Getting your head around the moons motion?

rccaulfield

So i noticed today the moon appears to go right to left as we look -the opposite to the suns apparent motion which goes east to west, never realised that before but i must admit i really struggle to visualise that the earths spin and the moons spin are such that the same face of the moon always faces us. Anyone any easy ways of thinking about this?

ThatDrGuy

As ever the diagram is your friend

Cú Giobach

rccaulfield wrote: »

So i noticed today the moon appears to go right to left as we look -the opposite to the suns apparent motion which goes east to west, never realised that before but i must admit i really struggle to visualise that the earths spin and the moons spin are such that the same face of the moon always faces us. Anyone any easy ways of thinking about this?

The moon rises in the east and sets in the west just like the Sun does, due to our rotation. Because it is orbiting west to east and rotating west to east (like us, which is why things appear to move the other way, east to west) it's like it's "rolling" around the Earth.
The moons actual west to east movement is slower than it's apparent east to west (since the former takes a month to go around as opposed the one day for the latter) but is discernible if you can see a star near it, it moves about one moon width east per hour.

Go outside face due north and point your arm straight and level in front of you, now raise it about 55 degrees (in Ireland), that is the line of the Earth's axis.
You are standing on a ball spinning once every 24 hours west to east around this axis, the moon is moving west to east making a full circle about once a month and also rotating once a month in the same direction..
Try to visualise these movements while looking south, remembering we are rotating the same direction the moon is orbiting, but we are turning faster and the moon is also rotating the same direction but only once per orbit (month).

rccaulfield

ThatDrGuy wrote: »

As ever the diagram is your friend

Great diagram, i pretty much have the light of the moon worked out its more the same side of the moon facing us all the time etc that has me struggling!;)

Cú Giobach wrote: »

The moon rises in the east and sets in the west just like the Sun does, due to our rotation. Because it is orbiting west to east and rotating west to east (like us, which is why things appear to move the other way, east to west) it's like it's "rolling" around the Earth.
The moons actual west to east movement is slower than it's apparent east to west (since the former takes a month to go around as opposed the one day for the latter) but is discernible if you can see a star near it, it moves about one moon width east per hour.

Go outside face due north and point your arm straight and level in front of you, now raise it about 55 degrees (in Ireland), that is the line of the Earth's axis.
You are standing on a ball spinning once every 24 hours west to east around this axis, the moon is moving west to east making a full circle about once a month and also rotating once a month in the same direction..
Try to visualise these movements while looking south, remembering we are rotating the same direction the moon is orbiting, but we are turning faster and the moon is also rotating the same direction but only once per orbit (month).

Thats gonna take awhile to work out in my head lol! Yea sorry should have clarified of course the moon does move across the sky the same way the sun does east to west if your not looking closely but as you say if its near a star you can see it move the opposite way west to east, this is the part i never noticed before, i wonder do many amatuers notice either?
The same face constantly facing is hard to visualise for me aswell, so the earths spin equals the moons rate of spin but in the opposite direction?

Cú Giobach

rccaulfield wrote: »

Thats gonna take awhile to work out in my head lol! Yea sorry should have clarified of course the moon does move across the sky the same way the sun does east to west if your not looking closely but as you say if its near a star you can see it move the opposite way west to east, this is the part i never noticed before,

Knowing your interest in space I did think that a bit odd.

i wonder do many amatuers notice either?

Good question. I first really noticed it watching out for occultations.

The same face constantly facing is hard to visualise for me aswell, so the earths spin equals the moons rate of spin but in the opposite direction?

The Earth and moon both spin in the same direction, and apart from being partly responsible for the rate of the moons rotation and orbit, the Earth's spin isn't really relevant to what you are trying to visualise.

A good way to figure these things out is to get a couple of balls, cups etc anything round that you can move around each other.
Forget about the Earth's rotation and just remember the moon turns exactly once on its axis for each orbit and all the movements of the Earth and Moon, orbits and rotations, are anti-clockwise. Go grab some balls and fiddle with them.

aido76

I found this youtube clip pretty good.

http://www.youtube.com/watch?feature=endscreen&NR=1&v=OZIB_leg75Q

An0n

You'd want a pretty big head o_O