As I glance at my watch, I see that it’s 7:02 pm EST. Says who? I went to www.time.gov and they have the time, to the exact second. How did “they” determine that it was exactly 19:03:25? When the first watch/wristwatch was invented, how did they know what time to set?
The time of day of any given spot on the Earth is determinded by it’s position relative to the sun. Noon is when its closest, midnight when its farthest.
GMT (Greewich Mean Time) is (for practical purposes of this thread) the time when the mean sun crosses the Greenwich meridian. http://greenwichmeantime.com/index.htm
No, Hail Ants, not really; noon is when the sun is at its zenith - a point where the sun will cast no shadow, and midnight at its nadir - directly opposite from noon, on the other side of the Earth. The Sun is closest around mid-winter and farthest mid-summer.
The sun casts shadows at noon, just not in an easterly or westerly direction. It is easy, then, to tell when noon is at a given point on the earth, assuming you know where west and east are. Originally, every place just went by their own “local noon.” This got difficult when railroads needed to coordinate with each other to prevent them crashing into one another, so time zones were invented. Timezones roughly follow lines of longitude such that noon in the time zone matches the local noon midway between the two boundaries of the zone.
No, Lodrain, not really *; midsummer is when the tilt of the earth is such that you get the maximum hours of sunlight in one day. It doesn’t have anything to do with the proximity of Earth to the sun otherwise the seasons would be the same in both hemispheres.
Coincidentally, I was having a discussion with some friends on this very point (how much of a contribution the Earth’s elliptical orbit makes to the seasons) just last night. We concluded that
a) The earths orbit is so close to circular that there’s probably no measurable effect
b) In any case, it would be a wild coincidence if the closest point was aligned with the Earth’s tilt so that it occurred in either hemisphere’s summer, and
c) If there’s any precession in the orbit, it wouldn’t even happen at the same time every year.
Not that this has anything to do with the OP…
True though this is, Lodrain’s statement is also true, for the Northern hemisphere. Note that Lodrain did not claim that the proximity to the Sun caused the seasons. In response to your conclusions:
a) That’s pretty much correct.
b) The closest point (so-called perihelion) does in fact occur in the Southern hemisphere’s Summer. It’s not a wild coincidence.
c) There is precession, but it occurs at a period of 21,000 years (I think). It will be a few thousand years before (b) is no longer true.
Oops. The main problem I had back there was a naming problem - I knew that in my neck of the woods, the perhelion occured in summer, in the middle somewhere, but forgot the exact date, so, I decided to be Shakespearean and say ‘midsummer’. Sorry.
All the definitions given so far ar interesting, but what really defines your and my, and everyone else on the planet’s time is the BIPM . They are the international organisation based in France, that is responsible for Coordinated Universal Time, which is the basis used by every time zone.