Astronomy/Expert Profile

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I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.

Experience in the area

I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.


Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)


I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors

(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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    K = Knowledgeability    C = Clarity of Response    P = Politeness
Jonathan02/01/17101010Thank you for such prompt, clear and .....
Apoorv09/04/16101010Thank you thank you. Thank you.....for .....
Andrew08/31/16101010Very nice. The link also really helped .....

Recent Answers from Courtney Seligman

2017-02-16 A Pure-Energy Universe?:

Since the total mass of the Universe is only a fraction of the so-called "critical mass", converting all of it to energy would simply add to the existing Cosmic Background Radiation. It would have no significant

2017-02-09 Rates of Rotational and Revolution of Planets:

The 1/20th of a second in 5000 years is a mistake (sorry). The actual amount is about 2 * 50 = 100 milliseconds, or about 1/10th of a second (or more accurately, since it is about 1.7 instead of 2 msec/day/century

2017-02-09 Rates of Rotational and Revolution of Planets:

I don't know whether any consideration has been given to what to do when the rotation rate becomes so slow that there is a significant fraction of a second error in the length of the day; and if there

2017-02-09 Rates of Rotational and Revolution of Planets:

I'm glad that you found the discussion of the Rotation of Mercury useful and interesting; but your remaining questions indicate that you didn't run across Rotation Period and Day Length (at http://cseligman

2017-02-01 Retrograde of Venus:

The beginning and ending of Venus' retrograde motion CAN be seen with the "naked" eye, but due to being in "crescent" phase at the time it passes between the Earth and Sun, it is not as bright as during


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