glenc
24-08-2006, 07:08 AM
Who Discovered M69?
It seems that Messier was the first to see it, but he didn’t know it.
Messier thought that Lacaille discovered it.
On August 31, 1780 Charles Messier described M69 as a:
`Nebula without star, in Sagittarius, below his left arm & near the arc; near it is a star of 9th magnitude; its light is very faint, one can only see it under good weather, & the least light employed to illuminate the micrometer wires makes it disappear: its position has been determined from Epsilon Sagittarii: this nebula has been observed by M. de La Caille, & reported in his Catalogue; it resembles the nucleus of a small Comet.' (diameter 2')
Messier thought that M69 was the same as Lacaille I.11 but this is unlikely for four reasons.
1. There is an obvious asterism at the position given for Lac 1.11 and it similar to other asterisms in Lacaille’s catalogue. (Lac I.13, III.9 and III.14.)
2. Lacaille’s positions are usually fairly accurate but M69 is 1.18d (or 70’) from L I.11 in pa 14d, and Lacaille does not make large errors like that. The largest error he made was 10 mins of time or 43 arc mins in RA with 47 Tuc, and that seems to be a typographical error. It is most unlikely that he would make a large error in both declination and RA at the same time.
3. M69 is too faint for Lacaille at magnitude 7.6. The faintest globular in his catalogue, apart from M69, is magnitude 6.9. He missed nine GCs brighter than magnitude 6.8, so it is unlikely he would include one that is magnitude 7.6.
4. M69 and L I.11 have different NGC numbers (NGC 6637 and 6634) because John Herschel listed them separately.
It seems that Messier was the first to see it, but he didn’t know it.
Messier thought that Lacaille discovered it.
On August 31, 1780 Charles Messier described M69 as a:
`Nebula without star, in Sagittarius, below his left arm & near the arc; near it is a star of 9th magnitude; its light is very faint, one can only see it under good weather, & the least light employed to illuminate the micrometer wires makes it disappear: its position has been determined from Epsilon Sagittarii: this nebula has been observed by M. de La Caille, & reported in his Catalogue; it resembles the nucleus of a small Comet.' (diameter 2')
Messier thought that M69 was the same as Lacaille I.11 but this is unlikely for four reasons.
1. There is an obvious asterism at the position given for Lac 1.11 and it similar to other asterisms in Lacaille’s catalogue. (Lac I.13, III.9 and III.14.)
2. Lacaille’s positions are usually fairly accurate but M69 is 1.18d (or 70’) from L I.11 in pa 14d, and Lacaille does not make large errors like that. The largest error he made was 10 mins of time or 43 arc mins in RA with 47 Tuc, and that seems to be a typographical error. It is most unlikely that he would make a large error in both declination and RA at the same time.
3. M69 is too faint for Lacaille at magnitude 7.6. The faintest globular in his catalogue, apart from M69, is magnitude 6.9. He missed nine GCs brighter than magnitude 6.8, so it is unlikely he would include one that is magnitude 7.6.
4. M69 and L I.11 have different NGC numbers (NGC 6637 and 6634) because John Herschel listed them separately.