Great question, Avery! As you know, a year is the time it takes for Earth to make one full orbit around the Sun. That orbit is elliptical (a slightly squashed circle) in shape, meaning that throughout the year, the distance between the Earth and Sun varies. (This influences seasons to a small degree--seasons are more controlled by the tilt of Earth's spin axis, but that's a different story.)

Anyway, it takes just over 365 days for Earth to complete one orbit around the sun, 365.2422 days to be precise. People like to keep their calendars consistent--that is, we in the northern hemisphere are used to having the shortest day of the year (winter solstice) in December, and the longest day (summer solstice) in June.

Since it takes .2422 of an "extra" day (roughly one-quarter of a day) beyond 365 days for Earth to complete one orbit, every fourth year we add one day to our calendar (February 29th) to keep the calendar and Earth's orbit coordinated. If we didn't do that, over time Earth would fall 1/4 of a day "behind" relative to our calendar.

For example, over an average lifetime, 80 years, a particular day of the year (October 31st, lets say) would fall 20 days early relative to Earth's position in its orbit--and therefore with respect to the seasons. The northern hemisphere's shortest day of the year is roughly December 21st, and we'd like to keep it that way. Without leap years, however, in just 80 years, the shortest day of the year would be roughly 20 days later, January 10th. Eventually, what is today the shortest day of the year, December 21st, would gradually shift to being the longest (you can do the math to see how long that would take--but it's less than 1000 years).

If that's not complicated enough, there's more. You'll notice that .2422 is not exactly 1/4. This requires adjustments too. Adding 1 day to the calendar every fourth year isn't perfect, since 4 x .2422 does not equal exactly 1. Lastly, days have gotten slowly longer over geologic time (a day is the time it takes for Earth to spin once on its axis), meaning that the number of days per year is going down.

1.4 billion years ago, a day on Earth—one full rotation—took 18 hours and 41 minutes, compared to 24 hours today. Millions of years in the future, therefore, there will be 365.00 days in the year, at which point we won't have to worry about leap year for a while. As you see, you've opened up all kinds of very interesting questions. Stay curious!

Wonderful answer here.

We have a leap year because it takes the earth 365.2422 days to orbit the sun. When we make our calendars, we want a year to be measurable in days, but we also want it to be as long as the earth's orbit so that we stay in sync with the seasons. The easiest way to do this is to make each year 365 days and add one day every four years. This catches our calendar up with Earth's orbit and actually puts us slightly ahead, because 0.2422 is slightly less than one quarter. This is where things get complicated. In order to get rid of the extra time, years that are divisible by 100 but not by 400 are not leap years (so 2000 was a leap year, but 2100 will not be).

In answer to your second question, Earth's rotation is what gives us day and night, which is very easy to observe. I suppose if the earth turned a bit more slowly, one orbit (year) would take the same amount of time as exactly 365 rotations (days), so we would not need a leap year.

Hi Avery, this is a very good question and I'm glad you asked because most people couldn't tell you why we have leap years. Fundamentally, the reason that we have leap years is that a day is very slightly longer than 24 hours. Now, over the course of a month, the excess time is not really observable. Over the course of years, however, it can start to add up. Ultimately, if you don't account for it, over the course of hundreds of years, you can start to have seasons shifting into different months because of that excess time. The way we avoid this is by adding in an extra day to year, every four years so that the excess time can be eaten up by an additional day that doesn't usually occur within our typical calendar year.

One pattern we observe because of the Earth's rotation is night and day! As our side of the globe rotates away from the sun, it is night. When we are exposed to the sun, it is daytime.

Leap years are years during which we add an extra day (Feb. 29) to our calendars. This is to better align our human-defined year ("calendar year") with the actual amount of time required for Earth to orbit the sun ("solar year"). One calendar year, as defined by mankind, lasts 365 days, but Earth requires approximately 365.25 of those days to travel around the sun. Therefore, one day is added every 4 calendar years to make up the difference. Leap years are not because of Earth's rotation.

For Earth, that internal axis is the one between the north and south poles, while the internal axis is one through the sun. Thus, leap years are (sort of) due to Earth's revolution and not its rotation. (But mostly because human calendars don't match up with the sun's calendar.) To identify a pattern observed because of Earth's rotation, think of an event which occurs because Earth is spinning sort of like a top.

(Side note, the procedure for adding leap days is more detailed than adding one day every 4 years. In fact, leap days are added every 4 years, except on years ending in "00" unless that year is also divisible by 400. This is because a more precise measure of the length of a solar year is 365.2422 days, slightly less than the 365.25 commonly used.)

We have leap years, because it takes a little more than a year for Earth to revolve around the sun. It takes about 365 1/4 days for Earth to revolve around the sun. When Earth revolves around the sun, we have different seasons. We have night and day because of Earth's rotation. Earth rotates on its axis every day.

Why Do We Have Leap Years? Leap days keep our modern-day Gregorian calendar in alignment with Earth's revolutions around the Sun. It takes Earth approximately 365.242189 days, or 365 days, 5 hours, 48 minutes, and 45 seconds, to circle once around the Sun. Leap year.

Earth's year is not exactly 365 days. We use the 24-hour day because it is a convenient period of time for human lives, but that means we need to add or remove days to the calendar because of the mismatch due to the period of the Earth's orbit.