[bhanna1]

I have been puzzled for years about the visible shape of the moon as it goes through its various phases. Specifically, when we see the moon in what I'll call 'a quarter moon' state, the visible part is always crescent shaped and continues to be that way as more and more of the moon becomes visible. After it reaches a 'half moon' state, the dark part becomes more and more concave, that is to say, the visible portion appears to bulge out and continues to do so until the moon becomes full. What causes this? Another thing I've wondered about is this: When we are in a quarter moon stage, the visible portion appears to rotate in a clockwise direction to some degree over the course of the night. Why is that? Hmmmmm?

[tickle]

Here is your explanation bhana1. The moon orbits the earth in a counter-clockwise direction.

By observing the Moon over a period of several weeks, one will notice that the Moon rises and sets at different times each night, and that there is a regular progression through lunar phases. In a synodic month, the Moon progresses through one lunar cycle and will vary between being a completely dark new moon and a fully illuminated full moon . The lunar phases are caused because the orbit of the Moon around the Earth will vary the Moon's position in relation to the Sun. Half of the Moon is always lit by the Sun, but the portion that we see will change depending on where the Moon is in its orbit. The synodic month “begins” at new moon. Because the Moon is in the same part of the sky as the Sun, the illuminated half of the Moon is not facing us and is not visible. During new moon, the Moon rises and sets at the same time as the Sun, and is therefore in the sky during the day. There is then a progression through the growing crescent phase until we see the right half illuminated; this is known as a first quarter moon. After the first quarter phase, there is another progression, this time through the waxing gibbous phases. The Moon becomes full about 15 days after new moon. During a full moon, the Moon is opposite the Sun and is fully lit. The Moon rises at sunset and sets at sunrise when it is full, so the Moon is always visible in the night sky while full. The Moon then begins to wane through another gibbous phase until it reaches its next phase called last or third quarter moon, and again proceeds through another crescent phase, ultimately returning to the new moon almost 30 days later. The saying “Once in a blue moon” is a referral to when two full moons occur in the same calendar month.

[bhanna1]

Thanks for the info Tickle! I think I understand, but am not sure. If what we 'see' of the moon is caused by the relationship between the location of the moon in its orbit and the sun... then why during the day when the sun and moon are both 'up', do we still the same phase of the moon that we do at night?

[tickle]

I dig up an astronomy book and find the Moon's relative brightness. The Full Moon has about 1/400,000th the brightness of the Sun. Yet the Full Moon sheds enough light that we can read a newspaper at night. It's over 33,000 times brighter than Sirius, which helps explain why, in the daytime, we don't see Sirius but do see the Moon.

I read that the Moon has one of the lowest reflectivities of all the objects in the Solar System. Curious. It looks bright and white even with an average reflectivity of an asphalt parking lot.

Almost every day in a lunar month, we can see the daytime Moon. The days we cannot are when the Moon is Full, when it's New, and a few days before and after the New Moon.

An exactly Full Moon is invisible (or at least not easily seen) during the day because then the Moon is opposite the Sun with the Earth in between. The Full Moon sets when the Sun rises (except near the poles) so we can't see the Moon during the day. It's below the horizon, shining brilliantly on the other side--the night side--of Earth.

When the Moon is New it will be dark and unseen and, for at least two days either side of the New Moon, the Crescent Moon will be "too faint and close to the Sun to be seen with the naked eye--although it might be glimpsed at sunset," says Robert Massey, astronomer at the Royal Observatory Greenwich.

The Moon is close to Earth and that's why we see bright sunlight reflected from its asphalt-parking-lot surface even in the daytime and even when part of the Moon is in shadow. Step outside on or about the 14th of June, and find the daytime Moon. It's there--a shiny half moon, high and West.

[bhanna1]

Thanks fro the additional info! I'm still puzzled by what causes the 'dark' part of the moon as we see it. I had foolishly thought it was caused by the earth being between the sun and the moon, but that can't be right because at times we see the moon during the day when the sun is up and it is always the same as it was at night... hmmmmm?

[tickle]

You have all the information I can supply on this particular topic. I don't know why you ae puzzled. The dark part of the moon is in shadow.

[ebaines]

The crescent shape is caused by the fact that the moon is basically a sphere being illuminated on one half only by the sun. The sun shines on one half of the sphere, and the other half of the spere is dark. Depending on how the sun, moon and earth are oriented, you perceive the moon as essentiall a flat disk with a crescent "cut out." You can see this for yourself if you try the following experiment. Set up a basetball (to represent the moon) on a table in the center of a large dark room, and set up a flashlight on another table that shines on the ball from far enough way for its beam to cover the entire ball - the flashlight represents the sun. The flashlight will illuminate one side of the ball, while the other side remains dark. Now walk around the basketball, and see how from various angles the half that is lit takes on a crescent appearance.

[FlyYakker]

tickle and ebaines are correct. See also Moon Phases / Lunar Phases Explained

The earth does occasionally cast a shadow on the moon, but only on a full moon, and only with specific earth/moon/sun alignments. Such an event is called a "lunar eclipse".

See: Lunar Eclipses for Beginners

[bhanna1]

Ebains & FlyYakker, thanks for the clarification! The experiment cleared it up for me. I suppose what I failed to consider was that everything (sun, moon and earth)is in constant movement or orbit which in itself is the major contributor to the moon phases. Thanks again!