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From: wyatt@cfa.UUCP (Bill Wyatt)
Newsgroups: net.astro
Subject: Re: Re: January Comet Ephemerides.
Message-ID: <160@cfa.UUCP>
Date: Tue, 31-Dec-85 10:14:50 EST
Article-I.D.: cfa.160
Posted: Tue Dec 31 10:14:50 1985
Date-Received: Thu, 2-Jan-86 04:40:05 EST
References: <1207@gitpyr.UUCP> <1865@teddy.UUCP>
Organization: Harvard-Smithsonian Center for Astrophysics
Lines: 61

> > COMET EPHEMERIS FOR ATLANTA
> > FROM 06:28 PM EST, WEDNESDAY, 01 JANUARY, 1986 UNTIL 06:53 AM EST
> >  
> >  TIME(EST)    AZ   EL    RANGE   LHA   RA/1950 D/1950 HDG  MAG 
> > 07:00:00 PM 237.4 35.4   01.18 02:53.8 22:12.4 -02D49 248 +04.6
> >   HALLEY'S COMET                                 D/D=0.55
.....
> 
> Could someone explain where to look in the sky to find these coordinates.
> Thanks.
> -- 
> 	Jim Galbiati,  GenRad Inc,  Production Test Division

Since AZ (azimuth) and EL (elevation or altitude) are given, these are
the easiest to use. Zero degrees azimuth is due north, increasing to the
east (= 90).  Since 270 degrees is due west, 237.4 degrees is WSW more
or less. Elevation is just that: the angle above the horizon at the
given azimuth, in this case 35 degrees. As the earth rotates, these
coordinates will change rather quickly, so they are valid only at the
given time and place (see discussion of RA and Dec, below).  BTW,
astronomers used to (something like longer than 30 years ago, I think)
measure azimuth from the south, not the north.

Right ascension and declination (the RA/1950 and D/1950 in the line
above) refer to a coordinate on the celestial sphere and are analogous
to longitude and latitude. The zero point of declination is the
celestial equator, and is measured in + and - degrees, just as for
latitude. For RA, the zero point is the `first point of Aries', and is
measured in hours of time (1 hour = 15 degrees) and is measured
increasing to the east, so that stars passing overhead later in the
night have a greater RA that those that passed overhead earlier. (Except
that 24 hours wraps around to 0 hours again). At one time the zero point
was in Aries. Now it's in Pices, heading for, and nearly into Aquarius,
hence `Age of Aquarius'.

For stars, RA and dec do not change, to first approximation. The RA and
dec coordinate system is apparently tilted relative to the Az/Alt system
for any observer not at the north pole, so the Az/Alt coordinates change
quickly as a star rises.  Precession of the earth's pole (a 17,000 year
cycle) does alter the coordinate system, however, so all coordinates are
given as they were in 1950.0, also known as the epoch.  Some
observatories are now converting to 2000.0 coordinates, and everybody
plans to sometime.

Solar system objects, such as comets, are so nearby as to have an
appreciable angular speed relative to the earth, so their RA and Dec
change also, albeit more slowly than Az/Alt. For this reason, the RA and
Dec given above are also only good for the time stated.

For a quick sky reference, the westernmost star in Cassiopea is near 0
hours, so a line between it and the pole star can be extended to define
the zero point. The Great Square of Pegasus (which Halley was
near for a while) is close to one hour or 15 degrees on a side, and the
two easternmost stars are also near 0 hours (thus extending the line).
Last, the top star in Orion's belt is on the celestial equator. Remember
that the lines of RA converge to the north pole, just like longitude,
and so are curved relative to declination.

-- 
Bill    UUCP:  {harvard,genrad,allegra,ihnp4}!wjh12!cfa!wyatt
Wyatt   ARPA:  wyatt%cfa.UUCP@harvard.ARPA