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I'm in the northern hemisphere, facing north, and notice that the moon is due north (azimuth 0) and is still above the horizon, albeit barely.

Does this mean the moon is necessarily circumpolar (doesn't set), at least for the next few hours?

As the moon moves away from due north, it's altitude/elevation normally increases. However, it's possible that the moon's declination is decreasing so rapidly that the moon's elevation actually decreases, and the moon sets.

Is this possible?

This question is tangentially related to my question "libnova odd behavior for 89.5 degrees north latitude" on Stack Overflow.

EDIT to answer questions: I am facing true north. I am referring to the Earth's true moon, but not necessarily in any given time frame. In other words, if something like this happens 10,000 years from now, that's fine.

EDIT: OK, I believe the moon CAN set even if it's up when due north. Example:

  • You are at latitude 89.5N

  • The moon's declination when due north is +0.5N. This means the moon grazes the horizon when due north.

  • If the moon's declination were constant, it's highest elevation for the day would be 1 degree.

  • Since the moon's declination can change more than one degree per day (average of 1.5 degrees/day), it's quite possible for the moon's declination to go below -0.5N, at which point it would have set (it will actually set before this point unless it happens to reach due south).

In theory, this could happen with any object whose declination changes, provide you're close enough to the North Pole.

I'll leave this update open for critiques.

EDIT: Actually, this whole question is just asking whether the midnight sun/moon can set at any position other than due north, which is fairly obviously true. Reasoning: once the moon/sun finally sets not due north, it was obviously still up the last time it was due north.

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Clarify if you are facing true north or magnetic north? –  Sparr Nov 15 '13 at 21:16
    
Are you asking about a hypothetical moon, or The Moon as in Earth's moon? –  Sparr Nov 15 '13 at 21:17
    
re your edit - you're right that the moon could set after being visible at due north in an extreme case such as in your example - in my prior comment I neglected to mention the small daily declination changes which enable the borderline case. But yes, the rare borderline case could enable rising and setting very close to due North. Now that the question is clearer, hopefully the answer is too. –  Moriarty Nov 17 '13 at 8:22
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2 Answers 2

Yes - here is a good explanation of the situation. Just as it's possible to have continuous daylight for days or weeks on end, you can have continuous moonlight too.

The Moon's orbit is tilted at (a maximum of) just over 5 degrees to the ecliptic, however this tilt has an oscillatory precession with a period of 18.6 degrees. This precession means that our "lunar arctic circle" will vary in latitude over time.

For example, if the moon is sitting exactly on the ecliptic (latitudinally speaking), it would remain above the horizon until it is at a sufficiently low phase of illumination that its current ecliptic longitude is below the horizon. Of course in reality the moon only crosses the ecliptic twice during each lunar cycle (it's these two points that precess). Stellarium is a cool program to visualize this on if you turn on the ecliptic grids.

One important thing to note is that this phenomenon is dependent on both the time of year and the phase of the moon. In the summer months, new and near-new moons are the ones that can remain visible for days or weeks on end. Conversely, at the North pole, you might not see a full moon at all for a few months!

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I think this just answers the question "can the moon be cirumpolar", to which I already knew the answer (yes). My question is about a much more specific case. –  barrycarter Nov 16 '13 at 16:11
    
@barrycarter If you are in the Northern hemisphere and the moon is due north and you can still see it, it will not set for at least 24 hours due to my explanations above. The turning points of all celestial objects, when they go from increasing to decreasing altitudes (or vice versa) are always true North and South. That's just the way the Earth rotates. Any differences due to the fact that the Moon slowly moves along the ecliptic are inconsequentially small. –  Moriarty Nov 16 '13 at 23:46
    
"Any differences due to the fact that the Moon slowly moves along the ecliptic are inconsequentially small"... that's what I'd like to see proof or explanation of. The moon's declination can change quite rapidly. –  barrycarter Nov 16 '13 at 23:58
    
@barrycarter "The moon's declination can change quite rapidly." - no, it cannot. The equatorial co-ordinate system is independent of Earth's rotation. Perhaps you are confusing declination with altitude. The Stellarium program (linked above) will help you visualize the situation. –  Moriarty Nov 17 '13 at 1:01
    
OK, can you put an upper limit on the moon's change in declination so we can have some numbers to work with? –  barrycarter Nov 17 '13 at 1:14
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up vote 1 down vote accepted

Answering my own question: yes, the moon can set after being up while due north, and this situation occurs quite frequently at far northern/southern latitudes.

Example: On 2013-08-03 21:45:21 at 72N, 0E, the moon is due north and up:

enter image description here

Less than 24h later, at 2013-08-04 at 20:51:08, the moon sets:

enter image description here

This situation is typical. If the moon has been up for more than 24 hours:

  • It will almost always set at a location other than due north.

  • In the 24 hours prior to moonset, it will have been due north and above the horizon.

Note: I used stellarium for dramatic effect, even though it tends to give inaccurate results compared to other libraries ( http://stackoverflow.com/questions/16293146/ ).

Other libraries give similar, but nonidentical results.

The general case is the same, however: if the moon's been up for over 24 hour and sets, it will have been up and due north some time in the last 24 hours.

I am downvoting and not accepting Moriarty's answer because:

  • He originally answered the wrong question.

  • He later answered the right question incorrectly.

  • He later claimed that this is a "rare borderline case". In fact, this happens almost anywhere/anytime the moon is up for more than 24 hours.

EDIT: it turns out this is known. All planets (incl the Sun and Moon) do NOT reach maximum elevation when they culminate: http://www.geoastro.de/TransitCulm/index.html

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