Astro Advisory
Notice of an upcoming
astronomical event.
PISGAH
ASTRONOMICAL
RESEARCH INSTITUTE
Contact: Bob Hayward, Astronomer/Educator
828-862-5554 ([email protected])
John Avant 919-848-1153 ([email protected])
FEBRUARY 29 - WHY ?
Rosman, NC (February 3, 2016) – Astronomers at the Pisgah Astronomical Research
Institute are as interested in the upcoming leap day on February 29 as the public since it
gives them a chance to show how smart they are regarding things astronomical.
The calendar and most all timekeeping up until the invention of atomic clocks has been
tied in closely to the motions of astronomical objects. Historically the year in most
civilizations began with the vernal equinox, the first day of spring. Back in the days of
the Romans the year began in March; that’s why the seventh month of the year,
September, was named after the Latin for seven. The Romans also noticed that the year
was not exactly 365 days long; rather it was closer to 365¼ days. So, they came up with
a scheme of adding a day to February, the last month of the year, every four years to
make the average year 365.2500 days long.
Now, fast forward to the year 1582 when the Roman Catholic Church realized that the
vernal equinox was slowly creeping earlier and earlier in the calendar. This threw
everything off with the actual seasons and with the Church’s rules for calculating the date
of Easter. So Pope Gregory XIII by decree dropped ten days from the calendar and
established new rules for adding a leap day to the end of February. This is the Gregorian
calendar, which we use today. There are three rules:
• Any year evenly divisible by 4 is a leap year.
• However, any century year, i.e., a year evenly divisible by 100, is not a leap year.
• However, any year evenly divisible by 400 is a leap year.
This makes the average length of the Gregorian Calendar year 365.2425 days. Since the
actual length of the year measured by the earth’s orbit around the sun is 365.2422 days,
we will have one day’s error in 3323 years. And, actually, there are some other more
complicated rules to correct even this small error.
The Protestant countries did not go along with Pope Gregory’s decree in 1582 and it
wasn’t until 1752 that Britain and her colonies changed over to the Gregorian Calendar.
At that time they also moved the first of the year to January making September, named
after “seven,” the ninth month.
PARI is a public not-for-profit public foundation established in 1998. Located in the Pisgah National
Forest southwest of Asheville, NC, PARI offers STEM educational programs at all levels, from K-12
through post-graduate research. For more information about PARI and its programs, visit www.pari.edu.
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