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With all the talk these days about "leaping years," we should probably be asking "what
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.