M22, a globular cluster in Sagittarius

Image by Chris Lasley
This was probably the first globular cluster discovered, by Abraham Ihle in 1665. According to Kenneth Glyn Jones, it is supposed (e.g. by Admiral Smyth) that Hevelius may have seen it even earlier, but Halley, De Chéseaux and Messier commonly acknowledge Ihle's original discovery. This globular was included in Halley's list of 6 objects published 1715, and observed by De Chéseaux (his No. 17) and Le Gentil as well as by Abbe Nicholas Louis de la Caille, who included it in his catalog of southern objects as Lacaille I.12. Charles Messier, who cataloged M22 on June 5, 1764, states that it is also included in John Bevis' English Atlas.

M22 is a very remarkable object. At 10,400 light years, it is one of the nearer globular clusters. At this distance, its 29' angular diameter, about the same apparent diameter as that of the Full Moon, corresponds to a linear of about 88 light years. It is visible to the naked eye for observers at not too northern latitudes, as it is brighter than the Hercules globular cluster M13 and outshined only by the two bright southern globulars (not in Messier's catalog), Omega Centauri (NGC 5139) and 47 Tucanae (NGC 104) - this is the ranking of the four brightest in the sky.