The Official Cricketweb Science Thread!

[silentstriker]

Originally Posted by zaremba

But the moon's lit face should always point towards the sun

It does....the part of the moon that's lit up is the part that is in direct sunlight without the Earth blocking it.

[zaremba]

Originally Posted by silentstriker

Well because they shine with the intensity of a hundred billion suns . I am not sure I quite understand the question though.

I suppose what got me thinking is that if you look at the Milky Way, yeah sure it's white-ish, but it's basically pretty dark. When you look at pictures of galaxies from much further away, they appear bright white.

[zaremba]

Originally Posted by silentstriker

It does....the part of the moon that's lit up is the part that is in direct sunlight without the Earth blocking it.

Well it doesn't really - or more precisely it doesn't seem to. It's the Moon Tilt Illusion and it does my head in every time

[silentstriker]

Well, we're in the milky way. So all the stars around us are in the milky way....

Everything else you see, not in that plane, are the galaxies. They are so far away that despite having hundreds of billions of stars, they'll appear as single or fuzzy white dots.

[zaremba]

Originally Posted by silentstriker

Everything else you see, not in that plane, are the galaxies. They are so far away that despite having hundreds of billions of stars, they'll appear as single or fuzzy white dots.

Well we see individual stars outside the plane of the Milky Way too, just a lower concentration of stars than are in that plane. But the point is, if the galaxy we see from so close appears to be basically pretty dark (as the Milky Way does), why should other galaxies resolve into white dots rather than much darker dots?

[silentstriker]

Originally Posted by zaremba

Well we see individual stars outside the plane of the Milky Way too, just a lower concentration of stars than are in that plane.

We cannot see any star that is not in the milky way (with the naked eye). The plane of the milky way is simply where most of the stars are oriented with respect to the Earth, but all stars that we see are in the milky way. I think that's what you might be saying too, but I just want to be sure.

Originally Posted by zaremba

But the point is, if the galaxy we see from so close appears to be basically pretty dark (as the Milky Way does), why should other galaxies resolve into white dots rather than much darker dots?

What do you mean darker dots?

We see the empty spaces because we're in the milky way and thus we can resolve the distance between two stars.

If you place two light bulbs right in front of you, you will be able to see two separate light bulbs. If you keep them the same distance from each other, but move them further back, at some point, with your naked eye, it will only appear as one light bulb. It'll be fainter but you won't see any of the darkness between the two bulbs, you'll just see one small flash of light.

[zaremba]

Originally Posted by silentstriker

We cannot see any star that is not in the milky way (with the naked eye). The plane of the milky way is simply where most of the stars are oriented with respect to the Earth, but all stars that we see are in the milky way. I think that's what you might be saying too, but I just want to be sure.

I know that we can't see any individual stars outside the Milky Way (using that term in the sense of our galaxy). What I'm saying is that we can resolve individual stars outside the plane of the Milky Way (in the sense of the broad pale smear across the sky). I don't think we're disagreeing on this.

Originally Posted by silentstriker

What do you mean darker dots?

We see the empty spaces because we're in the milky way and thus we can resolve the distance between two stars.

If you place two light bulbs right in front of you, you will be able to see two separate light bulbs. If you keep them the same distance from each other, but move them further back, at some point, with your naked eye, it will only appear as one light bulb. It'll be fainter but you won't see any of the darkness between the two bulbs, you'll just see one small flash of light.

Yeah I know but it's just a question of how bright these galaxies appear. The Milky Way doesn't appear that bright, but more distant galaxies do. I realise I'm talking crap here, but it's something I struggle to get my head round.

[silentstriker]

Originally Posted by zaremba

Yeah I know but it's just a question of how bright these galaxies appear. The Milky Way doesn't appear that bright, but more distant galaxies do. I realise I'm talking crap here, but it's something I struggle to get my head round.

Because we don't see the Milky Way. We see the individual stars in the milky way. We don't see the galaxy as a whole (e.g, we see a Sun, and another Sun and another Sun, etc....we don't see the combined power of a hundred billion Suns).

If we went a distance away where you could see the whole milky way galaxy in front of you...it'd be very very bright.

[zaremba]

Originally Posted by silentstriker

The moon will pull the water on Earth that's closest to it towards it, resulting in a high tide. However, there will also be a high tide on the opposite end of the earth because comparatively, it experiences a much less of a tidal pull.

This is fine, but why should that mean there's a high tide - ie sea levels above mean sea level - on the side of the Earth that faces away from the Moon?

[zaremba]

Originally Posted by silentstriker

Because we don't see the Milky Way. We see the individual stars in the milky way. We don't see the galaxy as a whole (e.g, we see a Sun, and another Sun and another Sun, etc....we don't see the combined power of a hundred billion Suns).

If we went a distance away where you could see the whole milky way galaxy in front of you...it'd be very very bright.

This is what does my head in though. Why should it appear brighter when you move away from it than it does when you're right amongst it?

[silentstriker]

Originally Posted by zaremba

This is fine, but why should that mean there's a high tide - ie sea levels above mean sea level - on the side of the Earth that faces away from the Moon?

Because of the gravitational difference. Maybe this is a better way to think about it: water is a fluid, so let's break it down into three components:

1. Water that's close to the moon.
2. The Earth (the solid parts)
3. Water that's on the opposite side of the earth .

In terms of gravity, the water that's closest to the moon experiences the most gravity, followed by the Earth, followed by the water on the opposite side of the Earth.

So it's obvious that there is a high tide on the side closest to the moon because the water is pulled towards the moon. However, remember that the Earth also experiences more gravity than the water on the opposite side of the moon. So the moon essentially pulls both the water that's closest to it and the Earth towards it more than it pulls the water on the far side of the moon. E.g, it's pulling the Earth away from the water on the other side of the earth. Hence, the second high tide on the other side.

[silentstriker]

Originally Posted by zaremba

This is what does my head in though. Why should it appear brighter when you move away from it than it does when you're right amongst it?

It doesn't appear brighter if you add up all the brightness of the individual stars that you see.

The further galaxies may appear brighter than an individual star in the milky way, but if you add up all the stars you see in the milky way and compared the 'combined' brightness, the milky way is way brighter.

[zaremba]

Originally Posted by silentstriker

Because of the gravitational difference. Maybe this is a better way to think about it: water is a fluid, so let's break it down into three components:

1. Water that's close to the moon.
2. The Earth (the solid parts)
3. Water that's on the opposite side of the earth .

In terms of gravity, the water that's closest to the moon experiences the most gravity, followed by the Earth, followed by the water on the opposite side of the Earth.

So it's obvious that there is a high tide on the side closest to the moon because the water is pulled towards the moon. However, remember that the Earth also experiences more gravity than the water on the opposite side of the moon. So the moon essentially pulls both the water that's closest to it and the Earth towards it more than it pulls the water on the far side of the moon. E.g, it's pulling the Earth away from the water on the other side of the earth. Hence, the second high tide on the other side.

Yep - we're there - I sort of knew the answer to that one before I asked it, but that's a damn good explanation.

[zaremba]

Originally Posted by silentstriker

It doesn't appear brighter if you add up all the brightness of the individual stars that you see.

The further galaxies may appear brighter than an individual star in the milky way, but if you add up all the stars you see in the milky way and compared the 'combined' brightness, the milky way is way brighter.

OK I think I'm with you now. Thanks.

Now, the Moon Tilt Illusion...

[silentstriker]

In like five years of hanging out in observatories, and learning and TAing for astronomy classes, I confess that question has never come up. And I've never noticed it when observing...so I can't tell you.