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I heard in a documentary that, in Svalbard (Spitsberg), Norwegian archipelago in the Arctic Ocean, the Moon never sets. Why? A drawing would certainly help.

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  • $\begingroup$ Given the far northern latitude, I'd suspect the Sun would be up for six months, then set for six months, but a) that would not apply to the moon, & b) the situation with the Sun is not 'continuous all the time', but merely 'continuous for 6 months'. $\endgroup$ – Andrew Thompson Sep 5 '14 at 17:48
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    $\begingroup$ I agree, it seems quite implausible. The moon is on a circular orbit around the earth. It's not geostationary. Also, the axis of the earth is tilted by 23.4°. If the moon is over the south pacific, you will not see it from Spitzbergen. However, it takes the moon a month to go around, so I suppose it may be visible for about half a month at a time. $\endgroup$ – Rikki-Tikki-Tavi Sep 5 '14 at 18:41
  • $\begingroup$ @Rikki-Tikki-Tavi - Your comment is interesting. In what plane is the circular orbit of the Moon around the Earth ? $\endgroup$ – Nicolas Barbulesco Sep 11 '14 at 14:00
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    $\begingroup$ The moon orbit's plane is inclined 5.1° from the plane on which the earth goes around the sun. So yes, it's entirely possible for the moon to be continuously visible to be for about half a month at a time, if the two bodies are in the right places. $\endgroup$ – Rikki-Tikki-Tavi Sep 11 '14 at 14:44
  • $\begingroup$ @Rikki-Tikki-Tavi - Thank you, but do you you know in what earthy plane the Moon orbits around the Earth? Our Equator's plane? $\endgroup$ – Nicolas Barbulesco Sep 12 '14 at 14:19
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You must have misheard it, or the documentary you watched wasn't presenting very precise information. It does set but it also stays on the night sky for several days during the polar winter (polar night) when the Moon if full. This is relatively simple to imagine, so I'll describe it;

So what's happening is that the Earth's axial tilt during the polar winters leans the whole Northern hemisphere towards the night side, away from the Sun. This tilt is big enough (~ 23.4°) that the night sky objects aligned with the Earth's equatorial plane stay visible relatively low on the horizon. With those regions being either relatively flat and/or with a view towards the sea, there's not many obstructions limiting the viewing angle, so the Moon (and analogous also the Sun during polar summers) stays "locked" low above the horizon. To help a bit with imagining this, here's an animation of the Earth's axial tilt, courtesy of Wikipedia:

                                                   enter image description here

If we imagine this animation of the Earth with the Sun in the distant left of the image, so during Northern hemisphere's winter (winter solstice to be precise), and the Moon to the distant right of the image (roughly 25 widths of the image away), so when it's either full or close to this lunar phase, it's not too difficult to appreciate that the northernmost polar regions have a direct line of sight of the Moon during Earth's full rotation on its axis, or a day. If you keep in mind that other celestials, including the Moon, are oblivious to the Earth's axial tilt (well, not exactly, but let's not nitpick about tidal effects that might take millions of years to make a difference), as the Moon moves farther in its orbit, in our case towards the viewer, this observation angle decreases further still and those northernmost latitudes hide to us for some part of the day. At lunar last quarter, it would be directly towards us relative to the image, so this direct line of sight relationship between the Earth and the Moon becomes reciprocal to how we're seeing places on the Earth on the animation.

Why when the Moon is full? Simply because that's when the Moon is also behind the Earth (but not in its shadow), so the relative angle between the observation point and the Moon would stay high enough to observe it. As it moves in lunar phase and in orbit around the Earth farther, this angle becomes lower and the Moon indeed does set also in the arctic region. For what is worth, this goes exactly the same for South pole, only with a half a year difference.

One other effect that plays a role here is the Earth's atmospheric refraction which also adds to the duration during which the Moon appears not to set. Meaning, that even when the Moon wouldn't be in direct line of sight, but only marginally so, it would still appear low on the skies due to optical effect (displacement) of the atmosphere. This effect would somewhat offset observing the Moon from lowlands with possibly shallower observation angle when compared to higher altitude observation points with less direct line of sight obstructions, due to denser atmosphere and thus higher refraction index.

