How do you figure out a moon phase?

Question

I can't upload my picture, but here is a very rough example:

If the sun is 0, the earth 1 and the moon *



-----------0
-----------------1
----------------*
(--- there to hold my characters in place)
what phase is the moon in? 1st quarter, half, etc.? and which way : light/dark, so you see the left of the moon or dark/light, so you see the right of the moon? how do you figure these kind of problems out?

Answer 1

One way to solve this type of question is to darken the half of the moon farthest from the sun. Then rotate your paper so that moon is at the top. Cover the half of the moon closest to the top (which is farthest from Earth and not visible), and you'll see whether the right or left half is lit and whether it's more or less than 50% lit.

Answer 2

The easiest way is the full moon and new moom being a gimmie. Now if you see half of the moon, draw a line along the straight edge . If you can form the letter "P" as in premier (or first) then you are looking at the first quarter. If it is a d (not a p) then it is last quarter, but think "PREMIER" when looking at the moon
Starting with the full moon - A day or two before the "full moon" you will see the
moon rise just before sunset. At full moon it will rise as the sun sets, and of course
after the sun sets as the time goes on. Each night after, notice how the light on the
moon shrinks from full to last quarter and then to nothing (a 2 week time period).
Also during this time you will notice the moon "traveling" closer to the sun.
Two weeks from full moon (day may vary) look for the sliver of the moon just before sunset.
As the moon moves into NEW moon and you cannot see at all, watch for it to become noticable
as a sliver in the other direction as the sun sets. Follow the light on the moon as
it expands to first quarter and then to full. So you can actually see the moon and the sun
at the same time virtually most of the month.

Answer 3

If you're mathematically inclined, the most precise way is to use some trigonometry. Draw a triangle with the Sun (S), the Earth (E) and the Moon (M) at its vertices. The angle SME gives the phase and it is what you want to solve for. To see why this is true, draw a picture with the sizes of the objects exaggerated. Colour in the dark side of the moon (the half not facing the Sun) and imagine yourself at Earth looking at it.

The following should become clear to you. If SME is 0, then the phase is Full. If SME is 180, then the phase is New. If SME is 90, then the phase is First or Last Quarter, depending on the orientation.

You can then calculate SME in two steps. First, you know the distances SE and EM. Second, I presume you know the angle SEM. The first step is to use the Law of Cosines to calculate SM. Next, use the Law of Sines to calculate SME from SM, SE, and SEM.

If you know the angle ESM instead of SEM, then this can be done without the Law of Cosines. You'll only require one application of the Law of Sines.

Answer 4

To clarify something for you punks, the key phases of the moon are as follows, conveniently separated by one week from one to the next:

New moon
First Quarter
Full Moon
Third Quarter

There is no "half moon". That's because one-half of the moon is always in sunlight, whether you can see it or not. What you are calling half moon is really the first or third quarter. ("Quarter" refers to which one-fourth of the moon's 28 day cycle it is in.)

The link below gives some more details and a good illustration that should help you to figure out the problem.

Answer 5

Well, start with the knowns. The full Moon is a lit circle and the new Moon can't be seen because it is between us and the Sun, and the Sun is lighting the opposite side.

Quarter Moons, Half Moons, and Gibbous Moons are pretty easy to identify. Half is obviously half. Quarter is obviously between new (0) and half. Gibbous is the only weird one. Where did they get that name from?

Ah, but is the Moon waxing or waning? If it is gibbous, will it soon be full, or was it full a few days ago.

Look at the Moon and look at the Sun (well, not directly at the Sun). Is the Moon to the east or to the west of the Sun? The Moon moves from west to east. If the Moon is west of the Sun, it is waning, and will soon be a new Moon. If the moon is east of the Sun, it is waxing, and moving toward full.

Answer 6

The easiest way to figure this out is to take a ball of some sort (tennis, basketball, whatever) and a flashlight. Place the flashlight where the sun is, yourself where the earth is, and the ball where the moon is going supposed to be.
In this case (assuming we're looking down on Earth's northern hemisphere) this would likely be first quarter, one of two phases known as half. From the northern hemisphere, the right side of the moon will be light while the left side is dark.

Answer 7

Precise illumination percentage is a tough one, but in that diagram, the moon would be waxing gibbous (between 1st quarter and full).