Light Years and Julian Years

This note was posted by Duncan Steel to the Calendar Mailing list (CALNDR-L).  I’ve performed minor editing (bullet lists, superscripts...), but have otherwise left it as posted.  It is used here with Duncan’s kind permission.

—Ivan Van Laningham

Subject: Julian Day usage
Date: Tue, 19 Aug 1997 16:27:26 +0900
From: Duncan Steel
Reply-To: East Carolina University Calendar discussion List <CALNDR-L@ECUVM1.BITNET>
To: Multiple recipients of list CALNDR-L <CALNDR-L@ECUVM1.BITNET>

This is a brief note replying to Rick McCarty’s request for information regarding possible civil uses of Julian Days (as an astronomer, of course I am used to using JDs much of the time).

A few years back I was trying to write my own pseudo-random number generator (OK, OK, no smart comments from folk who know the futility of such things) for use in the simulation of noisy data collection. Using a VAX/VMS machine, I was looking at using the computer’s timing system (number of ticks passing, in a multi-user machine, which I would then hit with various trigger functions and MODs and DIVs to get my p-r number). Anyhow, on digging out the time/date data block I saw that the days were numbered, and I thought the number looked familiar. Sure enough, it was the MJD (Modified Julian Day number = JD - 2,400,000.5).

BTW, I have seen people object to using the term Julian DAY in the above sense (although it is the common dictionary - e.g. Webster’s - definition; number of days elapsed since 1 January 4713 BC), preferring Julian DATE; but it seems to me that using ‘Julian Date’ allows a higher probability of confusion with the Julian Calendar, as mentioned by Rick.

On Rick’s other point about the length of the ‘year’ used in calculating a light year, again as an astronomer perhaps I might make a few comments. I’ll break these up for ease, although there is no great significance or ordering to the points, and this is not meant to be a complete list:

I am just really listing off things which one might need to think about, without any great seriousness. It’s just that in any situation one has to consider what is significant, and what not.

In fact I work on solar system objects, as opposed to more distant things. So let’s leave light years alone and think instead of where one might use the length of a ‘year’ in some solar system dynamics. The speed of an object in heliocentric orbit (like that of the Earth, as I gave above) is given by:

V2 = G Msun [(2/a) - (1/r)]


Now in evaluating that one could plug in various constants (like the astronomical unit, AU), but generally it’s easiest to use good old Kepler and recall that he told us that the cube of the period in years divided by the square of the semi-major axis in AU is a constant. My point is that when I do that I habitually use ‘1 year = 365.25 days.’ Any ‘better’ value does not lead to an improvement, as such, for the sort of simple sums that I’m doing. Of course, if I were doing a proper numerical integration of the solar system then things would be different (all double-precision numbers etc.), but not for a few sums on a pocket calculator. Even if I did diligently type in values of G, Msun, and the AU, still I know that the first two at least are not known to better than about six figures, so what the hell. On the other hand, when I calculate an ephemeris for an asteroid, all these things are significant, else I could miss it. It all depends, doesn’t it? For a ‘light-year’, 365.25 is near enough.

And another thing. What about the refractive index of space? It’s not a vacuum. Once I was making some measurements using a VHF radar (26 MHz actually) sited on a shingle (pebble) beach in New Zealand, and after a while I realized that near the ground over this shingle the speed of the radio waves was only 98% of the speed of light (i.e., over the shingle there was a region of ‘refractive index’ in the VHF of about 1.02). What’s it like all along the line to some distant star or galaxy? Interstellar reddening shows that there’s dust there. What else, say plasma-wise, and gas? And of course what does a ‘distance’ mean when light doesn’t travel in straight lines, at least so far as a simple geometer might think of lines.


Kind regards,

Duncan Steel

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