365 days a year?

The year is approximately 365 days because that is the number of days required for the sun to complete its excursion from one tropic, extrema of travel, to the other tropic, and back. Tropic of Cancer and Tropic of Capricorn.

But it turns out that just a bit more than 365 days is required. About, but not quite, 356 1/4 days is closer. But 1/4 days are hard for humans to live with. So we have leap year where we bundle up the 1/4 days and play them all at once as one extra day.

The not quite above is that it is actually seven hours and change longer. So once every 400 years, as in 2000, we DON'T have a leap year on a year divisble by 4.

All of this was determined several times over the years and the last major calendar change was done under the auspices of Pope Gregory. The Roman Catholic church ruled most of the European world then, so if the Church said jump, everyone jumped.

The reason for the Gregorian Calendar, Pope Gregory's calendar that most of the world uses, was to insure that Easter occurs shortly after the vernal equinox, because before lots of folks could read, or had watches, the farmers in Catholic countries would start their planting on the Monday after Easter Sunday. There are myths about planting under a full moon, so Easter is calculated to be the Sunday on or after the first full moon after the vernal equinox, basically March 21.

Now, modern, extremely accurate clocks have detected that the Gregorian scheme, while very good, also has some minor flaws. But someone else will worry about that fix because, as I recall, that adjustment needs to be made by adding or subtracing a day every 1,000 to 10,000 years. The next fix is due in about 200 years. I'd like to live that long, but I figure I'll be lucky to survie another seventy.

Kelly

Also, sometimes an extra second is added to Dec. 31st by international agreement, to fine-tune the system.

Yes the roational period around the Sun is likely to be getting faster since the dinosaurs were around.

From the Advanced Physics forums:

The Earth's orbit is eliptical, the amount it is out of round is only about 5,000,000 km in an orbit of radius about 150,000,000km. When you jump you push the Earth away from you slightly, but the difference in your mass and the Earth's mass is humungous. So the effect is very,very, small. However, there are people all over the Earth jumping at the same time, so the effects cancell each other.
There is also the pressure of sunlight on the Earth, and the moon. The Earth and the Moon are tied together by gravity so that it is their common centre of mass that orbits the sun. The orbit is decaying so in a few billion years the Earth will be closer to the sun. Meanwhile the Earth is slowing in its rotation and the day gets longer all the time. This is partly due to the drag of the tides.
By the way, space is a vacuum, there are only a couple of molecules per cubic meter.

PS

The sun is radiating alot of energy every day and e = mc^2 so its gravitational mass must be decreasing, causing our orbit to get very marginally bigger every second of every day!

http://curious.astro.cornell.edu/question.php?number=563

So the Sun loses 4,200,000,000 kilograms every second but it weighs 2 x 10^30 kilograms so that's a very small amount!

The Earth is also presumably slowing down as it collides with space dust as it travels through space every day - but given the Earth weighs alot more than dust, the collisions and decrease in speed is probably miniscule every century - so the effects maybe counterbalance!

PPS

http://www-istp.gsfc.nasa.gov/stargaze/Scalend.htm

Very precise atomic clocks nowadays tell us that the day is gradually getting longer. The culprits are the tides, twin waves raised in the Earth's ocean by (mainly) the Moon's gravitational pull. As the waves travel around the Earth, they break against shorelines and shallow seas, and thus give up their energy: theory suggests that this energy comes out of the (kinetic) energy of the Earth's rotational motion.

What then is the period of the Earth's rotation around its axis? A day, you say? Not quite.

Suppose we observe the position of a star in the sky--for instance Sirius, the brightest of the lot. One full rotation of the Earth is the time it takes for the star to return to its original position (of course, we are the ones that move, not the star). That is almost how the day is defined, but with one big difference: for the day, the point of reference is not a star fixed in the firmament, but the Sun, whose position in the sky slowly changes. During the year the Sun traces a full circle around the sky, so that if we keep a separate count of "Sirius days" and "Sun days", at the end of the year the numbers will differ by 1. We will get 366. 2422 "star days" but only 365. 2422 Sun days.

