The shape of the moon in the Pacific North West tonight was an exact semi-circle, like a cookie cut in half. How can that be? I thought that since the Earth casts it’s shadow on the moon, the shape of the shadow has to be cresent shaped as the Earth is round. But tonight the shape of the shadow is a straight line.
No, the earth only casts a shadow on the moon during a lunar eclipse. The edge between the illuminated part of the moon and the dark side is the “terminator.” The terminator is the shadow of the moon itself. The moon’s sphere is directly lit by the sun, it can only illuminate the “front” side of the moon.
Jeez, did that make sense? I wish I could draw a picture.
oops, correct that, I meant to say “the terminator is the EDGE OF the shadow of the moon itself.” Did that make it any clearer? Probably not.
Only during an eclipse is the shadow of the Earth over the moon. The rest of the time the shadow is cast by the side of the moon facing the sun over the rest of it. Imagine the sunlight was shining on the top of the moon. When you are standing between the moon and the sun the whole hemisphere is lit. As you move down the moon more and more of the unlit lower hemisphere is seen. When you are exactly half way down the moon, ie at right angles to the sun, then you will see exactly one half of the lit top hemisphere and one half of the unlit bottom hemisphere, a perfect hemispgere. When you’re at the bottom the moon is between you and the sun and the whole globe is dark.
Of course in reality the sun and moon are in nearly the same plane to us and we’re apparently moving around the moon, but the principal is the same.
Shine a flashlight on a tennis ball and move it around until the angle of the light from the ball it is at right angle to your eyes. You’ll see a perfect semi-circle of lit ball.
The phases of the moon are not caused by the earth’s shadow. You are just looking at an illuminated sphere from an angle - in the case of a half moon, the sun is directly to the side. A full moon occurs when the sun is directly behind us - i.e. when the earth is roughly between the sun and the moon. A crescent moon occurs if the moon is almost between the sun and the earth - most of what we see is the dark (unlit) side of the moon, and a little bit of the bright (lit) side.
When the earth is directly between the moon and the earth, the earth does cast a shadow on the moon. That’s called a lunar eclipse, and happens only a couple of times a year. Other times the alignment is not so perfect, so the shadow of the earth doesn’t hit the moon. Similarly, when the moon is directly between the earth and the sun, the moon casts a shadow on the earth. If you are in the shadow, what you see is a solar eclipse. You’re even less likely to see one of those, because only the people in the shadow can see a solar eclipse.
Check out this site for a pretty cool animated image of the Moon going through its phases, as well as the current Lunar phase. You can see that there are times when the dark portion of the Moon are definitely not round as seen from Earth.