What part of the earth gets the most moonlight?

October 6, 2012 3:49 PM

Is there any place on Earth that gets the most moonlight?

Hey astronomers! I don't know if this even makes sense to ask, but my partner and I have been kicking around whether there is any place on Earth that gets what could be described as the most moonlight. Moonlight only counts as times where it is only the moon showing, not also the sun (since it's reflected light from the sun, I know this seems arbitrary, but I'm not looking for just visible moon, since the light really does look different).

Is there any way to determine this?
Does the whole Earth get the same amount of moonlight per year?
How would one even start to figure this out (astronomy was many years ago)?

posted by zinful to science & nature (7 answers total)

I think it's the same everywhere. Consider this: the phase of the moon dictates how much light the earth gets from the moon. Calendars list the phases of the moon. Are there different calendars for different locations on earth? No, it's the same calendar everywhere. Therefore every place on earth has the same moon phases and therefore every place gets the same amount of moonlight.
posted by Green With You at 4:47 PM on October 6, 2012

The shortest answer is that every part of the earth has the same amount of hours of moon-above-the-horizon-while-sun-is-below-the-horizon time.

The slightly more complex answer is that if you're interested in intensity of light multiplied by time (similar to 'insolation' but for the moon--we could call it inlunation) then some areas of the earth definitely get more hours of higher intensity lunar light. Those places are exactly the same for the moon as for the sun, so look at articles and graphs about insolation.

It is different yet if you are interested in taking into effect clouds, surface features, etc. (Both answers above assume perfectly clear skies and a perfectly spherical earth.)

So those are the simplistic answers. Here are a few more details:

1. The moon follows the same general path in the sky as the sun, known as the ecliptic. (The moon's orbit is inclined only 5.1 degrees from the ecliptic, which is the sun's path in the sky--so in simplistic terms it only gets 5 degrees or so away from the ecliptic at maximum.) Sun goes around the ecliptic once per year whereas the moon goes around it once per month.

The consequence of the moon following the same path as the sun is, it shines in generally the same places and at the same intensities as the sun does. So there are times when the moon is visible from the north pole and below the horizon for the south pole, and similarly for every place above or below the arctic/antarctic circles. However, the moon will go on through its 'seasons' once per month rather than once per year, as the sun does.

2. Your restriction of counting moonlight only after the sun is down adds an interesting twist, in that the during the night in summer, that moon is in the 'winter' portion of the ecliptic and vice-versa during the summer. Meaning that on days when the northern hemisphere is getting more sunlight, the southern hemisphere is getting more moonlight, and vice-versa. However over the course of a year (or particularly, several years--there might be some slight random deviations in shorter time periods) both hemispheres will get the same amount of total moonlight.

2a. BTW, an interesting tidbit is that the moon is up exactly the same number of hours during the day as during the night. Of course, it is much more noticeable at night for two reasons--no competition from the sun and the moon itself is generally in its brighter phases (the further the moon is from the sun, in angular distance as seen from our point of view, the more full its phase)

The reason for this is: Both the sun and the moon illuminate exactly half the earth at once. (Again this assumes perfectly spherical earth and also infinitely distant moon & sun--there will be only a slight discrepancy from reality which is left to an exercise for the reader.) Illuminating half the earth at all times, combined with the relatively random combinations of orbit and rotation times means that over the long time any given spot on the earth is illuminated half the time by the sun and also half the time by the moon. Since the orbits of the sun and moon aren't 'locked' or coordinated at all with each other or the rotation of the earth, you end up with half the moon-up time being when the sun is up and half when sun is down.

3. Again, these are the somewhat simplified answers based on a perfectly spherical earth (mountains will make a difference in moon-shine time, particularly for places nearer the poles where the moonlight time would be reduced in deep values, or on high mountains near the equator, where moonlight time might be increased slightly even over equatorial flat areas), moon exactly aligned with the ecliptic (it is actually 5.14 degrees or so off), and lengthy average times (in any given year certain random places will end up with a few minutes more moonlight just because of the way the lunar months lined up with the solar years etc--however I can't see how this would favor any one particular spot over another, more different random places during different years).
posted by flug at 4:57 PM on October 6, 2012 [13 favorites]

As pointed out above, both the sun and the moon are above the horizon for the same numbers of hours everywhere on earth when totaled over a full year. This ignores some small variations due to diffraction of light by the atmosphere, the ellipticity of the earth's orbit, and the fact that the sun and moon are disks rather than points.

However, if you are interested in the longest number of hours of moonlight on any one night, you want the night of the full moon nearest to December 31, when the moon is highest in the sky of the year and up for the most hours. In the southern hemisphere you would want the full moon nearest to June 21. That night will be the longest moonlight everywhere in that hemisphere.

