Expert: Tom Whiting - 8/1/2010

Question
QUESTION: I have tables for Long & Declination etc but don't know how to convert that to RA.

could I have the Right AScension of New York  and Chicago please

ANSWER: Hi Keith,
Places on the Earth's surface do not have a "celestial" Right Ascension or RA.

I'm afraid you are confused here.  The coordinate system used on the surface of the Earth is
called longitude and latitude.  The coordinate system used up on the sky's celestial sphere (an imaginary dome over our heads) is called Right Ascension and Declination.  The two systems are
not related... except the declination (latitude up on the sky) parallels the surface of the
Earth, because the celestial equator is right over the Earth's equator and the N and S celestial
poles are right over the Earth's geographic poles.  But since the Earth rotates under "the
imaginary dome of the sky), the prime meridian (and all the other longitude numbers on the surface of the Earth)... are continuously moving eastward under the different values of RA in the sky.  So while there is a small parallel correlation between latitude and declination, there is no such correlation between positions of Earthly longitude and a star's Right Ascension, other than both lines run exactly north to south, perpendicular to their respective "latitude" coordinate.
Hope this helps,
Clear Skies,
Tom Whiting
Erie PA USA

---------- FOLLOW-UP ----------

QUESTION: obviously my knowledge is limited, but I thought RA was the combination of  Long and Dec which allowed one to pinpoint a position  with one measurement-- I thought it was Geo because it seems to Retrograde.
how then can one compare  the line of a star with Earth--- as the Earth rotates against the backdrop of the stars, how do we compare the position of the stars with places on Earth -- Declination ?

thanks Keith

Answer
Hi Keith,
There is zero (no) correlation between star positions (RA and Declination) on the celestial sphere, and any location on the surface of the Earth (Latitude and longitude). However, since the stars are fixed elements in our sky, we can use the star positions (and the time and the date) to determine your location on planet Earth using the stars if one is a trained navigator.
(And if you have an accurate clock, a sextant, and a table of star times on such and such a
date over Greenwich, England.)

I'm not trained in that field, but I do understand the basic theory behind it.  Sextant readings of Polaris' altitude above the northern horizon will give you your latitude fairly precisely and immediately. Whatever altitude Polaris is above the northern horizon corresponds to your latitude.  (The south celestial pole in the southern Hemisphere will do the same for south of the equator... the only problem is that there is no star that marks that spot in the sky... you just have to know where it is in the night sky.)
For longitude, Navigators take sightings on a bright star... for simplicity, lets say it's exactly on your meridian (it's highest for the night)... at say, 0600 hours GMT.  And you have a table of stars and times on specific dates that say that same star was on the Prime Meridian over Greenwich, England exactly 5 hours earlier at 0100 hours GMT.  Therefore, your position is 5 hours west of Greenwich, England, and since one hour of time equals 15 degrees of the Earth's rotation, thus you are at exactly 75.0 degrees W. Longitude. (15 degrees x 5 hours).
All this hard work of course, has been superceded by GPS satellites and modern inertial navigation systems, so the old celestial navigation method is becoming a lost art.  Plus, the modern forms of navigation also work in the daytime and cloudy nights too.

In the recent old days, flight navigators would actually sight on 3 bright stars with their sextant, and compute 3 separate LOP's (lines of position) on their maps, and where those 3 lines cross on their maps, that's their location at that instant in time.  In the daytime, they'd take 3 sightings of the sun at different intervals, and this would then give them their 3 LOP's on their maps, thus their position.  But like I said above, the modern day GPS systems and inertial guidence systems, makes all this obsolete now.
Clear Skies,
Tom Whiting
Erie, PA USA