# What makes the Moon and Venus shine?

Is there some substance on Venus or some kind of material, because neither bodies have any source of light.

• Virtually everything reflects light to some degree. – SE - stop firing the good guys Apr 8 '16 at 6:04
• The cheese. It's the cheese. – Ginasius Apr 8 '16 at 12:54
• – uhoh Mar 25 at 6:06

Or would we? The point is, the moon is a large rock. So large that its apparent angular diameter is $$1/2$$ a degree. That means it takes up an area $$1/2$$ a degree by $$1/2$$ a degree (give or take a factor of $$\pi/4$$). Now it doesn't matter how far away the moon is, what matters is the angular diameter, because that's how much light reaches your eye. To put this another way, the apparent brightness of any particular 1 meter square patch falls off as $$1/d^2$$, where $$d$$ is the distance to the moon, but the radius of the moon increases as $$d$$ if the angular diameter remains constant, so the number of such patches increases as $$d^2$$, which cancels.
Venus has a smaller angular diameter (it varies, but up to $$1/60$$ of a degree) but its albedo is 7-8 times greater. Again, it's the fact that Venus is big that makes it shine, even if it looks like a dot to the naked eye.
• +1 This is basically right. Keeping the Sun-Moon distance constant, the brightness of the Moon or any extended object per unit solid angle remains essentially constant as we move farther away from it. Walls don't get brighter when we walk towards them. It breaks down when the angular size decreases until it reaches our resolution limit and the object becomes unresolved or star-like. But until them it's nearly flat. – uhoh Mar 25 at 3:27