NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Right Ascension.
From: Bruce Stark
Date: 2005 Aug 26, 20:49 EDT
From: Bruce Stark
Date: 2005 Aug 26, 20:49 EDT
Right ascension is the order in which the sun, moon, planets, and stars "ascend" to the celestial meridian. "Right" was used here to mean "correct" or "true," as when a straight line was called a "right line." That's my notion, anyway.
If you know the RAs of ten or twelve stars, separated from each other by two hours or so, and are familiar with the sky around them, you'll have a pretty good idea what's overhead, day or night, any time of year. This can be handy if you're out under the night sky for the first time in months, and drifting clouds obscure parts of constellations, so that nothing you can see makes sense.
The sun's RA is zero March 20, and increases about an hour every half-month. For instance, it's been about five months since March 20, so the sun's RA should be roughly ten hours. Adding the time SINCE YOUR LAST NOON to the sun's RA gives the RA of your meridian. Drop twenty-four hours, if it's more than that.
It's about five PM here. Knocking off an hour for daylight time makes it four hours since noon. Adding the sun's RA tells me that the RA of this meridian is about fifteen hours. Arcturus, RA about fourteen and a quarter hours, and declination about nineteen and a quarter degrees north, should be overhead. About twenty-five degrees south of me and probably a little west of south. Can't see the star, since it's daylight, but it's there.
Bruce
If you know the RAs of ten or twelve stars, separated from each other by two hours or so, and are familiar with the sky around them, you'll have a pretty good idea what's overhead, day or night, any time of year. This can be handy if you're out under the night sky for the first time in months, and drifting clouds obscure parts of constellations, so that nothing you can see makes sense.
The sun's RA is zero March 20, and increases about an hour every half-month. For instance, it's been about five months since March 20, so the sun's RA should be roughly ten hours. Adding the time SINCE YOUR LAST NOON to the sun's RA gives the RA of your meridian. Drop twenty-four hours, if it's more than that.
It's about five PM here. Knocking off an hour for daylight time makes it four hours since noon. Adding the sun's RA tells me that the RA of this meridian is about fifteen hours. Arcturus, RA about fourteen and a quarter hours, and declination about nineteen and a quarter degrees north, should be overhead. About twenty-five degrees south of me and probably a little west of south. Can't see the star, since it's daylight, but it's there.
Bruce
