we cannot see The Moon as it is night time on Luna? Does the back side of The Moon ever get illuminated by The Sun? Does The Moon rotate on it's axis? Oh, I wish that there was a tutorial film explaining this.

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Just forget any shame. I'm with you. :grin:

I've got partial answers, but will happily wait for a fellow stargeezer with a more complete picture of how things are to come along.

Please? We're ready... :happy11:

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The Moon rotates on its axis and orbits the Earth. The speeds of both rotation and orbit are about the same, being about 27 days. This means that only one side of the Moon ever faces the Earth. This is known as tidal locking. The ‘dark side’ of the Moon never faces Earth. It does however face the Sun every now and again.

Imagine a golf ball orbiting a tennis ball - the golf ball rotates on its axis at the same speed as it orbits the tennis ball, so the logo always faces the tennis ball!

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The moon is tidely locked to the earth so only one face is ever visible. Because the orbit is eliptical and inclined WRT the equator we can see slightly more than 50% of the surface, though not at the same time.

The lunar farside gets as much sunlight as the nearside, allowing for the occasional eclipse.

Eventually, in about 2 billion years the Earth will become tidaly locked as well.

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Indeed one hemisphere of the moon is always lit but obviously we see various degrees of this as the moon travels around us.

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1 hour ago, PhotoGav said:

The ‘dark side’ of the Moon never faces Earth. It does however face the Sun every now and again.

 

I think calling the far side the dark side confuses people sometimes. 

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31 minutes ago, Dave Lloyd said:

I think calling the far side the dark side confuses people sometimes. 

Very true Dave, far side is far more accurate.

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7 hours ago, DaveS said:

The moon is tidely locked to the earth so only one face is ever visible. Because the orbit is eliptical and inclined WRT the equator we can see slightly more than 50% of the surface, though not at the same time.

The lunar farside gets as much sunlight as the nearside, allowing for the occasional eclipse.

Eventually, in about 2 billion years the Earth will become tidaly locked as well.

The reason we see 59% of the portion of the Moon that faces Earth is due to its own axis of rotation relative to its orbital plane and the Earth's orbital plane, called libration. The Lunar far side gets just as much light as the near side, due to its rotation relative to the Sun. The  Moon's orbit is tilted 5.145 degrees relative to the Earth's orbit around the Sun, and this tilt "rotates" once every 18.6 years. This is the reason we have periodic series of lunar eclipses rather than continuous lunar eclipses. The plane of Lunar orbit has to coincide with crossing the ecliptic at the New Moon in order to have a solar eclipse, and has to coincide with crossing the ecliptic at Full Moon to have a lunar eclipse. To illustrate this, Google the solar eclipse maps that were all over the 'net during the total eclipse last August in the US. This past eclipse was a "descending node" eclipse, the land track was from northwest to southeast. the next US total eclipse in 2024 will be an "ascending node" eclipse, travelling from southwest to northeast. If you look at any of the past or future eclipse tracks, they will follow one of these general paths at some point on the Earth's surface. Solar eclipses are easier to demonstrate because of the much smaller umbra than a lunar eclipse has.

The orbit of the Moon is fairly complex, it's much more than a circle around the Earth. Due to this orbital angle relative to the ecliptic, the Moon will also appear higher or lower in the sky  from time to time in its apparent track across the night sky.  Additionally, the difference in the Moon's period relative to the Earth's rotation means it rises each night about 50 minutes later than the night before. Add to this a natural "wobble" in the Earth's rotation and in the moon's rotation, their inclination to each other in rotation and their individual orbits around the Sun, it gets pretty complicated. If you  could look down from "solar north" at the Earth's orbit around the Sun, and the Moon's orbit around the Earth, it would appear that the Moon was moving in and out between Earth and Sun as the Earth orbited the Sun.

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The Moon also Nods and Rocks, which may sound silly, but it does, and allows us
to see more of its East and West  Limb areas, and the Northern and Southern regions.
 

 

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Some very nice information here, thanks everyone.  I really like that video Ron as, whilst they know it exists, most people, me included, possibly underestimate how much libration there is.

I think I buck the trend for most imagers in that I absolutely love our moon, and embrace its importance to us.  In fact it was seeing the moon through high powered binoculars that really sparked my entry in to AP.  I can image around the moon phases with a combination of targets and filters (NB) etc. but I can't image around the clouds, so they bother me far more.

There are several very good videos available with examples of what would happen if our moon wasn't there, or suddenly went off orbit, so really highlighting just how critical it is to us.  

Like the OP I have always struggled a bit with the moon phases, relying on software to tell me where it is going to be for planned imaging sessions, but the information in this thread has really helped.

 

Edited by RayD
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Sorry, I’m a huge Floyd fan and it will always be the ‘dark side’ for me, but you are absolutely right, it is really the ‘far side’.

Not to be confused with the fabulous cartoon series ‘The Far Side’ by Gary Larson, e.g.:

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Sorry, I digress!

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The Sun's gravitational pull on the Moon is nearly twice as strong as the Earth's, so it is actually more true to say that the Moon orbits the Sun with the Earth causing a perturbation in that orbit.

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Our Moon is the largest in the solar system relative to the size of its planet, but the 5th largest overall. Only Io is more dense than our Moon, of all the planetary satellites.

At some time in the not terribly distant future ( none of us will be around, though) total solar eclipses will all become annular; the Moon's distance from Earth is gradually increasing.

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