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.

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    $\begingroup$ I've tried to edit to make this more understandable, but it is still very confusing. The moon doesn't have a geographical location, as it is not on the surface of the Earth. It has a location in the sky. There is the location relative to the stars (right ascension and declension) and a position relative to the horizon (altitude and azimuth) and there is the sub-lunar point (the point on the Earth where the moon is directly above) What are you trying to find? Please edit to make your question clear. $\endgroup$
    – James K
    Dec 30, 2017 at 13:02
  • $\begingroup$ astronomy.stackexchange.com/questions/13488 and aa.usno.navy.mil/faq/docs/crescent.php may be helpful. You may also want to google "sublunar point" $\endgroup$
    – user21
    Dec 30, 2017 at 18:13
  • $\begingroup$ Thank you for your edit. I see many different questions: "How do I determine the points on the Earth that have the moon on the meridian at a given time?", "How do I locate a point in the sky given its ra and dec?", "What bright stars are (currently) close to the vernal equinox" These are three different questions. Please pick just one. It's also unclear what you mean by "finding a point on the celestial sphere". Do you want to point a telescope at it, or find it on a chart, or in software. $\endgroup$
    – James K
    Dec 31, 2017 at 0:57
  • $\begingroup$ To see the youngest New Moon you need to look towards the western horizon, just around sunset. It's very hard to see the Moon in broad daylight when it's that close to the Sun. (I'm assuming that solar eclipses don't count). $\endgroup$
    – PM 2Ring
    Dec 31, 2017 at 7:02
  • $\begingroup$ I think you are asking how to find the azimuth given the right ascension, location and time in which case see astronomy.stackexchange.com/questions/2507/… or astronomy.stackexchange.com/questions/8390/… $\endgroup$
    – James K
    Jan 1, 2018 at 18:51

1 Answer 1


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.


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