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On 9/16/2014 2:01 PM, Samuel L wrote:
> I'm beginning to understand it a little bit assuming that the
> Constellation of Aries is the general equivalent to the Greenwich
> meridian. Now if that's the case then I can figure it out.
>
> That something is located at/near/on the Vernal equinox might as well be
> anywhere in the universe to someone that doesn't know what to look for.
> Here's how I see things- this is September. Spring is a long time ago
> and also in the future. So it's impossible to locate something at this
> date based on that time....which I'm sure it's not.
>
Explanation from a Dummy:

As a starting point, Frank mentioned "precession." Google that.
Practically speaking where the north pole points star-wise changes over
time, with a 26,000 year cycle. Currently Polaris is the pole star in
the northern hemisphere. The pole does not point directly at it (note it
has declination greater less than 90 so it circles the pole) but it will
come closer to the pole (declination decreasing) over the next century
or so. Then it will drift away. 3,000 years ago it was not our pole star.

In essence the line for the equinox did pass through Aries at some point
in time thousands of years ago, but due to precession it does not now.
Back in the day (way back) when the man-made construct of degrees,
minutes and seconds (sexagesimal system) used for latitude and longitude
came into being, it did. The First Point of Aries became the landmark
for SHA and RA, which as others noted is almost as arbitrary as the
Greenwich meridian. It is, a least, and observable celestial event.
Greenwich vs Paris? Political event. ;-)

http://en.wikipedia.org/wiki/First_Point_of_Aries

With that landmark, we can locate a star at any given point in time.
Given other minor variables, the SHA of a star remains almost constant.
(Look at the star tables, pages 269-273 in the almanac, and you will see
a slight change in SHA and declination over the year). Much like the
Sun's GHA, GHA Aries changes widely on a daily basis. Simply put, GHA
Aries plus SHA star equals GHA star.

You might also notice the almanac gives the SHA of planets in the lower
right of the left daily pages, then wraps it up neatly with GHA Aries to
give you an hourly GHA for the planets on the left page. As Jonathan
pointed out, too much paper to do each star.

AS a beginner I was confused by SHA and RA being given either in time or
degrees and seemingly moving in opposite directions. (Hours are
convenient for astronomers.) I'll let you do your homework on that one.

http://en.wikipedia.org/wiki/Right_ascension
http://en.wikipedia.org/wiki/Hour_angle

   

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