This question comes from Matt who asks, "What would happen if the moon were replaced with an equivalently-masked black hole? And what would a lunar or holar eclipse look like?" I won't spoil the answer, but later on we will have to resort to the unit of millidegrees Fahrenheit if that tells you how little a black hole moon would affect the Earth.
A black hole the mass of the Moon would have an event horizon about the size of a sand grain.
Specifically, according to one of my favorite charts, a black hole moon would be a grain of fine to medium-fine sand, and could pass through a sieve of size ASTM No. 70 or larger.
I mean, I guess a black hole with the mass of the Moon would pass right through any sieve, destroying it in the process, but that's neither here nor there.
Literally, once you're inside a black hole, its singularity isn't a point in space at all. For you, it's now a point in time. It's neither here nor there.
Since the Moon's mass and position wouldn't change, the Earth's motion and tides wouldn't change, either.
When you're floating outside a spherical mass, its pull on you is the same regardless of whether the mass is concentrated at the center of the sphere or spread out throughout it.
If the Sun were replaced by a black hole of the same mass, the Earth's orbit wouldn't change, although life on Earth might.
With the Moon gathered into a point, there'd be no moonlight, which would affect the life cycles of all kinds of nocturnal animals.
But compared to a lot of the other things we humans have already done to mess with animals on Earth, this would be fairly minor. Our black hole Moon would be pretty low-profile.
