The easy moon question that stumps everyone

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rentcp
Asternaut
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Joined: Mon Dec 05, 2011 9:16 pm

The easy moon question that stumps everyone

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.

Chris Peterson
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Location: Guffey, Colorado, USA

Re: The easy moon question that stumps everyone

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.)
Chris

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Chris L Peterson
Cloudbait Observatory
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neufer
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Location: Alexandria, Virginia

Re: The easy moon question that stumps everyone

rentcp wrote:
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?
At any given time of night you see half of the celestial sky.

But as the hours go by from sunset to sunrise you keep seeing a somewhat different half of the celestial sky (assuming that you are not at the North or South Poles).

By the time the night is over you have viewed a large percentage of the celestial sky.
Art Neuendorffer

BMAONE23
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Location: California

Re: The easy moon question that stumps everyone

Roughly 50% of the 24 hour day the sun isn't shining on the side of the Earth that you are viewing from. Roughly 50% of the 24 hour day it is shining on the side you are viewing from. The moon is visible in the sky all of the time, even during the day time. During the time that it spends closest to the Sun, generally the Sun is too bright to notice the much paler Moon, you won't notice the moon in the sky. Then you begin to see it Wax as it leaves the proxcimity of the Sun or Wane as it nears the Sun.

So I guess the short answer is that the angular size that the Sun covers in the sky is small and the location of that coverage with respect to the location of the Moon changes every day.

So you can see the moon in the brightening morning sky or darkening evening sky even when it is close to the Sun.

Neufer's answer is shorter and somewhat better

charlieo3
Ensign
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Joined: Fri Sep 17, 2010 3:53 am

Re: The easy moon question that stumps everyone

Take the shade off a lamp, (turn the lamp on) and let the bare bulb be your "sun." Place an earth globe a few feet away (a basketball will do in a pinch). Take a 2" styrofoam ball (hey, it's Christmas time, use an ornament), make it orbit your globe with the same side facing the earth. You'll figure it out for yourself. Simple.