Expert: Jayendra Upadhye - 7/13/2008

Question
Hi,

i am looking for the maximum and minimum angle of incident of sun rays on the surface of the Moon. As the day on moon consists of about 14 days of earth, so then what are the variation in intensity of solar rays??

shakeel

Answer
Hi Shakeel,
I will answer this question at length.
The simple 2 sentence answer would tell you little.

You might be knowing that the moon turns around itself once every orbital period ie once roughly 28 days.

Due to this reason, The half that always points to the earth gets its mid-day sun on full moon day. (Pournima). (provided there is no lunar eclipse).

The other half gets its share of mid-day sun when it is new-moon for us. (Amavasya). As that time, the earthward pointing half is in its nightside.

Now as the moon rotates around the earth, atleast half of it (not the "halves" pointing to and away from the earth, but "some other" half!) is always having daylight.

In its orbit around the earth, if you have the mid-point of its earthward pointing half as your reference point, it will start having daylight on its horizon, the moment the moon appears as "half day/half night from earth and is mid-way in its waxing phases.

As the moon advances further in its orbit, towards full-moon, the sun will come up for this reference meridian of the moon, from that side of the moon's horizon and progressively rise to its mid-day hieght at full moon day.

Then on the sun will begin setting for that point, till it completely sets when the moon is mid-way to the new-moon day, in its waning phase.

This for this chosen point,
1 - sun rises mid-way between new-moon and full-moon.
2 - it is mid-day at full-moon-day.
3 - sun sets mid-way between full-moon and new-moon.

For a point exactly opposite on the moon's "unseen face",
1 - sun sets mid-way between new-moon and full-moon.
2 - it is mid-day at new-moon-day.
3 - sun rises mid-way between full-moon and new-moon.

At all other meridians of the moon, this cycle is as many degrees out of phase as the meridian is from our reference moon-meridian.

Got it?

The maximum angle is always 90 degrees, at mid-day, if you keep the east west line as reference.

A slight complication is that the moon's orbits is not coplanar to the ecliptic. (plane of the solar system).
It has a 5 degree inclination to it.

This means that on full-moon day, the sun is not eactly at 90 degrees elevation, but slighly off to the south or north, depending on the location of the earth-moon pair in their orbit around the sun, and the orientation of the axis of inclination of the moon's orbit.

this will ensure that:-
1 - Each point on the moon's surface gets its maxima of solar intensity once every orbit.

2 - This maxima on any given full moon will vary as we change lunar latitude.

3 - This maxima will vary cyclically at all latitudes on any given full-moon-day over a range, over a period of 1 year.

4 - That this maxima will never exceed the solar constant of radiant energy in the vicinity of the earth's orbit, of approx. 1.4 kw /square meter.

The range being a function of the orientation of the inclination axis of moon's orbit to the ecliptic.


references:-
1 - http://en.wikipedia.org/wiki/Moon
2 - http://en.wikipedia.org/wiki/Solar_constant

Hope i could put it through.

regards
Please do rate the asnwer if you find it helpful.
Jayen