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    $\begingroup$ Maybe he didn't mishear. After all, we say the sun never sets at those latitudes, yet it clearly does. You're saying the moon is doing basically the same thing--up for days at a time. If we say the sun never sets would it not be reasonable to also say the moon never sets? $\endgroup$ – Loren Pechtel Sep 6 '14 at 21:50
  • $\begingroup$ "Those northernmost latitudes hide to us" what ? There seems to be a word missing. $\endgroup$ – Nicolas Barbulesco Jan 29 '15 at 12:48
  • $\begingroup$ Is atmospheric refraction doing more up north than anywhere else on Earth ? $\endgroup$ – Nicolas Barbulesco Jan 29 '15 at 12:53
  • $\begingroup$ @NicolasBarbulesco No word is missing. "What" are those very same northernmost latitudes of the Earth from the vantage point of the observer. I'm not sure what you mean with the refraction question. North pole has an antipode in South pole, so no. $\endgroup$ – TildalWave Jan 29 '15 at 12:55
  • $\begingroup$ Let's be more precise. Is there more atmospheric refraction in the polar regions ? $\endgroup$ – Nicolas Barbulesco Feb 17 '15 at 15:36
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For the practical side of TidalWave's excellent answer, here's a moon almanac for the place: http://www.timeanddate.com/moon/norway/longyearbyen

In 2014 the moon is above the horizon for a maximum of about 9 days at a time.

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Why does the Moon never set in Svalbard, Norway?

Lunar_Orbit_and_Orientation_with_respect_to_the_Ecliptic.tif

Source

As can be deduced from the diagram or knowing that the Moon's orbit is inclined by only 5 degrees with respect to the ecliptic while the Earth's axis is inclined by more than 23 degrees, the moon can not always be above the horizon, nor always be below it. Over the course of about 20 years all combinations are possible, and so whatever is possible at Svalbard must also be possible at the antipodal point Adelaide Antartica, and the Moon can't "never set" at both places unless it's always visible from one and simultaneously never visible from the other, and that's some kind of symmetry breaking.

For more on this see answers to:

Here's a calculation for the year 2020 and for August 2020 only, adapted from this answer. The Moon oscillates between periods of being up for days, being down for days, and rising/setting daily, depending on it's position along its orbit. It cycles through these states a little more than 13 times a year. You can take your pick from the long list of types of lunar months in this answer. For something simple like the sidereal month of 27.3 days there are about 13.4 of those in a year.

elevation of the Moon in 2020 as seen from Svalbard

from skyfield.api import Loader, Topos
import numpy as np
import matplotlib.pyplot as plt
from skyfield.api import load

halfpi, pi, twopi = [f*np.pi for f in (0.5, 1, 2)]
to_degs, to_rads = 180/pi, pi/180

load = Loader('~/Documents/fishing/SkyData')  # avoids multiple copies of large files
ts = load.timescale() # include builtin=True if you want to use older files (you may miss some leap-seconds)
eph = load('de421.bsp')

earth, sun, moon = [eph[x] for x in ('earth', 'sun', 'moon')]
AS = earth + Topos('90.0 S', '0.0 E', elevation_m = 2835)
Svalbard = earth + Topos('79.0 N', '18.4 E', elevation_m = 1000.) # elevation is variable

days = np.arange(0, 366, 0.1)
times = ts.utc(2020, 1, days)

malt, maz = [thing.radians for thing in Svalbard.at(times).observe(moon).apparent().altaz()[:2]]
salt, saz = [thing.radians for thing in Svalbard.at(times).observe(sun).apparent().altaz()[:2]]

days31 = np.arange(0, 31, 0.02)
times31 = ts.utc(2020, 8, days31)

malt31, maz31 = [thing.radians for thing in Svalbard.at(times31).observe(moon).apparent().altaz()[:2]]
salt31, saz31 = [thing.radians for thing in Svalbard.at(times31).observe(sun).apparent().altaz()[:2]]

plt.figure()
plt.subplot(2, 1, 1)
plt.plot(days, to_degs * malt, '-', linewidth=0.5)
plt.xlim(0, 366)
plt.xlabel('days in 2020')
plt.ylabel('Moon evel (deg)')
plt.suptitle('from Svalbard')
plt.subplot(2, 1, 2)
plt.plot(days31, to_degs * malt31, '-')
plt.plot(days31, np.zeros_like(days31), '-k')
plt.xlim(0, 30)
plt.xlabel('days since Aug. 1, 2020')
plt.ylabel('Moon evel (deg)')
plt.show()
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