Messier 21

Discovered by Charles Messier on June 5, 1764.

Messier: M21.

June 5, 1764. 21. 17h 50m 07s (267d 31' 35") -22d 31' 25"
Star cluster, near the preceding [M20]: The nearest neighboring known star to these two clusters is 11 Sagittarii, 7 mag, according to Flamsteed. The stars of both these clusters are of 8-9 magnitude, enveloped in nebulosity.

[Mem. Acad. for 1771, p. 443 (first Messier catalog)]
In the same night [June 5 to 6, 1764] I have determined the position of two clusters of stars which are close to each other, a bit above the Ecliptic, between the bow of Sagittarius & the right foot of Ophiuchus: the known star closest to these two clusters is the 11th of the constellation Sagittarius, of seventh magnitude, after the catalog of Flamsteed: the stars of these clusters are, from the eighth to the nineth magnitude, environed with nebulosities. I have determined their positions. The right ascension of the first cluster, 267d 4' 5", its declination 22d 59' 10" south. The right ascension of the second, 267d 31' 35"; its declination, 22d 31' 25" south.
[p. 456] 1764.Jun.5. RA: 267.31.35, Dec: 22.31.25.A. Cluster of stars near the preceding.

Bode: Bode 50.

A star cluster.

William Herschel

[Unpublished Observations of Messier's Nebulae and Clusters. Scientific Papers, Vol. 2, p. 652]
1786, May 26 (Sw. 556). A rich cluster of large [bright] stars.

John Herschel (1833): h 1993.

h 1993 = M21.
Sweep 275 (July 28, 1830)
RA 17h 54m 25s +/-, NPD 112d 30m 2s (1830.0) [Right Ascension and North Polar Distance]
A tolarably rich, sc, coarse cl; one star 9m, the rest 10....12
A tolarably rich, scattered, coarse cluster; one star of 9th magnitude, the rest of 10th to 12th magnitude.

Smyth: DCXXXII [632]. M21.

DCXXXII. 21 M. Sagittarii.
AR 17h 55m 01s, Dec S 22d 30'.6
Mean Epoch of Observation: 1835.55 [Jul 1835]
A coarse cluster of telescopic stars, in a rich gathering galaxy region, near the upper part of the Archer's bow; and about the middle is the conspicuous pair above registered, - A being 9, yellowish, and B 10, ash coloured. This was discovered by Messier in 1764, who seems to have included some bright outliers in his description, and what he mentions as nebulosity, must have been the grouping of the minute stars in view. Though this was in the power of the meridian instruments, its mean apparent place was obtained by differentiation from Mu Sagittarii, the bright star about 2deg 1/4 to the north-east of it.

John Herschel, General Catalogue: GC 4367.

GC 4367 = h 1993 = M21.
RA 17h 56m 13.8s, NPD 112d 30' 8.6" (1860.0) [Right Ascension and North Polar Distance]
Cl; pRi; lC; st 9...12 2 observations by W. & J. Herschel.
Cluster; pretty rich; little compressed; stars from 9th to 12th magnitude.

Dreyer: NGC 6531.

NGC 6531 = GC 4367 = h 1993; M 21.
RA 17h 56m 14s, NPD 112d 30.1' (1860.0) [Right Ascension and North Polar Distance]
Cl, pRi, lC, st 9...12; = M21
Cluster, pretty rich, little compressed, stars from 9th to 12th magnitude.