On December 21, 2018 at 2:23pm PST I want to be standing as directly under the sun as humanly possible. Obviously the latitude of that point will be the Tropic of Capricorn. Assuming I have the right ascension for the sun at that time how would I know what line of longitude to stand on? Please use simple non-technical terms if possible.
Easiest is to find calculators on the web for it.
I used NASA's WebGeocalc. It's not the easiest interface, but it generated for me:
2018-12-21 22:23:00.000000 UTC 203.80673189 -23.43541272
That's in degrees east and degrees north. The figure of -23.4 looks pretty good for the correct latitude. Also, you know that it's a couple hours after noon on the US west coast, so the longitude of 204E (156W) is also reasonable.
Actually, much easier is timeanddate.com page on sun position. Gives a nice graphic and it's easily reached from their page on the solstice.
Unfortunately both agree that the position is in the Pacific ocean, in the vicinity of some small islands. Not the easiest travel destination.
In case someone stumbles across this question and wants to know the math here's what I've been able to figure out.
All right ascensions(RA) are distances, in time, east of the vernal equinox given in hours, minutes, and seconds. The location of the vernal equinox depends on the standard used when giving the RA. For instance J2000 uses the location of the equinox at noon on Januar 1, 2000. The equinox is the imaginary line formed by the intersection of the equatorial plane and the ecliptic plane on the side of the planet where the sun was moving from below the equator to above it.
To begin finding the longitude of an object you need to compute the GMST or GAST for the date and time the measurement was given. You then subtract the GMST from the RA and multiply the result by 15 to get the longitude of the object.