What If You Replaced the Moon With a Black Hole?

A question from a viewer of XKCD’s What If? series: “What would happen if the Moon were replaced with an equivalently-massed black hole? And what would a lunar (“holar”?) eclipse look like?” The answer to the first part of the question is: not that much. But the explanation of why that is is fascinating.

It’s worth reading the comments on the post as well…XKCD brings out the nerds and their interesting observations:

Imagine if a species grew up on a planet that had a black hole moon the mass of the moon. They’d have tides, they’d have an unobstructed view of the night sky, and they’d have no clue about this behemoth out there and would be unable to explain these bizarre perturbations in Earth’s orbit when they finally worked out Earth’s orbit.

EDIT: To everyone mentioning lensing effects: no. The eye can discern about 1 arc minute which at the distance of the moon is 280km. The lensing effect is detectable generally about double the event horizon. If the event horizon is about the size of a grain of sand, doubling it is not going to come close to being detectable with the naked eye from Earth. It is probably safe to assume that the same would be true of captured dust — that the particle size is too small to be detectable to the naked eye.

Another commenter points out that the video never explicitly answers the second question:

It never answered the part of the question about the eclipse. A grain of sand passing in front of the sun wouldn’t be visible, but if it’s a black hole, would lensing effects do anything weird?

The consensus in the comments seems to be that the effect would be minor and nearly imperceptible:

Lensing is dependent on two things: Mass of the object around which light passes, and how close by light passes. Since the black hole is one lunar mass, a very small mass on gravitational level, the lensing would be minor. Light could get a lot closer to the black hole, though. You might see a very slight “shimmer” at the edge of the sun when the black hole passes by the edge, but not much more than that. If the black hole happened to perfectly pass in front of a star that you’re observing with a telescope, you might very very briefly see a small ring instead of a point of light, but that’s about it.

Science!

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