Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 03/10/2008 12:07:00
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What causes a half-moon, crescent moon, new moon and so on?
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In the most simple and basic terms, the varying degrees of crescent moon are caused by the earth coming between the sun & the moon casting a shadow. The new moon is caused by the moon being completely in the earth’s shadow.
The full moon is the result of either the moon being completely clear of earth’s shadow or a bunch of drunken louts one their way to a footy match dropping their trousers (pants US) and exposing their posterior.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.freesmileys.org%2Fsmileys%2Ftaunt007.gif&hash=d80297e48bad104f1b4b9df43fdf0233) (http://www.freesmileys.org)(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.freesmileys.org%2Fsmileys%2Ftaunt005.gif&hash=891d1ff9e398f75797a93c736026f28f) (http://www.freesmileys.org)
Be fair, you asked for that.
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To put it politely I think this explanation is somewhat in error
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Don - you are referring to a Lunar Eclipse, which can be seen only occasionally.
The Moon's phases are seen all day, every day (somewhere on Earth).
Go out in your garden with a football and a torch at night. Get someone to shine the torch on the ball (or hold it in a clamp of some sort) and wander round the garden. Depending where you are standing, the ball (or, at least, illuminated parts) will look just like the Moon does.
We see it at different angles, as it goes around Earth and it looks different every day / night.
It's amazing just how many people think it's what you said. You are (were) in good company.
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Oooo yes! So sorry, I was far to intent on mooning and not thinking about the real answer.
Slap my wrists!
It's phases are dependant on the moon's attitude to the earth and the sun, and how much of the illuminated side that is visible from earth.
Can I moon again now?
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The Moon's phases are seen all day, every day (somewhere on Earth).
It would be incorrect to state that all the different phases of the moon are visible from different points on the Earth on the same day.
The Earth-Moon distace is approximately 30x Earth's diameter, so the parallax effect is minor:
like looking at your torch-lit football from 3 meters away, first with one eye then the other.
The phase of the moon looks essentially the same from anyhere on the Earth on a particular day.
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I didn't say that; my grammar may not have been perfect but I was drawing the distinction between that and the Eclipses, which are visible for a short time and in a localised area when they occur.
Trousers back up please Don_1.
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What causes a half-moon, crescent moon, new moon and so on?
The moon orbits the earth once every 28 days (relative to the sun). So there's a 28-day cycle of positions of the moon relative to the incident light from the sun. This causes the "phases" of the moon.
The world then spins once every 24 hours beneath the moon, causing the moon to appear to rise, track across the sky, and set (roughly) daily - in a similar way to the sun.