[AstroPy] Rise/set/transit times

Nordgren, Bryce L -FS bnordgren at fs.fed.us
Fri Oct 17 19:45:17 EDT 2014

I made an Ipython notebook to walk through Example 15a in "Astronomical Algorithms", 2nd ed. [1] This was a debugging tool for me, and I hope it's interesting to you too. I try to do things one step at a time and compare against the "book answer" at each stage.

The process does expose some ambiguities in the Astronomical Algorithms text, at least to a non-astronomer like me. This mostly involves what units should be assigned to the arbitrary constants. Could I pick your brains on this topic?

Assuming you said "yes", there are a number of places where Meeus converts between fractional days and degrees. I've been trying to make his "fractional day" variables work out to "u.sday", converting to degrees where instructed by multiplying by 360 u.deg/u.sday. All is well and good until we hit "input block 21" on my notebook. He then wants to multiply his fractional day variable by 360.985647. I tried assigning the units u.deg/u.day, but then my sidereal time (theta0 in notebook) is off by a degree.

I'm guessing that as long as theta0 is OK, all the subsequent calculations will be OK and the unit system will convert hourangle to real time for me. However, this seems an awful lot like subverting the unit system. Anyone have any ideas what Meeus is doing, and maybe how I could express it better using astropy.units?


[1] http://nbviewer.ipython.org/github/firelab/met_utils/blob/master/tests/ex15a.ipynb

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