Question

Dear Mitch, 

Why do we have an extra day tagged onto the end of February every four years?  I know it has to do with "leap year' but is the whole thing really necessary?

Brad C 

Anchorage, Alaska

Answer

Dear Brad, 

This is one of those questions that can be answered in many different ways – from simply to astronomically complex.  It seems that there are more places to find the detailed answers with lots of astronomy in them, all of which come down to the basic fact that most days are not exactly 24 hours long (as most of us were taught in school).  Now I will try to give you the simplest answer I can.

Based on the earth's rotations and its movement through our solar system, a full rotation takes less than 24 hours; it really takes about 23 hours and 56 minutes.  We say a day is 24 hours because that is a close number to round 23-hours-and-56-minutes to, and it is an easier number to use.  Clocks, for example, have twelve numbers on them, and when the hour hand makes its way around the clock-face twice we have 24 hours.  BUT to make up for all those extra minutes that we round into a day, every few years those little bits of time add up to an amount that is close to a full day.  SO, almost every fourth year (BUT NOT EVERY SINGLE FOURTH YEAR) we have an extra day to make the whole system of time measurement more accurate.   And, as you probably know, this is one of those special years, sop we have 29 days in February instead of the usual 28!

Enjoy the extra day! 

Hope that helps,

Mitch