does anybody know where i can find out how we tell time based on astronomy?

Answer 1

Well, fundamentally, chronography started out by tracking the sun through shadows on the ground, but now we mostly use mechanical, electrical, and atomic properties to tell time.

However, a very large project is in development at the moment to gauge the oscillation of pulsars (neutron stars with a jet of high-energy matter and EM energy jutting out at a different angle from the axis), which have been discovered to be the most accurate clocks in the universe that we yet know of. Our radio telescopes have been able to pick up the high-energy streams from pulsars and count them. However, the dilemma lays in counting them accurately. If we don't have an accurate clock on Earth to regulate the processor frequencies governing the radio telescopes, it's hard to use anything they pick up to be a time piece. However, it's a tempting prospect because they strike between .0015 and 8.5 seconds apart, which is fairly easy for us to convert into standard time units.

Answer 2

You purchase a sextant. You observe the sun rising over the horizon and when it stops rising, you've got the moment of the meridian passage of the sun. Knowing your longitude, you translate that into time by dividing it by 360 then multiply by 24. That will be your Greenwich Hour Angle (GHA). Then you adjust for your time zone. Of course, it isn't very accurate because the day is not exactly 24 hours all year long and the exact time of the meridian is not easily found since the sun appears to stay immobile for a while on the horizon but you can find correction tables in the almanac and, for the sextant, you can measure an elevation say one hour before noon, at noon, then set back your sextant to the first elevation. When you see again the sun kissing the horizon, you'll note the time passed and half of that will be the time of the sun's meridian passage, or noon.
All this is assuming you know you longitude. If you don't, the only alternative left is a good almanac table and watching the moons of Jupiter with a telescope. That was, unfortunately, the only way seafarers could find (but not too accurately) their longitude before John Harrison invented the chronometer. Incidentally, it was when trying to build such a table of Jupiter's moon occlusion that the Danish astronomer Rømer discovered that that it was not constant during the year thus concluding that the speed of light varied with distance and came to an good estimate of that speed. That was in the 17th century.

Answer 3

Basically, the way it's done now is to observe the Earth's rotation relative to several pulsars. Every time the Earth rotates once is called a "sidereal" day. The time it takes to do this is about 23 hours, 56 minutes, and 4.091 seconds.

The exact measurement is multiplied by 1.00273781191135448, to get a value of about 24 hours. The difference from exactly 24 hours is called "Excess Length of Day", or sometimes just LOD. This is subtracted from a running total called "UT1-UTC", and is published every week -- see link #1. Right now, this total is -0.267640 seconds. When it gets close to -0.7 seconds, the Time Lords will announce a leap second.

This is the way its been done since 2003. Before that, the method was different and more complicated.

Link #2 below has more information, but is a little bit out of date.

Link #3 has a graph of LOD.

Answer 4

during the day the sun is what we use... rises in the east and settles in the west. if your formiliar with the constellations you'll see that stars rise and settle as well if your know where they are but that is harder because the seasons will also change star patterns.

Answer 5

Try searching for the atomic clock. Here's a link to Wikapedia that explains it a bit.

Hope this helps

Answer 6

http://encyclopedia.thefreedictionary.com/Epoch+(reference+date)

Answer 7

sun dial