The Science of the Leap Year

The Science of Leap Years

Humans like round numbers like 365 days in a year, 24 hours in a day, and 60 seconds in a minute.  Over time we have been able to get more precise with how long it actually takes the Earth to orbit the Sun. In fact, it takes 365.2422 days (365 days, 5 hours, 48 minutes, and 46 seconds) for the Earth to make one complete revolution around the Sun. The extra quarter day is dealt with every four by adding a full day, February 29th, into the calendar. We call this a leap year.

It’s still not a perfect system because we’re overestimating the years by a fraction each year (365.25 is greater than 365.2422). To make up for this overestimation a rule states there can only be 97 leap years over the span of 400 years, and not 100 like you may have anticipated. However, there is an easy way to remember the rule. At the turn of the century (ex. 1800, 1900, 2000) the year must be divisible by 400 to have a leap year. If not, no additional day is added to the calendar. For example, there was no leap year in 1700, 1800, or 1900, but there was one in 2000. Pretty cool!

I have a complete lesson on seasons that includes notes, a lab, PowerPoint, homework, assessment, student choice project, and more.