# If we pick any point on the moon (except possibly the poles), is the sun visible for 13.66 days, and then not visible for 13.66 days?

The moon is tidally locked to the earth. The moon's orbital period is 27.32 days.

Does that mean that if we pick any point on the moon (except possibly the poles), the sun should be visible for 13.66 days, and then not visible for 13.66 days?

The reason I ask is because it would appear to present a huge problem for anyone setting up a permanent settlement on the moon.

• If they wanted to use solar power, they would apparently need to have a huge battery that could last through 2 weeks of darkness.
• Likewise growing crops in a greenhouse would face the same problem. I don't think most food crops do well with 2 weeks of continuous sun followed by 2 weeks of darkness. Although, there might be some algae or similar that would tolerate the intermittent light.
• The concept of microwave beaming from satellite panels could work quite well on moon. And if the panels have to be brought from Earth, it takes significantly less fuel to put them in moon orbit than to land them on the surface.
– jpa
Commented Nov 21, 2021 at 17:56
• @jpa how about orbital mirrors, then? If you need light anyway, there is no point of converting it to microwave and back to light. Commented Nov 21, 2021 at 20:47
• @jpa The weight of 22000km of 10AWG uninsulated aluminum wire is in the neighborhood of 330 metric tons. Assuming Starship could meet its goal of being able to carry over 100 metric tons, then they could carry enough wire to circle the moon in just four trips. Assuming regolith is insulating they can just bury the bare wires (or lay on surface) and run them at like 1MV. Then just lay panels and high voltage converters along the wire in say 8 places or so. A big project, but a proven technology on earth at least. Commented Nov 22, 2021 at 5:31

Yes, the light cycle is about one month.
29.53 Earth days, to be precise. (the difference with your figure is because the moon is also orbiting the sun along with the Earth, and after a month the sun's direction is 1/12th way further around the celestial background)

Yes, these days consist of one unbroken sunlight period, followed by one unbroken darkness period. The exact ratio of the two would depend on local geography, your latitude on the globe, etc, same as day vs. night length on Earth varies by location and season.

And yes, it would present immense problems for greenhouse growing of plants.

Another of the problems is the radiation environment on the Moon, both in terms of ionizing radiation and unfiltered solar light.

Yet another is the thermal environment. The moon's surface temperature varies from as high as about 100C down to as low as -180C. Obviously this is very bad for any life, including your greenhouse plants.

A realistic greenhouse on the Moon is an underground structure, shielded from the extremes of temperature, radiation and the vagaries of sunlight.
Illuminated purely by artificial lighting in the form of grow lamps.

The power source for the grow lights could be from solar panels and storage batteries, but is more likely to be nuclear.

• The answer shoud start with "No" instead of "Yes", because any point in the Moon is under sunlight for 29.53/2=14.765, not 13.66 days.
– Pere
Commented Nov 21, 2021 at 22:25
• @pere no, it isn't, except by extreme coincidence. The moon, like the Earth, is a sphere. Day and night duration on a sphere are not exactly the same length unless the sun passes exactly overhead. But then, you seem to have not read anything except the first word. The OP asks if the day/night cycle is really that long. My response is "Yes, the light cycle is about one month.", and I then proceed to correct the total duration, ratio of day and night, and other misconceptions. It's all in the answer, I do not know how you missed it. Commented Nov 22, 2021 at 6:16
• Doesn't the Earth also block the Sun from the moon for some time during each cycle? Obviously the percent would be different for any particular point. Commented Nov 22, 2021 at 16:24
• @HannoverFist Yes, Yes it does. Read all about it here: en.wikipedia.org/wiki/… ...On average, about 3 hours per year Commented Nov 22, 2021 at 17:44
• This is one of the main reasons the poles are likely to be the most congested (and contested?) regions of the Moon. See also permanently-shadowed regions, ISRU, etc. Commented Nov 22, 2021 at 19:51

It’s interesting that you excluded the poles from your question. If the Moon’s axis was perpendicular to the ecliptic, you could (in theory) mount solar panels on a vertical mast at the poles and continuously track the sun 24/7.

However, the Moon’s orbit is tilted 5.14° to the ecliptic and the axial tilt of its spin axis is 6.68° to its orbit. To add complexity, the nodal points (the points where the Moon’s orbit crosses the ecliptic) rotate retrograde at about 19° per year. As a result, the Moon’s “tropical year” is only 347 days long.

On an idealized smooth Moon, the polar “day” and “night” would each be 173 days but twilight (sun partially visible) lasts 18 days.

However, because of surface contours, the Shackleton crater at the south pole is in perpetual "night" and the peaks on its rim are in perpetual "daylight".

This is potentially important for ISRU.

• The "surface contours" will also include solar panels once you start erecting those vertical solar farms. Mutual shadowing will start to be an issue if you want to expand polar solar power to any great degree. Commented May 7, 2023 at 23:04

The Earth has large areas around each pole where the sun stays up, and down, for months. This is because of the axial tilt.

The big difference is that the Moon's asis is only tilted a little bit. It still has an arctic circle, but the latitude where that occurs is proportionately smaller.

Also, there are deep craters and steep walls. You have have heard about ice being found on the Moon: this is because there are spots where the sun does not shine.

So, if you were to pick a random point on the moon's surface, you may find it to be in the arctic/antarctic circle, the irregular surface might make the horizon nearer or farther away than it would be on a perfect sphere, and you might be in a deep hole.

First, a slight correction noted by others. The solar day cycle on the Moon is 29.53 Earth days, not 27.32. The difference comes from the Sun moving relative to the distant stars when viewed from the Moon during its rotation/orbit around Earth. Thus the average daylight period at low latitudes across the lunar surface would be about 14.76 Earth days alternating with an equal nighttime period.

We might consider craters near the poles, where the Sun is never visible at all because shadowing from the higher surrounding ground. Such locations are characterized by the presence of persistent ice, which has been considered a possible source for hydrogen and oxygen released via solar energy.

The flip side of this situation is the peaks or ridges, again near the poles, where the light that would otherwise reach the shadowed regions is captured. Such areas are considered candidate locations for capturing solar energy. Although no 100% sunlit area has been found, tabulated data from Ref. [1] show areas around Shackleton, De Gerlache and Malapert craters (all near the lunar South Pole) that received 74% or more sunlight year-round in 2020. The De Gerlache Crater ridge peak has a minimum 64% daylight "hours" during the winter season.

Reference

1. Bussey D. B. J., McGovern J. A., Spudis P. D., Neish C. D., Noda H., Ishihara Y., Sørensen S.-A. (2010). "Illumination conditions of the south pole of the Moon derived using Kaguya topography". Icarus 208 (2): 558–564. Bibcode:2010Icar..208..558B. doi:10.1016/j.icarus.2010.03.028.