by Chris Peterson » Mon Dec 05, 2011 10:14 pm
rentcp wrote:I've asked a number of people and can never get a satisfactory answer. 3 facts, followed by the question:
1. The moon's orbit is roughly co-planar with the orbit of the earth around the sun
2. I experience night time when my side of the earth faces away from the sun
3. For 14 of every 28 days the moon must be on the sun-side of earth as it follows its lunar cycle/orbit around earth
Q: Why can I see the moon on +95% of nights if my side of the planet faces away from the moon on 50% of nights?
I'm looking for the simple answer to this seemingly confounding and ?inaccurate? set of assumptions + question.
Each day, the Earth rotates once about its axis (that's the definition of a day). What this means is that each day, the Moon passes over every line of longitude (really, it's a bit less because the Moon's orbit takes it about 15° in the same direction the Earth is rotating... but we're talking approximations here). The only time you can't see the Moon at all is when it is very close to the Sun... a day or so on either side of new. Otherwise, figure you will see it for about half a day- 12 hours- each day. Except for very close to the new Moon, when that entire 12 hours matches the daylight hours, at least part of the period will be when the Sun is down- night. So most nights, you can see the Moon, even if for just a short time near sunset or sunrise.
In practice, all of this is tweaked by the motion of the Moon around the Earth, but the fact that it's orbital plane is inclined, and by the seasonal shift of the ecliptic. But none of those change the basic argument.
(BTW, it isn't just the Moon. Most of the stars and constellations of your hemisphere are visible at some point in the evening year-round as well.)
[quote="rentcp"]I've asked a number of people and can never get a satisfactory answer. 3 facts, followed by the question:
1. The moon's orbit is roughly co-planar with the orbit of the earth around the sun
2. I experience night time when my side of the earth faces away from the sun
3. For 14 of every 28 days the moon must be on the sun-side of earth as it follows its lunar cycle/orbit around earth
Q: Why can I see the moon on +95% of nights if my side of the planet faces away from the moon on 50% of nights?
I'm looking for the simple answer to this seemingly confounding and ?inaccurate? set of assumptions + question.[/quote]
Each day, the Earth rotates once about its axis (that's the definition of a day). What this means is that each day, the Moon passes over every line of longitude (really, it's a bit less because the Moon's orbit takes it about 15° in the same direction the Earth is rotating... but we're talking approximations here). The only time you can't see the Moon at all is when it is very close to the Sun... a day or so on either side of new. Otherwise, figure you will see it for about half a day- 12 hours- each day. Except for very close to the new Moon, when that entire 12 hours matches the daylight hours, at least part of the period will be when the Sun is down- night. So most nights, you can see the Moon, even if for just a short time near sunset or sunrise.
In practice, all of this is tweaked by the motion of the Moon around the Earth, but the fact that it's orbital plane is inclined, and by the seasonal shift of the ecliptic. But none of those change the basic argument.
(BTW, it isn't just the Moon. Most of the stars and constellations of your hemisphere are visible at some point in the evening year-round as well.)