How do we know how many galaxies are in the Universe?

"The human mind is not capable of grasping the Universe. We are like a little child entering a huge library. The walls are covered to the ceilings with books in many different tongues. The child knows that someone must have written these books. It does not know who or how. It does not understand the languages in which they are written. But the child notes a definite plan in the arrangement of the books---a mysterious order which it does not comprehend, but only dimly suspects."
-Albert Einstein

Earlier today, I had the pleasure of visiting a high school astronomy class via Skype, answering some very good questions from some very enthusiastic and curious students.

But there was one question that we ran out of time for.

(Image credit: 2P2 Team, WFI, MPG/ESO 2.2-m Telescope, La Silla, ESO.)

How do we know how many galaxies are in the Universe?

While that's a great topic for a blog post, I thought it would be an even better topic for an animated video! So for anyone's high school astronomy class, or anyone with five minutes to spare, hope you enjoy it! (And don't hesitate to full-screen it.)

For those of you curious about the original sources of these remarkable images and videos used in the creation of this, I couldn't resist sharing these amazing resources with you from around the world, in order of appearance.

Video of Milky Way rise, by William Castleman.

Image of the southern Milky Way, by A. Fujii.

Image of the Orion Nebula, by Ioannidis Panos.

Video of a flythrough from Earth to the Virgo Cluster, by R. Brent Tully.

Image of the Milky Way, by Derek Rowley.

Image of Hickson 68, wide-field, by Gimmi Ratto.

Image of NGC 7331 by Paul Mortfield and Dietmar Kupke/Flynn Haase/NOAO/AURA/NSF.

Image of Stephan's Quintet of Galaxies, by Adam Block/NOAO/AURA/NSF.

Image of the star field near M31, retrieved from here.

Deep image of distant galaxies with foreground stars, retrieved from here.

And all the other images and videos are derived from these two fabulous data sets:

Images courtesy of R. Williams (STScI), NASA and the Hubble Deep Field Team. And perhaps most spectacularly...

Images and videos courtesy of NASA, ESA, S. Beckwith (STScI) and the HUDF Team.

And here we are, less than a century after learning that the Milky Way wasn't the only galaxy in the Universe, we now know that there are at least 100 billion of them, and possibly even more than that!