Why does the Moon never set in Svalbard, Norway?

  • 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.

    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'.

    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.

    @Rikki-Tikki-Tavi - Your comment is interesting. In what plane is the circular orbit of the Moon around the Earth ?

    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.

    @Rikki-Tikki-Tavi - Thank you, but do you you know in what *earthy* plane the Moon orbits around the Earth? Our Equator's plane?

    That depends on the season. The earth precesses very slowly, so you can say it always points the same way (towards polaris), but the moon's orbit precesses much faster, every 18.6 years. So at one time the moon's orbital inclination must be added to the earth's axial tilt, and 9.3 later it must be subtracted. (I had to think this up right now, so please someone check it)

    The part with the season is wrong in the comment above is a mistake. I had originally thought that the precession matches the solar year, but it doesn't. It's 18.6 years. I didn't read the comment again before posting it.

    http://www.cbsnews.com/news/a-visit-to-the-doomsday-vault/ might be the documentary. The quote is "It's an otherworldly place, a twilight zone, where, sometimes, the sun never rises and the moon never sets". The qualifying word here is "sometimes". Of course, that also applies to any other place in the world if we limit sometimes to, say, 2 minutes (where I am now, the sun won't rise and the moon won't set for 6 hours!)

  • TildalWave

    TildalWave Correct answer

    8 years ago

    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.

    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?

    "Those northernmost latitudes hide to us" **what** ? There seems to be a word missing.

    Is atmospheric refraction doing more up north than anywhere else on Earth ?

    @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.

    Let's be more precise. Is there more atmospheric refraction in the polar regions ?

    @NicolasBarbulesco On average probably yes, slightly. But that would be mainly because of low altitudes, proximity to large bodies of water (even frozen) and near 100% air saturation. On the other hand, less pronounced layering between troposphere and stratosphere, or even tiny and hygroscopic airborne ice particles decrease that effect (but you might see a single object, such as the Sun, as a deformed circle because of it, just like in large cities with a clear carbon oxides boundary). So it's a mixed bag, difficult to say in a general sense. Especially for polar environments.

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