Years
and years
The year that we are most familiar with has 365 days, divided into
12 months or just over 52 weeks. The problem with this definition of
a year is that it is based on three different time units, derived from
different astronomical movements.
A day is the time for one rotation of the Earth on its axis. A week
is approximately a quarter of a lunar month. A month is approximately
one orbit of the Moon around the Earth, while a year is the time taken
for the Earth to orbit the Sun.
The movements of the Earth, Moon and Sun are not astronomically
connected and thus years, months, weeks and days do not relate to each
other in whole number quantities as the first indications might suggest.
The year
The term "year" is generally understood
to be the time required for the Earth to travel once around the Sun,
which is about 365 1/4 days. There are different definitions of a year
(see below), but the most commonly accepted one is called the Tropical
Year. Its length is 365 days 5 hours 48 minutes 46 seconds, and is defined
as the time between two vernal equinoxes, when the Sun, as seen from
Earth, appears to cross the equator moving north.
Leap years
The extra quarter day in the solar year requires a periodic adjustment
of the calendar to keep it in touch with the seasons. Julius Caesar
was the first person to introduce the idea of a leap year. His leap
year rule added a day to February every four years. Our current calendar,
the Gregorian calendar, only differs from the Julian calendar in that
the leap year rule was changed. Every fourth year is a leap year except
for centuries, which are only a leap year if they are divisible by 400.
Let's look at the last 400 years. 1700 was not a leap year, but 1704,
1708 through to 1796 were leap year, 1800 was not made a leap year again
and so this pattern continued until the year 2000, which became a leap
year because it is divisible by 400. Over 400 years, this evens things
up a bit, but the calendar is still about 2.8 hours ahead of the seasons.
Although not specifically part of the Gregorian calendar, it has been
suggested that the 2.8 hours per 400 years be compensated for by declaring
each year divisible by 4 000 a normal year and not a leap year, the
next being the year 4 000! What about the 4 hours we are still short
of each 4 000 years, I hear you say? I wonder if we will still be using
the Gregorian calendar in 2000 years time?
Some other years
Two other more technical measurements of the year are the Sidreal Year
and the Anomalistic Year. The Sidereal Year of 365 days 6 hours 9 minutes
10 seconds, is a measure of the Earth's orbit against the stars. The
Anomalistic Year of 365 days 6 hours 13 minutes 53 seconds, is the time
taken by the Earth to pass around one orbit of the Sun from one perihelion,
the point in its orbit nearest the Sun, to the next.
Another motion involving the Earth, is the Solar System revolving around
the centre of our Milky Way Galaxy, completing a revolution about every
225 million years. This year is called the Cosmic Year.
|