Expert: Courtney Seligman - 10/18/2010

Question
Hi.  I am doing a research project that involves ancient Egypt and Greece.  In my
Starry Night program it indicates that beta and alpha Kentaurus are visible from
Athens for a few days per year.  Other research sources indicate that they are
not.  Can you please tell me which is correct because perhaps the other sources
are referring to current visibility, not visibility in ancient times.  Thanks!  Erin

Answer
The stars are not visible from Athens at this time. Athens is nearly 38 degrees north of the Equator, and as a result, Athenians cannot see stars that are less than 38 degrees from the South Celestial Pole. Alpha and Beta Centuari are less than 30 degrees from the Pole, so they would always be more than 8 degrees below the southern horizon.

However, as you suspected, the precession of the Earth's axis of rotation can change this. In the last five hundred years the stars have moved two degrees closer to the South Celestial Pole, so in the 2000-plus years since Hellenic times they could have moved eight or more degrees lower in the Athenian sky. So they probably were (barely) visible in Athens in ancient times; but they are no longer visible from that location.

Of course, since southern Egypt is considerably to the south, ancient Athenians would have known of the stars' existence even if they had never been visible from there. But given the way you put your question, I presume you wanted to know if they could be directly observed.

Unfortunately, the precession calculator I use is only good for the last five hundred years, and I don't have Starry Night. But you might try setting Athens as your location, and setting the time so that Centaurus is on "your" southern horizon. That should show that the southern part of the constellation is not visible in the current era. Then try resetting the date to -500. If that doesn't cause an error message, it should allow you to see what the sky would have looked like to ancient Athenians.

(NOTE ADDED AFTER ORIGINAL POST)
I have installed Stellarium software on this computer, to allow me to directly check the positions of the stars. Resetting the year is not as easy as it should be, but allowed me to see that in 500 BCE, Hadar (Beta Centauri) would have been easily visible from Athens, and Rigel Kentaurus (Alpha Centauri), though a little farther south, would have scraped above the southern horizon. Unfortunately, the Stellarium software does not show the southern horizon correctly, and unless the "ground" view is turned off, hides the stars. So it occurs to me that Starry Night might have a similar problem with visibility, which would prevent you from directly seeing that the stars were observable in ancient times. Hence this addition.