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Lunar Arctic Circle?

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one_nation_underdog

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I was in Finland the other day reading about a town above the arctic circle and the phenomena of winters where the sun doesn't shine for weeks and summers where the sun does not set. I was wondering what would be the nature of the moon sightings above the arctic circle. Will it too be gone for weeks in the winter and/or visible all day in summer? Is there a lunar arctic circle delineating how far north one needs to go befoe not seeing the moon or seeing it circle the sky? Or is this phenomena too erratic to mark due to the makedly different orbits of the sun and moon in relation to earth? I'm curious to hear some ideas on this subject. Thanks, Doug
 
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neilsox

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One of the posters who is rarely wrong said that the moon is visable rarely near the poles in summer, but frequently in winter. I did not understand why and it seems counter intuitive. Like the Sun I would expect the moon to rarely be more than 22 degrees above the horizon. Max would be about 45 degrees for Northern Finland, for the Sun on about June 21 each year, and the sun does not set for several days including June 21. Neil
 
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MeteorWayne

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sigh....neilsox, do you ever actually look at the sky?

The moon, like the sun and all the planets, travels along the ecliptic (the plane of the solar system) in our skies, +/- 5 degrees more because the lunar orbit is inclined relative to our equator.

So in the summer, the ecliptic is high in the sky in the daytime (where the sun is) and low in the sky at night, where the summer constellations of Sagitarrius, Scorpio, and Capricorn are.

In the Winter, the ecliptic (and the sun) is low in the daytime, so it is high in the sky at night, when the full moon, and the winter constellations of Taurus, and Gemini are.

So the highest moons are in the winter, and the lowest in the summer.

(All of the above is for the northern hemisphere, the seasons are reversed for the southern hemisphere.)
 
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SpaceTas

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Solar arctic/antarctic circles are set by tilt of Earth's axis with respect to our orbit ie at latitude = 90-23.5 = 66.5 deg.
So when Sun is at it's furthest south (soltice in Dec) it will just get to the horizon at North 66.5 deg and so never be visible from further north. At the pole the sun won't be visible for half a year because the Sun is south of the celestial equator. In summer when the Sun is at the celestial equator (Declination = 0) then it will at the horizon 24 hours at the pole. When it is furthest North it will stay on or above the horizon for all latitudes north or the Arctic circle. Reverse direction for southern hemisphere.

It is very similar for the Moon.

Moon's orbit is tilted 5.14 deg with respect to the Earths orbit and so the ecliptic in the sky. ie has a tilt of 23.5 + 5.1 = 28.6 deg with respect to the Earth's and celestial equator). So the lunar arctic/antarctic circles are at 90 - 28.6 = 69.4 .

For example when the moon is at it's northernmost in the sky it will be above the horizon for anybody north of 69.4 deg. or invisible for anybody south of -69.4 deg.

One complication: atmospheric refraction lifts Sun or Moon up by about 34 arc minutes (just over one Sun or Moon angular diameter). So the above values are out by 1/2 deg. Add half a degree to get effective arctic/antarctic circles.
 
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MeteorWayne

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SpaceTas":2hqhyeat said:
Moon's orbit is tilted 5.14 deg with respect to the Earths orbit and so the ecliptic in the sky. ie has a tilt of 23.5 + 5.1 = 28.6 deg with respect to the Earth's and celestial equator). So the lunar arctic/antarctic circles are at 90 - 28.6 = 69.4
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It's also 23.5-5.1=18.4 degrees....90-18.4= 71.6. :)
 
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rfoshaug

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All my life I've lived north of the arctic circle (at about 69° north, in Norway), and this phenomenon with the full moon being constantly above the horizon for days in the winter and being constantly below in the summer is something I've seen many times.

:)
 
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neilsox

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Thank you rfrohaug for the first hand report. Should I assume the full moon (or almost full) is frequently visable near December, but not near June when crescent moons are visable, but hard to see due to the sun also being above the horizon? Neil
 
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