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Recently I inherited quite a few old QuickBasic programs which perform various astronomical calculations. I'm attempting to understand these programs and rewrite some of them in Python. I do not have a deep background in astronomy.

A number of the programs take a parameter file as input, YEAR.DAT. Below are 5 years of these files (each column represents one file). I need help in figuring out the various data values.

YEAR.DAT

year        2001         2008         2009         2010         2011 
delta t      66           65           66           66           67 
tilt         23.43909     23.43818     23.43805     23.43799     23.43786 
dow           1            2            4            5            6 
gst           6.71430      6.66860      6.71839      6.702509     6.68659 
x1          105.690      330.340      310.959      291.631      272.303 
bs           84           90           88           87           86 
fs          301          300          298          304          303 
x2          357.765      356.959      357.689      357.433      357.177 
x3          354.289      193.159      335.720      105.105      234.489 
jd      2451910.5    2454466.5    2454832.5    2455197.5    2455562.5 

I believe that all the values which are time dependent are for 0:00 hours on Jan. 1 of the year given.

Here are the values I think I've figured out:

tilt is the obilquity of the ecliptic
dow is the day of the week, where Monday is day 1
bs is the number of the day of the year when British Summer Time (BST) begins
fs is the number of the day of the year when BST ends
jd is the Julian day number (of 0:00 hours Jan. 1)

Values I'm unsure about:

delta t is some sort of time delta, but I don't know what
gst seems to be Greenwich Mean Sidereal Time, but for what moment?
x1, x2, and x3 I'm clueless about

Here are my questions:

  1. What might delta t be?
  2. Is gst in fact Greenwich Mean Sidereal Time? For what moment?
  3. What are x1, x2, and x3? (This is low-priority question.)
  4. How can delta t, gst, and, perhaps other values, be determined for 2018, 2019, ...?

Any help will be greatly appreciated.

Roger House

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    $\begingroup$ Could you give an example of what the programs calculate? Maybe that might help. Alternatively, do you have the BASIC listings, do they offer any clues? $\endgroup$ – adrianmcmenamin Nov 6 '16 at 19:53
  • $\begingroup$ You're asking us to debug a program without even giving us the code. Have some mercy! $\endgroup$ – Carl Witthoft Nov 7 '16 at 13:39
  • $\begingroup$ Mostly guessing: delta t is the number of elapsed leap seconds (although the values you have seem incorrect-- it should always increase over time), gst is probably GMST at the start of the year or 12 hours earlier (see aa.usno.navy.mil/faq/docs/GAST.php for details), x2 appears to be the Earth's degree position compared to perihelion (which occurs shortly after the new year), clueless on the other two but one may have something to do with the moon's position. $\endgroup$ – barrycarter Nov 7 '16 at 16:44
  • $\begingroup$ I feel your pain. I can't tell you how many times I've scratched my head while pouring over decades old programs with variable names like xi, jas, and rp2. Your best bet is to see how they're used and try to glean the information from context. You might also try changing some values and see how it affects the output. It's hard for us to determine these without the full code. $\endgroup$ – zephyr Nov 7 '16 at 17:44
  • $\begingroup$ For delta T perhaps: eclipse.gsfc.nasa.gov/SEhelp/deltaT.html, Since the Earths axis tilt is 23.5 deg you might look at these numbers on context of the Earth. $\endgroup$ – scrappedcola Nov 8 '16 at 15:53

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