Degree of difficulty 2 (of 5)

Minimum aperture Naked eye

Designation NGC 2422

Type Open cluster

Class III2m

Distance 1600 ly (K2005) 1620 ly (Hipparcos, 1999)

Size 14 ly

Constellation Puppis

R.A. 7h 36.6min

Decl. –14° 29′

Magnitude 4.4

Surface brightness –

Apparent diameter 30′

Discoverer Hodierna, 1654

History M 47 was discovered before 1654 by the Sicilian astronomer Giovanni Batista Hodierna. He described his find plainly as “a nebulosa between the two dogs.” Hodierna's observations were forgotten until the booklet with his observing notes was rediscovered in 1984.

Hence, Charles Messier made an independent discovery of M 47 on the 19th of February 1771. He noted that night: “Star cluster, not far from previous [M 46]. The stars are larger; the cluster does not contain nebulosity.” In the calculation of the position, however, Messier made a sign-related mistake and, consequently, M 47 became a missing object for many observers after him. Oswald Thomas finally identified M 47 with NGC 2422 in the year 1934 – quite possibly, by some good luck. It was left to T.F. Morris in 1959 to fully resolve this case by going through Messier's calculations and finding his mistake. An interesting catch is that the missing M 47, the “ghost object,” received an NGC number of its own: 2478.

William Herschel did not know of all this, when he independently discovered M 47 on the 4th of February 1785. His son John later commented on this cluster: “a large, pretty rich, straggling cluster.” Smyth saw “a very splendid field of large and small stars, disposed somewhat in lozenge shape,” and he also noticed the brightest (7th magnitude) cluster star with a companion at about 20” distance. Reverend Webb commented on M 47: “grand broad group visible to naked eye, too large even for 64×.”

Astrophysics The open cluster M 47 has a visual appearance which differs strikingly from M46, its apparent neighbor in the sky, which is actually three times as distant.