# What is the approximate number of black holes behind the Moon seen from Earth?

Asked by mattbrowne (31537) November 27th, 2011

Let’s assume the average distance between the Earth and the Moon. So when we look up the sky and see the Moon, it does hide a portion of space which contains lots of galaxies including black holes. How many?

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I’d be willing to bet that no one knows. The moon obscures about 1/10 of the night sky, so if you knew the total number of black holes in the universe, you could compute it that way, but so far as I know, no one knows that number. Sorry!

CaptainHarley (22409)

I would think it would depend on where in the moon’s orbit we are. We’re not in the center of the universe, so at some times the moon is between us and the center of the universe, and that would be the direction of the most black holes. At other times, the moon will be between us and the edge of the universe, and that would be the direction of the fewest black holes.

In any case, my answer would be a very, very large number of black holes.

wundayatta (58492)

is this a riddle?

In the visible universe there are roughly 200 billion galaxies. Most of these are probably home to a massive black hole.

The moon is roughly 30 arcminutes across and so takes up roughly 1 over 300,000 of the total night sky that is northern and southern hemispheres combined. Dividing one by the other gives the result of one million black holes behind the moon.

The universe may contain many smaller black holes perhaps 1,000 in our galaxy alone which would mean that figure would increase to one billion and there may be lots more than that.

flutherother (25730)

Why are you asking this @mattbrowne?

Dutchess_III (34039)

We don’t know (since the distribution of black holes is more a field of question than knowledge), except in terms of the percentage of the sky (and thus, presumably, the universe) that is occluded by the moon from Earth.

Zaku (17413)

To estimate i will make two assumptions:
1)There is one black hole at the center of each galaxy
2)Galaxies are somewhat smoothoy distributed in space.

Bases upon those two assumptions, I will look at the Hubble Deep Field image. It identifies 10,000 galaxies in an angle 1/70 of the angle the moon subtends. Squaring that for area, I get 70×70=4900 x 10,000 = 49,000,000 black holes .
That’s my answer and I’m sticking to it.

LuckyGuy (33521)

@worriedguy I thought that not every galaxy had a black hole at the center. Am I mistaken?

ANef_is_Enuf (24523)

@ANef_is_Enuf

All the ones we know about do.

CaptainHarley (22409)

@ANef_is_Enuf

There’s actually a hypothesis that supermassive black holes create galaxies.

IIRC, the program I saw about this last night said that some astrophysicists believe that galaxies with bright, bulging centers have SBH cores.

Brian1946 (22235)

@Brian1946 I know the theory, but I thought that it had not yet been proven that all galaxies have a black hole at the center, they just believe that most do. I could be wrong.

ANef_is_Enuf (24523)

@Brian1946

Since there is no current way to establish that all galaxies have a black hole at their core, yes, it is still a theory.

CaptainHarley (22409)

If the universe is a sphere of debris with the site of the big bang in the center, then there would be a finite number of black holes (one in the center of each galaxy, depending on which cosmological model you believe).

OTOH, if the universe is flat and infinite in all directions, then there would be an infinite number of black holes along a line in any given tangent from earth.

HungryGuy (15966)

It would depend on how long you look at the moon and from where you look. But the estimate is between 100 and 1000 black holes.

bkcunningham (18424)

ZERO!!!!!!!!!!!!!!!!!!!!!!!!!! We can’t SEE them because they’re behind the moon!!!! Fist bump!

Dutchess_III (34039)

@Dutchess_III I totally agree, Dutchess Hawking! ;-)

Brian1946 (22235)

Heh!

Dutchess_III (34039)

Don’t be ridiculous. Read the question again.

CaptainHarley (22409)

Here is the question: “What is the approximate number of black holes behind the Moon seen from Earth?

Now apologize @CaptainHarley!

Dutchess_III (34039)

@Dutchess_III

No.

“Letâ€™s assume the average distance between the Earth and the Moon. So when we look up the sky and see the Moon, it does hide a portion of space which contains lots of galaxies including black holes. How many?”

Operant word: “HIDE”

: P

CaptainHarley (22409)

@ANef_is_Enuf I am picking an average. There is a theory that every large galaxy has one. Some have multiples . Some have none. My assumption of 1 per galaxy is probably not far off.
By the way, there is a cool video video of stars whipping around the black hole in the center of our own galaxy. I ‘ll see if I can find it.

LuckyGuy (33521)

Approximately 150 galaxies are hidden behind the moon at any one time when we look up.

bkcunningham (18424)

I found it. Video of stars moving around a black hole in the Milky Way galaxy.
The one I saw was on a NASA site. It was a 16 year time lapse study of star motion. You can see some stars whip around an unseen object.

LuckyGuy (33521)

@Dutchess_III , @CaptainHarley , @Brian1946
I know it could be a trick question but it is so much more fun to figure it out as if it is real. I love Fermi questions.

We all learn something.

LuckyGuy (33521)

Here’s the video clip I wanted to show you.

LuckyGuy (33521)

@worriedguy agreed. Plus, I was just taking advantage of the wording of the question and disregarding the details. @CaptainHarley‘s no fun!
Going to look at the video

Dutchess_III (34039)

What happened to Matt Browne? He asked and then disappeared like he was sucked into a black hole.

bkcunningham (18424)

@Dutchess_III

Hah! If you only knew! LMAO!

CaptainHarley (22409)

I can only imagine, @CaptainHarley! :)

Dutchess_III (34039)

TGIF! Sorry for my very late reply. Gee, what a week… I will read your answers in more detail this weekend. Yes, some huge office black hole sucked all my energy the whole week. Came out of nowhere. Well, first things first.

mattbrowne (31537)

@RealEyesRealizeRealLies – No, it’s not a riddle. Besides, riddles are only allowed in the social section, right?

mattbrowne (31537)

@Dutchess_III – Why am I asking this? Well, I feel there are not enough science questions on Fluther. During one of my commutes on the freeway recently I was staring at this beautiful crescent moon enjoying the sight and letting my thoughts wander. And the question popped into my head. I was also listing to some of the Naked Scientist downloaded podcasts…

mattbrowne (31537)

I couldn’t find some good estimates about the average number of black holes per galaxy and galaxy type. So I’d like to not just count the supermassive black holes at the center. Most stars in any galaxy are red dwarfs, right? So we can’t expect any as a result of their death (besides, the universe is still too young for any of them dying). We have to look at heavier stars. Do we know the distribution of star types per galaxy and galaxy type? How many of them eventually lead to a supernova resulting in a black hole? Right away and later? Which includes neutron stars of binary systems accumulating mass. Let’s discount speculation about tiny black holes from the aftermath of the big bang.

So we know about the 30 arc minutes and the rough total number of the galaxies of the universe? What about the average number of black holes per galaxy (supermassive in the center plus X)?

mattbrowne (31537)

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