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» Online Classroom   »   » Public Discussion of Cel Nav   » GHA of Aries

   
Author Topic: GHA of Aries
OLowry


 - posted August 15, 2007 10:00 AM      Profile for OLowry           Edit/Delete Post 
In all of my readings I keep coming across the term "GHA of Aries" What is so important about Aries or can you give a good definition.
From: Indianapolis, IN
David Burch


 - posted August 15, 2007 11:37 AM      Profile for David Burch           Edit/Delete Post 
Aries is explained in the Glossary. Also a search in the electronic course book for " aries" will find all the discussion of it, which (as far as definitions are concerned) are primarily

-- page 7-1 (which also then directs to the star finder book)

-- glossary (as mentioned above)

-- see also in glossary "ecliptic"

and in the Star Finder Book see page 10.

note the letter space before " aries" in the search is to prevent finding the word varies and so on.

========

in short, Aries is a reference meridian in the sky (pole to pole) equivalent to the Greenwich meridian on earth. The longitudes of all star positions in the almanac are relative to "Aries."

One way to think of where it is in the sky, is the sun crosses aries on the vernal equinox... also in Emergency Navigation we think of it as the meridian defined by Polaris at the pole and Phecda in the cup of the Big Dipper on one side and on the other side it goes roughly through Caph, the leading star of Cassopeia.

From: Starpath, Seattle, WA
OLowry


 - posted August 15, 2007 12:26 PM      Profile for OLowry           Edit/Delete Post 
I read all of those and even more on the internet but still did not understany why it was refernced so much. I guess you had to start mapping stars from somewahere and Aries was/is the starting point from what you said. Thanks
From: Indianapolis, IN
David Burch


 - posted August 15, 2007 01:15 PM      Profile for David Burch           Edit/Delete Post 
Yes. It makes sense on some level as it is a unique meridian, being the intersection of the ecliptic and the celestial equator (projection of earth's equator into the sky).... though in our course we tend to stay on earth as much as possible and rarely mention the "celestial sphere."

... i once was invited to give a talk on cel nav at Kepler College to the inaugural class of future astrologers (yes, that is astrologers), when astrology was first accepted as an accredited degree in the state of WA. During the talk i mentioned that we can completely ignore the celestial sphere as it has no practical application at all and just tends to confuse the subject. To which i was greeted by much snickering and laughter. When i asked what was so funny, they told me they have a 3-hour class devoted to the celestial sphere tomorrow.

From: Starpath, Seattle, WA
DustyDan


 - posted August 24, 2007 08:29 AM      Profile for DustyDan           Edit/Delete Post 
Does the celestial position of Areis at the summer equinox change over time?
From: North Dakota
David Burch


 - posted August 24, 2007 12:50 PM      Profile for David Burch           Edit/Delete Post 
Yes indeed it does, which accounts for why it is called the first point of Aries when in fact it is located in Pisces. It has shifted from Aries to Pisces since it was originally named, i guess in Babylonian times. it moves at a rate of about 1° along a great circle arc in the sky every 70 years due to the precession of the equinoxes.

See:

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

and

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

From: Starpath, Seattle, WA
HHEW


 - posted August 25, 2007 12:56 PM      Profile for HHEW           Edit/Delete Post 
Here's a way to visualize what Aries is:

As you look out at the night sky what you seem to be looking at is the inside surface of an enormous black sphere on which the stars are pasted.

This sphere has an equator located directly above the earth's equator and poles located directly above the earth's North and South poles. The sphere rotates slowly from east to west, carrying all the stars with it.

The reference line for the stars' declinations is, naturally enough, the sphere's equator and that relates directly to latitude on earth. Stars' declinations change extremely slowly. You don't need an hourly tabulation. Once every few days is plenty good enough.

Now, you could list a GHA for each and every star, but just limiting it to the navigational ones would mean 57 additional columns of numbers in the Nautical Almanac.

An alternative way is to pick one star as a longitudinal reference and simpy tabulate the other stars' meridians as such and such degrees and minutes of arc west of the reference star's meridian. Then all you'd need is a column of GHA's for the reference star. You can think of Aries as that reference star.

Actually though, Aries is a point on the equator, the point where the sun crosses the equator at the Spring Equinox. Since it's on the equator, Aries' declination is zero. No need to tabulate that.

The longitudinal coordinate of Aries is listed in the Nautical Almanac column headed GHA Aries and if you look in any almanac around the end of the third week of March, you can find the moment when the sun's declination changes from S to N. At that instant the sun and Aries are over the same latitude and longitude on earth, so GHA Sun and GHA Aries are identical.

Since the stars are virtually stationary on the inner surface of a sphere, once you know your position relative to Aries - i.e. your latitude and LHA Aries - the positiions of all the stars relative to you are given. This fact is the basis for the 2102-D Star Finder and Volume I of Sight Reduction Table 249.



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