Finding a location geographically of a celestial object, in this case the moon at one of its quarters Years ago it was mentioned to me the New Yorker magazine had a Young Moon “contest”; who could find the youngest moon; the thinnest crescent after a new moon. I knew enough to know that I would have to know exactly when the new moon occurred and more importantly where. The Thin Crescent wouldn’t be too hard as it would follow closely the sun. Still, not knowing where the new moon happened meant that I could see anything from nothing to a one day old moon. I knew the moon traversed about 13 degrees of arc a day in so doing about 7% Of its disc was illuminated so my chance of seeing a very thin crescent depended on the new moon happening to the east of me by some 6 to 12 hours, as the “record” was something in the realm of 9hours.

So the question arose in me “Where on the globe would an observer see any of the quarterly phases exactly over their meridian?” This question led to almanacs that used hours, minutes, seconds and hundredths of seconds of measurements, all starting from the celestial spheres Greenwich or zero longitude, the Vernal Equinox, that semi-fixed(a 26,000 year cycle) starting place for celestial longitude. My question is, how can I find and use that “exact” beginning zero “longitude?

I am being hobbled here by being forced to use the very nomenclature I don’t understand. I mean I understand it, it’s simple enough, I just don’t know where to start. How do I find right ascension 18h23m52.34s on the celestial sphere? Is there a bright star at the vernal equinox that is used to fix that spot in the sky?

I begin to think I’m communicating with a machine that can’t extrapolate...no matter, as long as I get an education. Ok then here’s a question: how can I find right ascention(ra)18h 23m 52s standing out in the desert some night? I don’t need declination as the object is large and I’ll know it when I see it.
How do I find right ascension 18h23m52.34s on the celestial sphere?


You use a star chart that shows the lines of right ascension and declination. Based on the stars that the line passes by, you can estimate the line in the real sky. It is similar to looking at a map of the Earth and using latitude and longitude to locate a city. (By the way, 0 hours right ascension, 0 degrees declination is the location of the vernal equinox on the celestial sphere. The bright star at that location is the Sun at the moment of the vernal equinox ;-)

Perhaps your question should be "How do I locate the young crescent Moon?". The answer to that question is that it would be better to calculate the azimuth of the Sun at sunset, and the altitude and azimuth of the Moon at 10 minute intervals afterwards. Based on knowing where the Sun was located on the horizon at sunset and the difference between the Moon and Sun, you can estimate where the Moon should be in the sky. There is no need to know where right ascension and declination are located among the stars (other than calculating the position of the Sun and Moon); the sky will be bright, so it is unlikely that there will be any stars visible that will help you locate the line of right ascension.