If you want the place with the longest continuous moonlight around December 31, that would be someplace above the arctic circle where the moon would never set and the sun would never rise. The exact distance above the arctic circle, within 5 degrees, varies over an 18.6 year cycle. You would see the full moon in the sky move in a full circle around the horizon. The moon would remain up continuously for the next week, all the while in darkness, and you would see it gradually shrink to a half, quarter moon before going below the horizon. Once every 18.6 years on that date, you would have the darkest sky and the brightest moon.
posted by JackFlash at 5:36 PM on October 6, 2012

tl;dr of the above is at the top of a tall mountain on the equator will definitely give you the greatest amount of inlunation over the course of a year.

Being on a tall mountain will also give you a minor edge in total hours of lunar visibility--because being on a tall mountain maximizes lunar visibility when the moon is near the horizon.

How much difference does a tall mountain make? Well, besides any benefit from getting above things that block the local horizon (which you will have to figure out for each particular locality), and for purposes of inlunation, that you may be above some of the atmosphere and cloud cover, it turns out that increasing your altitude by 1.5km gives you about another minute of moon visibility* at moonrise and moonset. So for instance if you were on Mount Chimborazo in Equador you would get about 8.5 minutes extra moontime per day vs. being at sea level (given your restrictions that will be somewhat less because either moonrise or moonset will usually be during daylight hours).

Assuming you're some kind of moon worshipper or something, that extra 8.5 minutes per day could be quite significant.

In terms of inlunation, the equator is clearly the best location for your tall mountain (see the graph here--equator clearly the best).

In terms of total hours of lunar visibility, however, I'm not sure if it makes any difference where you put your tall mountain. It is just possible that you will get more benefit from your tall mountain if it is near or perhaps just on one side or the other of the arctic or antarctic circle, because the moon will spend relatively more time near or just below the horizon in those places. The effect of a tall mountain is, basically, to turn a just-below-the-horizon-moon into a visible moon. But someone would have to do some pretty serious calculation to figure out for certain whether the benefit there is greater than for a mountain in more temperate or equatorial regions.

*The linked page is about sunrise/sunset but again the exact same calculation holds for the moonrise/moonset.
posted by flug at 5:51 PM on October 6, 2012

NB to the the above: Being on a tall mountain will also expand your sunrise/sunset times by the same amount as your moonrise/moonset times. Since you don't want to count moonlight during sunlight hours, it's possible that those two effects will basically cancel each other out. Again, some calculation will be necessary to figure out what the net effect will be.
posted by flug at 5:58 PM on October 6, 2012

Following up on Jackflash's idea, here is what happened at full moon at 80 degrees latitude, above the arctic circle, in December, 1934:

The moon rose on Dec 14th, 12:23 and remained above the horizon, with no sun visible at all, until Dec 25th, 17:02.

That was over 11 days straight of nothing but pure moonlight, and has got to be pretty close to the all time record for such a thing!

Full moon that year was December 20th and it was near the January 1934 "lunar standstill", both of which helped to maximize the amount of time the moon was above the horizon and the moon's height above the horizon. Being that it was around the winter solstice, the sun was well below the horizon and since was it full moon, the moon was as as fully lit and bright as possible.

That is a lotta moon!

Check it out yourself using this sky simulation by entering these values:

• Under Location, Longitude: 9:00
• Latitude: 80:00
• Under Time, Date: 14/12/1934
• Local time: 12:00
• Then click "+" under "Increasing minutes" and watch what happens over the next 11 days

posted by flug at 7:01 PM on October 6, 2012 [2 favorites]

Investigating a little further into what is the longest possible period of uninterrupted lunar visibility, it looks like if you go straight to the north (or south) pole, you'll maximize the period of uninterrupted visibility.

The other factors are finding a full moon and the major standstill of the moon that both align best with each other and with a winter solstice. There are probably some other factors having to do with the particulars of the lunar orbit etc.

The best alignment I could find was December 1915, which gives a period of uninterrupted moonshine at the north pole of 14 days, 11:00 hours (12/13/1915 19:26 to 12/28/1915 6:26).

Next best was December 2048 at 14 days 8:43 and Dec 2012 was close behind with 14 days 8:27.

Those are not the result of a very systematic search, so it is possible there are some others that beat these by a few hours--but not by much more than that, given that the lunar orbit is just a touch over 27 days, so our longest uninterrupted views are already a good bit over half that.

So in general: Your longest period of uninterrupted lunar visibility is a little over 14 days.

These are all calculated using the sky simulation at astronomes.com with locationat 89 degrees 59 minutes (it won't let you enter 90 degrees for some reason). You would get similar if not identical results by using the south pole and full moons near the June solstice.

And . . . I realize this may not be exactly the question you asked, but one conceivable interpretation of "the place on earth that gets the most moonlight" is the place that gets the longest possible stretch of uninterrupted moonlight--and here it is!
posted by flug at 8:32 PM on October 6, 2012 [1 favorite]