February 21, 2008
All right, admit it – you’ve been looking ahead to March Break, and counting off the days. But this year, like 2004 and 2000, is cheating you by adding another day before that glorious spring time off. What gives?
Simply, most humans like whole numbers, but when we measure nature we find lots of decimals. A year is defined as the amount of time it takes a planet to go around the central star once – a revolution.
For Earth, this takes about 365.25 days, not 365. So we “save” these quarter days and every four years add a leap day to the calendar. The technical term for a leap day is an intercalary day.
Did You Know?
The word “calendar” comes from the Latin “to call out,” as the first day of the month, calends, used to be announced publicly in Ancient Rome. Seems simple enough, but you may have heard that not every four years is a leap year. Recently, the leap years have been
. . ., 1992, 1996, 2000, 2004, 2008, . . .
but a hundred years ago they were . . ., 1892, 1896, 1904, 1908, . . .
and a hundred years from now will be . . ., 2092, 2096, 2104, 2108, . . ..
What’s up with that?
Well, a year is not exactly 365.25 days long, but more like 365.24219. We get this number by measuring how long it takes the Sun to return to the exact same part of the sky. This is called a tropical or solar year, and essentially measures the seasons.
With leap years every 4 years, our days drift from the time counted out by the Sun, but only measured over centuries. So leap years are lessened to non-centuries divisible by 4 (like 2004 or 2008) and centuries divisible by 400 (like 2000, but not 1900 or 2100).
When did all this start? In 46 BCE, Julius Caesar, to control the Roman Empire, declared the addition of an extra day in the calendar every 4 years. His Julian calendar fixed the days from drifting throughout the seasons.
But after 1600 years, the residual drift made holidays annoying to schedule, so the Roman Catholic church’s Pope Gregory XIII “fixed” the calendar by requiring the centuries-divisible-by-400 bit. This Gregorian calendar is what we use today.
Did You Know?
The Julian-Gregorian switch was made in 1582 by having October run Thursday October 4, Friday October 15 – but only in Catholic countries. Britain stayed Julian until 1752, and astronomers still use the Julian calendar. Some calendars still in use today don’t go by the Sun’s position in the sky, but by the phases of the Moon. Since a lunar month is about 27.32 days, lunar calendars need to include leap months. This is also why some cultural or religious holidays move around so much in the Gregorian calendar.
Examples include Chinese New Year, Diwali, Easter, Ramadan, and Yom Kippur. The Roman calendar also used to be lunar, adding its leap month between February and March. This is why Caesar added his leap day at the end of February; the Gregorian switch made it “February 29th.”
Did You Know?
Venus has the weirdest calendar in the solar system. Its day is longer than its year! We now measure time accurately by counting electrons jumping around atoms. Atomic clocks are accurate to 1 nanosecond per day. We now know that the Earth’s rotation is slowing down thanks to the Moon’s gravity, so the clocks that run GPS and communication networks get leap seconds added occasionally. But in this age of Gregorian calendars, guess what element is used for atomic clocks? Caesium – named after Julius Caesar (or cesium for Cesar).
Figuring out calendars can get really complicated. Although this article talks about the Sun and the Moon, stars give us a sidereal year that’s arguably more accurate – it is 365.25636042 solar days long. Check out these books and links for more information on measuring years, orbits, and time:
Gleick, J. (1999, 2000). Faster: the acceleration of just about everything. Vintage, New York.
Mitton, J. (1991). A Concise Dictionary of Astronomy. Oxford, New York.
Singh, S. (2004). Big Bang: The Most Important Scientific Discovery of All Time and Why You Need to Know About It. Fourth Estate, London.
Darcy J. Gentleman (writer) is a Toronto-based science writer. In a previous life as an astronomy student, he remembers needing lots of coffee to calculate the switch from Gregorian to Julian. Fortunately, he now knows how to use the magical internet(http://www.csgnetwork.com/juliandatetime.html) to figure out that 02:29:08 on February 29, 2008 is 2454525.6035648147 in Julian time.