but have you ever wondered why you can only have a birthday once a year?

Hi, I'’m astronomer Amy Mainzer.

The reason we only have birthdays once a year is because that'’s how long it takes for the Earth to go around the Sun.

♪ ♪ We call the motion of a planet around its sun its orbit.

The length of a planet'’s year is determined by how fast it goes around the Sun, and that depends on how close it is to the Sun.

Planets that are close to the Sun take less time to go around than planets that are really far away.

For example-- it only takes Mercury 88 days to go around the Sun.

That'’s because Mercury'’s a lot closer to the Sun than the Earth is.

If you were on Mercury, you'’d have a birthday every three months.

But if I were on Neptune, it takes 165 Earth years just to complete one orbit around the Sun.

That'’s because Neptune is so incredibly far away.

That means that even the oldest person on Earth is not even one Neptune year old.

The Earth is traveling around the Sun incredibly fast in its orbit.

It'’s going 67,000 miles an hour.

And at that incredible speed, it takes the Earth 12 months, or about 365 days, to go around the Sun one time.

And one day, if you go to live on Mars, its orbit is almost twice as long as Earth'’s, so you would only have half as many birthdays on Mars.

So the next time you need a reason to have a party, just tell everybody you'’re celebrating your Mercury birthday, and you need to have a party every three months.