It is the "star day" (sidereal day) which gives the rotation period of the Earth, and it is about 4 minutes shy of 24 hours. A clockwork designed to make a telescope follow the stars makes one full rotation per sidereal day.

The clocks we know and use, though, are based on the solar day--more precisely, on the average solar day, because the time from noon to noon can vary as the Earth moves in its orbit around the Sun. By Kepler's laws (discussed in a later section) that orbit is slightly elliptical. The distance from the Sun therefore varies slightly, and by Kepler's second law, the motion speeds up when nearer to the Sun and slows down when further away. Such variations can make "sun-dial time" fast or slow, by up to about 15 minutes.

PPPS

http://www.lpi.usra.edu/education/skytellers/day_night.shtml

Good news for overachievers: Earth's days are getting longer!

Researchers examining ancient corals noted that annual growth patterns suggested there were more days in a year in Fossil corals from the Devonian Period (380 million years ago) recorded 400 daily cycles. About 290 million years ago in the Pennsylvanian Period, there were 390 daily cycles each year. Assuming that Earth's revolution around the Sun has not changed dramatically, this means that the number of hours per day has been increasing and that Earth's rotation has been slowing. Today, the length of a day is 24 hours. During the Pennsylvanian Period a day was ~22.4 hours long. In the Devonian Period, a day was ~21.8 hours long. Earth's rotation appears to be slowing approximately 2 seconds every 100,000 years. Why are Earth days getting longer? Some scientists suggest that tidal cycles create a "drag" on Earth, causing it to slow down.

You may also find this interesting...

Here's the algorithm to determine Easter Sunday according to the Gregorian Calendar.

Res: residue (I'm not sure if that's right in English, it's supposed to be the leftover from the exact division of the numbers).

X: the year you want to know.

a: Res(x,19)
b: Res(x,4)
c: Res(x,7)

M=19a + 24

d=Res(M,30)

N=2b + 4c + 6d + 5

e=Res(N,7)

now you need to choose whichever makes sense from

d + e - 9 (April)
and
d+3 + 22 (March)

------------------------
Ex: 2004

a = Res (2004,19) = 9
b = Res (2004,4) = 0
c = Res(2004,7) = 2

M = 19(9)+24 = 195
d= Res(M,30) = 15
N = 2(0) + 4(2) + 6(15) + 5 = 103
e = Res(103,7) = 5

Now we gotta choose between

15 + 5 - 9 = 11 --------> April 11th
15 + 5 + 22 = 42 -------> March 42nd

The answer is April 11th
---------------------------

And since Easter Sunday is always, well, sunday... you are able to find exactly which day of the week it was on any* given day.

I don't know why the system works that way, but it does.

* too lazy to think if it works b.C.

Extending g_day's explaination, and what I would have included if I hadn't been interupted at home while posting....

The result of the lengthening day is that eventually there will be no need for a leap year because the rotation of the Earth around its axis will be in synch with the Earth's orbit around the Sun. Of course, extend that far enough an soon, a few million years, we will have to shorten the years occassionally by deleting a day from the year. I hope to live a long time, but I don't expect to live long enough to see any serious discussion of changing the current calendar.

Kelly

Interesting isn't it, our days get longer whilst our years get shorter!

i thought i was smart but apparently I have not lived long enough to compare to some of you guys... heck any of you lol... thanks for all the replies

Don't sweat it Seed. Engineering school is a great source of a horrendous amount of seeming unrelated information until you start practicing engineering in most of its glory. Then you find out you don't know anything and have to hit the books twice as hard as you did in school, without the pizza runs.

Kelly

man I dont think i could without the pizza runs... shoot... college is all about 3 AM trips to waffle house... but I have been goin through the forum and I am just baffled by the amount of intellegence within this forum... this is a great place :-D WOO HOO!