NavList:

David P, you wrote:
"Am I correct in thinking RA in degrees plus SHA in degrees = 360degrees, and SHA in hours, minutes, and seconds using the 'Arc to Time' table plus RA = 24hours?"

Yes, that's exactly right. As angles, SHA and RA are two pieces of the same full lap around the sky. So:
   SHA = 360° - RA, and
   RA = 360° - SHA,
and, yes, if you want RA in traditional "time" units, it's converted at exactly 15° per hour.

Your example: "if SHA star is 90degrees, RA star is 18hours"

That's exactly right. The official [modern Nautical Almanac listed] navigation star Eltanin, in the head of Draco, not too far from Vega, has RA equal to 269° to the nearest whole degree. We can also write that angle in hours and minutes: 270° would be exactly 18 hours and every single degree is four minutes of time, so 269° yields 17h56m. Same angle, different way of writing it. And we can subtract from 360 to get the SHA, which is 91°.

Details: those coordinates, 269° for RA and 91° for SHA, are the standard in astronomy (year 2000 epoch and equinox) given to the nearest degree, but navigation requires coordinates "of date" (in other words, how do they map to the globe today accounting for precession and other factors), and we need more digits, of course. Adding a couple of digits past the decimal point, the SHA of Eltanin in year 2000 coordinates is/was 90.85°. Today's SHA for Eltanin is 90.71°. 

In my recent navigation workshops (within the past 3-5 years), I try to mention the connection between RA and SHA explicitly. Right ascension has no real place in celestial navigation, but for anyone interested in astronomy, even as a little hobby, RA is everywhere. Modern star-finding apps will usually display RA. So the simple "360 minus" relationship, and the fact that they are two different expressions of the same geometric position on the sky, are worth understanding. Both SHA and RA are "sky longitudes" measured from the same "sky prime meridian", which we call Aries.

The only real difference between SHA and RA is that one increases toward the left of Aries as seen in the northern hemisphere, and the other increases toward the right. Which one is which?? Pick a favorite star and memorize it at least crudely. Betelgeuse is great for this! Its RA is nearly 6h, equivalent to 90°, while its SHA is nearly 270°. You also need to know that you can see Betelgeuse and Aries in the sky together at the same time with Aries on the right (northern hemisphere) and Betelgeuse on the left. That separation from Aries to Betelgeuse is increasing towards the left. SHA goes the other way around.

Frank Reed