Co-worker has everyone wondering why the half moon shadow is a straight edge when the moon, sun and earth are orbs, while all other moon phases have arced edges.
In: 11
Look at a sphere.
Now shine a light on it, at a right angle from where you are looking at it.
Half of the sphere is illuminated and from where you are looking at it, the border between the illuminated and dark parts looks straight.
It is curved but since we’re in the same plane as the curve at half moon, it appears straight.
As analogy, curve a sheet of A4 paper from to to bottom and look at one of the long edges with one eye. From most angles you’ll see the curve but if your eye is in plane with a long edge of the paper then that edge will look straight.
The Moon is pretty much a sphere. When you shine a light on a sphere, it lights up half of the sphere. All that changes to cause the phases of the Moon is the angle at which we view the Moon from the Earth. If we see the Moon at an angle of 90° from the Sun, we see a straight line between the two halves. Other angles show a curve.
Ahh!!! Finally, a topic I can speak on with a mild amount of authority. I’m a new elementary teacher, and I made this silly video for one of my classes in my masters. Basically, it is because (as mentioned above) the moon is a sphere. If you stand directly 90 degrees from a sphere and a light source, you see everything lit UP TO the widest part of that object, aka half of the object.
The reason you see a curved line in all other cases is because you are seeing this “half” from other angles, so you wind up seeing different amounts of this half
The silly video I made might help visualize this
[Phases of the Moon Video](https://youtu.be/H1sBbCZas7Y)
The shadow on the moon is always an arc, but at half moon the arc is pointing at you perpendicular, and you see a line.
Take a roll of toilet paper, draw a line around it and look at it from different angles. You can see it as ann arc but at a certain angle you see it straight. It’s a matter of perspective.
As it changes phase from mostly in shadow (crescent) to mostly illuminated (gibbous) it must pass through a point where half the visible surface is illuminated. What shape could the boundary take at this moment other than a straight line? Additionally the shape of the boundary must flip from convex to concave as viewed from the dark side, what shape can the curve take at this moment other than a straight line?
I suppose it could be an S shaped curve, but when do you ever see that on continuously curving surfaces?
Take a ball, draw a line from the top to the bottom. Rotate the ball observing the line.
The more the line to near the sides as you rotate the ball, the more curved the line appears, the more the line is near the center, the more straight it appears.
The line is always curved, its your POV that makes the line appear more or less curved.
Thanks everyone for your explanations. We tried to explain it in our own ways but none of it made sense to him. All of this will help so much! I’ll update in about 10 hours!
Because the moon is round. You are still seeing an arced edge, just from an angle that makes it look like a straight line. Like if you look at a coin, from the edge you cant see the curvature and it just looks like a straight metal line.
The shadow you see on the Moon is the Moon’s shadow. It’s the night side of the Moon. During a full Moon, your looking at its mid-day (noon) surface. During a new Moon, you’re looking at its mid-night surface.
During a half Moon you’re looking at the line separating the daytime side from the nighttime side (called the terminator). This traces a circle around the whole circumference of the Moon’s surface, so when you face it, it bisects the Moon.
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