Last updated: 15 July 2003

Clock & Calendar settings

Sent:	Sunday, July 13, 2003 18:16:23
From: (Marvin C. Sumner)
I've seen a few notes in Mike's pages that suggest some folks don't
understand this subject, so here is a little tutorial:

Realize that as computers in telescopes and other places calculate the
positions of celestial objects (stars, planets, satellites, etc.) the
date & time reference must be Greenwich Mean Time (GMT).  Thats because
latitude & longitude and celestial time-keeping (and other astronomy
factors) start there.

Now, most folks have clocks and calendars that can be quite accurate
enough for our needs, but they read LOCAL time (the time that you get
up, feed the chickens and go to work).  Your calendar flips to the next
day at LOCAL midnight.  Also, your clock includes the use or non-use of
daylight savings time (DST).

The folks at Meade know all this, so they have the Autostar asking for
LOCAL date, time, time-zone and whether or not  DST is in use.  The
date, time and DST are entered at start-up and the Time-Zone is included
in the description of the observing site.

The Autostar will use your input data to convert local time/date to GMT,
then proceed with its calculations.  Remember that when folks in North
America (and the rest of the Western hemisphere) are setting up for an
evenings observing, its already the next day GMT (in England).  The
scope's computer will do the time conversion math first, then if the
answer is greater than 24 hours (military time) it will add ONE to the
date and subtract 24 from the time.  (Month and year turn-overs
complicate it a bit).

The bottom line: Don't try to out-guess the Autostar's time-keeping
system.  Give it the parameters it needs as accurately as you can &
"turn it loose".

Marv Sumner

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