Which is true? A light year is the distance light travels in vacuum in         - a tropical year         - a sidereal year         - a Julian calendar year         - a Gregorian calendar year         - 365 days?

Which is true? A light year is the distance light travels in vacuum in         - a tropical year         - a sidereal year         - a Julian calendar year         - a Gregorian calendar year         - 365 days? A lightyear = 9.46 x10^12 kilometers, the speed of light is 299,792.458 kilometers/second. You work out the details and get back to us.

Which is true? A light year is the distance light travels in vacuum in         - a tropical year         - a sidereal year         - a Julian calendar year         - a Gregorian calendar year         - 365 days? Well, it's not 365 days anyway.... Otherwise this doesn't matter, for several reasons: 1. Professional astronomers do not use light years.  Instead they    use parsecs, which already is precisely defined.  Light years    are only used in pop astronomy . 2. The relative differences between these possible different light    years is smaller than the relative error in even our best stellar    parallaxes, including the Hipparcos parallax for Aplha Centauri,    the star system closest to our Sun. It won't matter much whether the distance to e.g. Betelgeuze is, say, 502.12345 Julian Light Years or 502.13417 Gregorian Light Years, if we cannot measure this distance any better than, say, +- 50 light years. Right?

Which is true? A light year is the distance light travels in vacuum in         - a tropical year         - a sidereal year         - a Julian calendar year         - a Gregorian calendar year         - 365 days? I don't think it matters; light-years are not in common use in the astronomical community, and astronomical distances aren't usually accurate enough for it to make much difference anyway. The 1996 Jul _Review of particle physics_ gives it as 9.461... Pm, suggesting that the number is not accurate to more than four significant figures.  Since c is exactly 299 792 458 m/s, this means that the year in a light-year is 3.156 x 10^7 s, or 365.3 d.  After rounding to four significant figures, that's consistent with most of these measurements of the year.

Which is true? A light year is the distance light travels in vacuum in         - a tropical year         - a sidereal year         - a Julian calendar year         - a Gregorian calendar year         - 365 days? A lightyear = 9.46 x10^12 kilometers, the speed of light is 299,792.458 kilometers/second. This would imply 1 year = 365.221799 days .....   The 1 ly = 9.46E+12 km should most likely be considered a rounded figure rather than an exact definition....

Paul Schlyter wrote in message <69labd$ This e-mail address is being protected from spam bots, you need JavaScript enabled to view it <snip 2. The relative differences between these possible different light   years is smaller than the relative error in even our best stellar   parallaxes, including the Hipparcos parallax for Aplha Centauri,   the star system closest to our Sun. I thought the closest star is Proxima Centauri. Proxima Centauri is one of the three stars that make up the star system we call Alpha Centauri. (I believe it's still uncertain whether Proxima is actually gravitationally bound to the other two components.) It is slightly closer (about 0.13 ly, or about 0.04 parsec, according to  http://monet.physik.unibas.ch/~schatzer/Alpha-Centauri.html) to us than the other two, much larger components. The system looks like a single star to the naked eye, hence the single name.