on 4/25/2002 2:10 PM, M R at [EMAIL PROTECTED] wrote: > In the astronomical studies they say light year, the > light is universal, but the year is based on earth's > revolution and not universal.
The distances measured in light years are all sufficiently approximate that it matters very little which definition of the length of a year one chooses. it could be just 31.6 megaseconds. It need not be measured by counting revolutions of the Earth around the Sun. If and when these distances are measured with sufficient precision, the light year will need to be defined with greater care. It is hoped that, before that happens, astronomers will have abandoned the light-year entirely in favor of units like terametres: 1 light year equals about 10 terametres (10 Tm). > Can there be an unit of time which is independent of > any celestial object. Yes, of course there can be (and is). SI time (the second) is based on the oscillations of the caesium atom (specifically the isotope 133). All caesium-133 atoms in the universe oscillate the same way. > Also how do the people in space > station calculate time when they get light all the > time. People in space stations don't "calculate" time, the measure it. (And what does "getting light all the time" have to do with it?) People on space craft measure time with atomic clocks based on (or synchronized with) caesium clocks. If they don't carry a caesium clock along (or some other type of clock that is sufficiently accurate for the length of the mission), they can keep synchronized with caesium clocks on Earth because they are constantly in contact with Earth. (For missions involving the great distance to other planets, such synchronization is complicated by the time delay of any signal between Earth and the space craft, but that delay is well understood and is easily compensated for.) Regards, Bill Hooper retired physics professor, Florida, USA -------------------------------------- "Simplification" begins with "SI" --------------------------------------
