Harold Corwin

IC 1012 = IC 4431. This is another of the nebulae found by Safford in 1866 using the 18.5-inch Clark refractor at Dearborn Observatory (while testing this telescope, Alvan Clark found Sirius B). Safford's position is +2 arcmin off in declination, and +4 seconds of time off in RA -- not too bad, especially given that there are no other nebulae nearby.

Javelle found the galaxy 31 years later. When reduced with respect to the Tycho-2 position for his comparison star, his measurement of the galaxy is off in RA by just about a second, and is spot-on in declination. All in all, there is no doubt about the identity. So why are there two IC numbers?

Javelle's published position was reduced from the BD position for his comparison star. The BD position is off by about an arcminute to the south. Safford's position is north of the galaxy, so Dreyer saw a published difference (assuming he checked; I'm sure he did -- he was very careful) of nearly three arcminutes. This was enough that he must have thought that there were two nebulae here.

By the way, this is one of the few nebulae for which Javelle has more than one measurement. His two observations differ by 0.78 seconds of time, and 1.3 arcseconds. This gives an idea of the internal accuracy of his data -- not too bad, but not the best that was done visually using micrometers.

― IC Notes by Harold Corwin

Drawings, descriptions, and CCD photos are copyright Andrew Cooper unless otherwise noted, no usage without permission.

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