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Q & A: Adding days

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Most recent answer: 10/22/2007
I've heard that the year is 365 and 1/4 days.Is this true? If so What does this mean?
- Zandi (age 19)
University of the WEstern Cape, South Africa
We say that a year is how long it takes the earth to go around the sun. A day is the time it takes for the Earth to spin around so the Sun goes from being straight overhead (or at least not to the East or West) to straight overhead again. (One day is actually a little longer than the time it takes the Earth to spin once on its axis, because the yearly rotation also changes the Sunís apparent position in the sky.) However, the earth doesn't take an even number of days to go around the sun. The normal calendar year is 365 days. But the earth, like you said, takes about 365.25 days to go around the sun.

If we tried to add a quarter day each year, our times would become messed up and noon could come when itís dark at night, not when the Sun is overhead. Likewise, if we always ignored that 1/4 day, it would eventually be summer when our calendars said it should be winte.

So what happens is that every 4 years, they add an extra day at the end of February to make up for the 4 quarters of days that we ignored. That year is called a leap year.

It goes a bit further. The earth doesnít take exactly 365.25 days to go around the sun. There is a couple minutes worth of difference. Every hundred years (if the year is not divisable by 400), you have to skip adding that extra day in order to make up for the few minutes that we ignore each year.


(published on 10/22/2007)

Follow-Up #1: Born on February 29

I was wondering.. what if you would be born on that specific day (the one that occurs each 4 years) would you have your birthday on the day befor or the day after that day?
- Joey (age 16)
Zoetermeer, Zuid Holland, The Netherlands

  Hi Joey,

  Iíll assume youíre talking about February 29th, which almost comes every four years, but it was skipped in 1900, and the skip was skipped in 2000, so there was one. Itís the day that picks up the little discrepancy between one year and 365 days, as a year is close to 365 and a quarter days.  If we didnít have it, the seasons would slowly crawl through the calendar over the course of hundreds of years (an "aliasing" effect if frequencies arenít exactly divisible).

  Anyhow, birthdays are entirely convention.  If youíre born on February 29th, you can celebrate your birthday as you wish.  You could just count groups of 365 days from your birth  like everybody else and celebrate on March 1.  If youíre eager to eat birthday cake a day early, no one will stop you Iím sure.

  I recommend seeing Gilbert and Sullivanís "Pirates of Penzance" for a fun story involving being born on this day.  Gilbert and Sullivanís operettas often make fun of mindless conformance to arbitrary social conventions.


(published on 10/22/2007)

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