| MadSci Network: Earth Sciences |
A good question, because behind the apparently simple answer there is
actually a lot going on!
The simple answer is that the Earth takes one day
to turn on its axis, and one year to go around the Sun. But what do we mean by
day, and how long is a year?
The best way to explain this is to make a model. You can use a globe to represent the Earth, but anything circular will do (eg a cup with a handle). If possible use a lamp or a torch to represent the Sun. Place the "Earth" and the "Sun" on a table, turn on the "Sun", and note how it only lights up one half of the "Earth". In the illuminated half of the "Earth" it is daytime, and on the other side it is nighttime.
Now turn the "Earth" without moving it, keeping your eye on one point (eg the place where you live, or the handle of the cup). As this point crosses from the illuminated side into the dark side imagine that you were standing on this point - you would see the Sun setting. When this point crosses back from dark into light you would see the Sun rising.
Start with your point on the "Earth" pointing towards the Sun. If you turn the "Earth" exactly once on its axis your point will pass "sunset", nighttime and "sunrise" and end up facing the Sun again. From your position in "outer-space" looking down on the "Earth" and the "Sun" it is easy to see what is going on - the Earth has rotated once on its axis. However, standing on the "Earth" (or the real Earth) it is not difficult to work when a complete revolution has occurred - the Sun is exactly overhead again[1]. This is the usual definition of day - the amount time between noon[2] yesterday and noon today
Things would be simple if the only thing happening was the Earth rotating on
its axis. However, the Earth is also orbiting the Sun - as it takes a year to go
all the way round, the amount it moves in a day is quite small (roughly one
degree). From your position in "outer-space" it is quite easy what is going to
happen using your model. As well as turning the "Earth" round once, move it a
little way around the "Sun" in the same direction (see the diagrams). Now,
although the "Earth" has turned exactly once on its axis, the
movement around the "Earth's" orbit means that the "Sun" isn't quite overhead -
the "Earth" needs to turn a little bit further:
So for an observer on the Earth, it takes slightly more than one revolution for the Sun to go from noon to noon. This it what we usually mean by day - the 24 hours[3] between noon yesterday and noon today. The time it actually takes the Earth to rotate once on its axis is called the sidereal[4] day, and is 23 hours 56 minutes and 4 seconds.
A year is the time it takes the Earth to complete exactly one orbit of the Sun. The definition is easy, but the length of the year is not - it is 365 days, 5 hours, 48 minutes 46 seconds (365.2422 days). This causes problems for calendar systems - ideally the year would be a whole number of days long, with no extra hours left over. So how does the calendar cope?
The idea the calendar makers came up with was to make most years 365 days long, but every so often to add an extra day, to make leap year with 366 days. [Julius] Caesar assumed that the year was 365.25 days, and decreed that every fourth year was to be a leap year.
The trouble with this Julian calendar is that it over a long time the small error in the length of the year (0.088 days) mounts up - a day every 128 years may not seem much, but from its introduction in 46BC until 1582 the calendar had slipped 13 days, which was important to the Pope, as this meant that the date Easter was too early. Christoph Clavius devised an improved calendar which assumes that the year is actually 365.2425 days long (still slightly too long) by having a leap year every four years, except when the year is divisible by 100 but not 400. This means that 1800 and 1900 are not leap years, but 2000 is.
In 1582 Pope Gregory XIII ordered this calendar to be introduced - to correct for the errors already accumulated since Easter's date was fixed (since 325AD) he ordered 10 days to be dropped from the month of October. Protestant England (and her colonies, including the American colonies) did not adopt the Gregorian calendar until 1782 - by this time they had observed a (Julian) leap year in 1700, so they needed to loose 11 days to get back in step. Hence the calendar for September 1752 looks like this:
September 1752
Sun Mon Tue Wed Thu Fri Sat
1 2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
The Russians retained the Julian calendar until the revolution - by 1917 the difference was such that what we refer to as the "October revolution" is known there as the "November revolution".