Globular cluster in Sagittarius
M70
A near twin of M69 two and a half degrees away, and the accidental site of a famous discovery: Alan Hale and Thomas Bopp were both looking at this cluster in July 1995 when they independently noticed the comet beside it.
Hale had been observing this cluster from a driveway in New Mexico and Bopp from the desert outside Phoenix, and both noticed the same smudge beside it within hours of each other. The comet went on to be the most widely observed of the twentieth century, and the cluster it was found next to is a middling globular nobody would otherwise mention.
- Catalogue
- M70
- Type
- Globular cluster
- Constellation
- Sagittarius
- Distance
- 29,400 light-years
- Apparent magnitude
- 7.9
- Surface brightness
- 21.0 mag/□″
- Apparent size
- 8′
- Right ascension
- 18h 43m
- Declination
- −32.30°
- Found by
- Charles Messier, 1780
Small enough that its 7.9 magnitudes arrive concentrated rather than smeared: 21.0 per square arcsecond, which is why it stands out from the sky better than its brightness alone suggests. Why surface brightness is the number that matters.
Could you see it?
Straightforward with 10×50 binoculars from Bortle 5.
| Sky | Eye | 10×50 | 80mm | 130mm | 200mm |
|---|---|---|---|---|---|
| Bortle 1 21.9 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 2 21.7 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 3 21.4 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 4 20.9 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 5 20.3 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 6 19.3 mag/□″ | No | Challenge | Challenge | Challenge | Challenge |
| