On Mon, 10 Dec 2018 13:19:53 -0000 (UTC), Mike Collins
wrote:
I find it hard to format my brain in minutes and seconds of arc. I
think of
degrees and decimal degrees.

Do you feel the same about time? So you use hours and decimals of
hours instead of hours, minutes and seconds? "I'll see you at 9.835"
- such a statement would be wilder most people...

When I wrote planetarium software in the 80s I used decimals and
only
converted to minutes and seconds for the final display.

That's natural. You want to use one unit instead of mixing different
units internally in the software. For angles that unit could be
degrees. Or radians, so the built-in trig functions work without any
need for unit conversion. For time, hours could be that unit. Or,
perhaps even better, days counted from some reference date. All with
fractions to full machine precision of course. For display purposes
you convert angles to whatever you want: degrees with decimals, or
degrees and minutes with decimals, or degrees, minutes and seconds
perhaps with decimals. The day count is converted to the calendar
date followed by hours, minutes and seconds. If there's any input,
the opposite conversion needs to be done.