I have a theorhetical question about the Moon ..

[ollypenrice]

no, everyone sees the same side. the moon rotates once for every orbit around the Earth so the same side always faces all of the Earth (approximately). it's a bit like one of those lazy knobs (brodie knobs???) on a steering wheel. imagine you are holding it and turn the steering wheel one revolution; your hand/the knob never changes position but relative to the bolt that holds it on the steering wheel it has turned a single revolution.

Shane, where were you with this analogy when I was busting my head to explain synchronous rotation at an adult night class?? Brilliant. Definitive.

The moon did spin far faster on its axis than it does now but as with all satellites its rotation lost energy due to tidal forces until it became synchronous.

The dark-light side thing can be demonstrated sweetly like this; put a white football on a table and then shine a well focused torch beam at it from the other side of the (darkened) room. Then walk around the football (moon) to see all phases and even align your head to create an eclipse or half blind yourself at 'new moon' (or new football...) This is a demo that has always worked for me, leaving no one in any doubt.

Olly

[SeedyF]

What is gonna do ya head in next is that the moon is 400 times smaller than the sun and 1/400th the distance so we get a total solar eclipse, amazing coincidence eh, now I'm thinking if the orbit of the moon is not perfectly circular and the moon is sometimes closer in it's orbit and somtimes further away so we can see the 55% of it, does it depend where it is in its orbit as to how much of the eclipse we see, or if it's further away do we not get a total eclipse... ...ahh B@@@cks my brain's hurting, now stop thinking Matt...........

That's an annular eclipse where the sun's disc isnt fully covered and you see a ring of sun all around the moon. I learnt that doing my astronomy GCSE you know

[themos]

Shane, where were you with this analogy when I was busting my head to explain synchronous rotation at an adult night class?? Brilliant. Definitive.

But isn't it wrong?

The steering wheel knob presents a different side to the centre as the wheel rotates and the knob is free to pivot so that your hand doesn't have to twist as you go around.

The moon is like a fixed knob without a pivot, always presenting the same face to the centre. Such a knob will make your arm twist as you turn the wheel, thus showing that the moon rotates around itself.

[ollypenrice]

But isn't it wrong?

The steering wheel knob presents a different side to the centre as the wheel rotates and the knob is free to pivot so that your hand doesn't have to twist as you go around.

The moon is like a fixed knob without a pivot, always presenting the same face to the centre. Such a knob will make your arm twist as you turn the wheel, thus showing that the moon rotates around itself.

I think you're right. I first got this clear in my own head when instead of thinking about a circular lunar orbit I thought of the ogee curve of the moons trajectory around the sun-and-earth over the year. Draw that ogee line plus the earth's orbit and then you see that the moon does have to turn on the ogee to keep the same side facing the earth. Would it be true to say that in Shane's anaolgy we have to turn the steering wheel into the ogee line? Groan.

Olly

[The Warthog]

The dark-light side thing can be demonstrated sweetly like this; put a white football on a table and then shine a well focused torch beam at it from the other side of the (darkened) room. Then walk around the football (moon) to see all phases and even align your head to create an eclipse or half blind yourself at 'new moon' (or new football...) This is a demo that has always worked for me, leaving no one in any doubt.

Olly

Except that shows you the Earth as seen from the Moon. But if you hold the football at arm's length, with your hands top and bottom, and rotate, keeping the "Wilson" stamp towards you, you will see the phases of the Moon, while anyone outside your arms will see all sides of the football as it rotates. Of course, you will produce a football eclipse at full football.

Or, walk around somebody keeping your face towards him. He will see only your face, but people outside your circle will see all sides of your head. To do this entirely correctly, he would have to rotate 30 times while you were going around him, and then fall down like a drunk. Could be a great party game.

[Moonshane]

But isn't it wrong?

The steering wheel knob presents a different side to the centre as the wheel rotates and the knob is free to pivot so that your hand doesn't have to twist as you go around.

The moon is like a fixed knob without a pivot, always presenting the same face to the centre. Such a knob will make your arm twist as you turn the wheel, thus showing that the moon rotates around itself.

quite right matey! I suppose a better analogy is someone on a roundabout, standing near the edge and turning an umbrella with a white dot on it at one turn per revolution of the roundabout in the same direction as the roundabout. this way, the dot will always face someone sat at the centre assuming the dot starts pointing at the centre as the motion starts. I suppose it's not as straight forward as this though as Olly suggests as the Earth is turning too but once every 24 hours not once every 28 days although this does not affect the part of the moon that faces it when the moon can be seen from the Earth. I think this works? (my head hurts).

[ollypenrice]

Oh heck, how confusing. Warthog, I am going to protest that my torch model does not show the POV from the moon but maybe I explained it badly. When you walk aroud the football you walk around it on an orbit interior to the sun moon (torch football) distance. This is certainly the earth-equivalent view.

Olly

[The Warthog]

Confusion, conflict, misunderstanding - my work here is done.

[brantuk]

And once in a blue moon.... it's red. Usually when the Earth's in between the sun and moon.

[brantuk]

Forget the footballs and torches and steering wheels and walking round it - am I the only one here with an orrery? lol

[matteboy]

Blue moon? Two full moons in a calander month?

I remember when I was very little seeing a red moon, I got really scared cos I though it was mars with aliens coming to attack us!

[SeedyF]

Blue moon? Two full moons in a calander month?

I remember when I was very little seeing a red moon, I got really scared cos I though it was mars with aliens coming to attack us!

It's OK, the duvet will protect you