What is the greatest percentage of the Moon can be lit?

Question

I thought it was 50%. Half the Moon would be lit and it would be "full". A program I have that shows the current stage of the Moon has gone past 50%.

Answer 1

The illuminated fraction of the Moon will be slightly more than half. If the Sun were a point source at infinity, it would illuminate exactly 50% of the Moon. But consider if you're at the spot on the Moon's equator where the center of the Sun is right on the eastern horizon. At the same moment, 180° away on the other side of the Moon, they see the center of the Sun on the western horizon. But since the Sun appears half a degree in diameter, you can go 1/4° east and west of these two points before the Sun completely disappears. So the actual illuminated "hemisphere" is more like180.5°.

Answer 2

Being an optimist, when the moon is between the earth and the sun, the side facing the sun is lit, and the back side is lit from the reflected light from the earth. How much light does it take to be lit? On a full moon night with no clouds, I can see pretty good from the reflected light from the moon. I would say that under the right condition, the first one, all the moon can be lit or lighted.

Answer 3

Depends on what you're referring too.

If you refer to the entire surface of the spherical moon, then of course 50% of it is lit at all times (excluding lunar eclipses, if you want to get really picky).

However, if you refer just to the side of the moon that faces the Earth, then of course that side can be lit anywhere from 0% (new moon) to 100% (full moon).

I would think that any program that talks about the moon being over 50% lit is referring only to the side that's visible from Earth.

Answer 4

It depends - do you mean the percentage of the Moon's whole surface, or the part we can see? 50% of the Moon is always lit, except when it's eclipsed by the Earth. The side we can see is anywhere from 0% to 100% lit depending on its phase.

Answer 5

The sun is an illuminous body of greater diameter than the moon, so the moon would mathematically be more than half lit.

Put a little ball bearing up against a light bulb to understand this.

Bark seems to believe that the moon has an atmosphere.

Answer 6

Bramblyspam and injanier are both correct. If you want to get really picky though you can include Earthshine - sunlight is reflected from the Earth and illuminates the moon. It's most noticeable at new moon and is sometimes called the old moon in the new moon's arms.

Answer 7

light bends so it can wrap around a bit
illuminating more of the moon. Think about
it like this... before you can see the sun in
the morning, there is light.

EDIT: So far EVERY answer below is wrong!
Your program is correct. Light bends! On Earth,
when you can't see the sun you are on the opposite
side (people on the sun would not be able to see you),
but you can be lit just before sunrise and just after sunset.
Go out and look sometime! And the ball bearing argument
is not right... the sun casts a small cross section of its total
rays on the moon.

Answer 8

50 perecnt be cuase u can only see one half of the moon