Question: I’m in 8th grade and get home-schooled by my parents. They’ve been giving me assignments to use Sky Lights for answering science questions. This time I’m supposed to ask you a question. So here it is: Why does February only have 28 days, when the other months have 30 or 31? Thank you in advance for your help. — JT, Utica, New York
Answer: Great question JT. The answer relates to the original Roman calendar, established by Romulus around 753 BC. These days we use a modified version of that calendar in most (but not all) of the countries on Earth. Watch the animation above for a few orbits to get an idea of what happens as Earth goes around the Sun. Remember that one orbit around the Sun is what we call one year. And one rotation (spin) of the Earth is what we call one day.
[Note: The animation is NTS, and shows Earth spinning 30 times in one year for demonstration purposes. Having it spin 365 times in one year would make it harder to see what’s happening. And watching it might make you dizzy.]
When Romulus created that calendar (with the help of his astronomers), they knew there were going to be some problems with the number of days and months. What causes those problems is the fact that Earth doesn’t spin around exactly 360 times in one year. If it did, we could have 12 months each with 30 days.
The fact that it’s even close to 360 is pure coincidence. For example, a “year” on Mars has 687 days (counting Earth days) and 668 days (counting Mars days). Because Mars spins more slowly than Earth, one day on Mars lasts about 24 hours and 40 minutes. It gets even weirder with some other planets — on Venus, the day is longer than the year!
The problem faced by calendar makers is that one year on Earth lasts 365.2422 (Earth) days. Try dividing that up evenly. You can’t. So calendar makers just got as close as they could. Some months have 30 days, some have 31 days, but then there’s still 28.2422 days left over, and you have to put them somewhere.
February got stuck with that odd number of days simply because it was the last month in the Roman calendar. Their first month was March, chosen because the Spring Equinox occurs in that month. And the first day of Spring was important for agricultural, military, and religious purposes. It seemed as good a place as any to “start” their year.
You’ll notice that 28.2422 days is pretty close to 28.25 days. That’s why, every four years, we add a leap day to keep up, and February gets 29 days. It’s a little more complicated than that, but if you want to know more about leap years see my post from December 26, 2011. It goes into more detail about leap years, and uses the same animation as this post.
You might well ask: Why do we need to divide the year into 12 months? That’s another good question, and the answer is: There are (approximately) 12 Full Moons in one year. I discussed that in my August 22, 2012 post.
I hope you get a good grade on this assignment. There’s other good ideas for home-schoolers on my For Educators page. Best wishes for success with all your lessons. And I also hope you noticed that I didn’t say “good luck.”
Next Week in Sky Lights ⇒ The Setting Crescent Moon
I am a huge fan of Sky Lights! The articles are interesting and informative. Keep ’em coming!
Thank you sir. I endeavor to educate and entertain, but never oversimplify.
Why not thirteen months of twenty eight days with an unnumbered day inserted between July 14th and 15th with a leap day as required between the 28th day of the thirteenth month and January 1st. Yes, the moons and their phases would shift giving the calendar printers an excuse to print a new calendar every year, but the holidays would remain the same with the exception of Easter. I’m not sure about the start of the seasons because I’m too lazy to do the research.
Bob
An excellent question, Bob, since 13 months of 28 days each would yield 364 days in a year, and that’s pretty close to reality. Over the years, many “revised” calendars have been proposed. Of course, each has its advantages and disadvantages. There can be no “perfect” calendar. And when needed, where the “leap day” goes is purely arbitrary … having the seasons off by up to 0.75 days is no big deal. But the rationale behind 12 months is very compelling, just in terms of the mathematical factors of 12, definitions of “quarters,” etc. Even more compelling is the fact that, before artificial lighting (and especially before controlled fire), the Full Moon dominated the night. It allowed activities that couldn’t be done safely, or at all, using only the light of stars. That’s why, after establishing the obvious day-night cycle as a base unit of time, many of the earliest calendars were synced to the Moon.