A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2014 Sep 16, 11:31 -0700
Sam, you wrote:
"That's what I was afraid you'd say. There has to be a reference point for 0hr. Is the constellation Aries what I should use as a visual 0hr?"
There is a reference point for 0h right ascension (and equivalently for 0° SHA). Of course there is. The concept would be useless without one. This point has been chosen for the convenience of Earth-based positional astronomy and celestial navigation. The reference meridian on the celestial sphere that acts as a sort of astronomical "Greenwich" is known as "Aries" but it is not in the constellation Aries. You can find it easily enough on any star chart. If you want the navigation-based coordinates, you can use the star chart in the Nautical Almanac; look for the line marked 0° SHA. There are thousands of star charts online and there are sky simulation tools like "Stellarium" which can easily show you the coordinates more commonly used by astronomers; look for the line marked 0h RA. Both navigation coordinates, using SHA, and astronomical coordinates, start at the same meridian. Look for 0h RA or 0° SHA. Find that meridian stretching across the sky from the north celestial pole to the south celestial pole. Notice that you don't actually need that single location where the ecliptic crosses the celestial equator. You don't need that single spot any more than you need the exact location where the Greenwich meridian crosses the Earth's equator. All you need is a few familiar points near that "stripe" that runs all the way across the sky from pole to pole. There is a star that's fairly close to the actual zero point, omega Piscium. That's about as good as you can get for finding something right at the fiducial point. It's a small fraction of a degree away from 0° SHA, and it's declination is just under 7° N. The bright star Caph (beta Cas) in Cassiopeia --at the end of the W-- is relatively close to the line, and also the trailing side of the Great Square of Pegasus is relatively close to the line. If you can visualize a line across the sky running from the end of the W of Cassiopeia and passing just inside the Great Square near the trailing side, then you have visualized the zero line of SHA.
Bigger question: why do you need to know? It's nice for visualization sometimes, and it's re-assuring to know when you're learning all this that, yes indeed, there is a very specific meridian on the celestial sphere from which SHA is measured just as there is a specific starting meridian on the Earth for longitudes. But in practice this information is nearly useless. You will get little practical benefit from being able to point out the zero line of SHA/RA. It's good for your comprehension, but it will not make you a better practical navigator.
PS: So why is it called "Aries" if it's not in the constellation Aries? For centuries, astronomers used the zodiac signs, still used in astrology. The "sign" Aries was aligned with the constellation Aries thousands of years ago, but precession has caused them to separate. The sign Aries still has a very specific, well-defined meaning, but it does not coincide with the constellation today. That key "crossing point" that defines the "First Point of Aries" is now located in the constellation Pisces. And as described above, it's just a short distance below that star omega Piscium. So "Aries" is just a name for an arbitrary point in the sky. Ignore the constellation.