Why do some years get an extra day?

One trip around the sun takes about 365 days and 6 hours. Our calendar only counts 365, so each year we fall six hours behind. After four years, that adds up to a whole day. So every four years we tuck in an extra day at the end of February to catch up. That's a leap year.

Is a day the same length on every planet?

Every planet spins at its own speed and takes its own time to go around the sun. Venus spins so slowly that one turn takes 243 Earth days. But it gets all the way around the sun in just 225 days. So Venus takes longer to spin once than to finish a whole year!

How long is a day, and how long is a year?

Zib answers for ages 8–11, then goes 2 steps deeper each time a child asks why.

Zib says

Earth is always spinning. When your side turns towards the sun, it's daytime, and when it turns away, it's night. One whole spin takes about 24 hours, and that's a day. Earth is also travelling around the sun, and one whole trip takes about 365 days. That's a year.

For ages 8–10

Checked against Earth facts (NASA Science)
Why do some years get an extra day?

One trip around the sun takes about 365 days and 6 hours. Our calendar only counts 365, so each year we fall six hours behind. After four years, that adds up to a whole day. So every four years we tuck in an extra day at the end of February to catch up. That's a leap year.

For ages 9–11

Checked against What is a leap year? (NASA Space Place)
Is a day the same length on every planet?

Every planet spins at its own speed and takes its own time to go around the sun. Venus spins so slowly that one turn takes 243 Earth days. But it gets all the way around the sun in just 225 days. So Venus takes longer to spin once than to finish a whole year!

For ages 10–11

Checked against Venus facts (NASA Science)