is a year, anyway?" Ignoring all cultural history, a year is basically the period of

time that it takes for one physical body to orbit another. So an earth month could equally

be called a "moon year."

But to under-simplify things, even if we stay on earth, there are several different ways

to define a year — either the time it takes for the tilt

of the earth's axis to come back to the same angle relative to the sun (called a tropical

year) — or the amount of time before we come back

and see the same stars rising behind the sun (called a sidereal year - pronounced like

ethereal) — or, since the earth follows an elliptical

orbit which precesses, we could measure the amount of time between closest approaches

to the sun (this is called the anomalistic year).

Of course, all three "years" have slightly different lengths. But our day-to-day civil

calendar does its best to follow the tropical year, since its duration is defined by the

tilt of the earth's axis which also determines the passage of the seasons.

There are non-earth years, too: Mercury's year is roughly 88 earth days, Neptune's is

165 earth years, and the "Galactic" year – which is how long the whole solar system takes to

orbit once around the Milky Way – is around a quarter of a billion earth years! But not

all galactic years are long: check out this timelapse of stars orbiting the supermassive

black hole at the center of the Milky Way! Even though you can't see the black hole,

it's pretty obvious that it's there, what with all those stars zipping around in orbits

taking only ten earth years! Astrophysicists have even used these orbits to figure out

that the black hole is as massive as 4 million suns.

But enough about interstellar timekeeping… if you want to learn more about leap years

here on Earth, check out Grey's new video over on his awesome channel, "cgpgrey"

And a big thanks to the UCLA Galactic Center for doing sweet supermassive black hole research

with the Keck telescope in Hawai'i, and for sharing their images with the public.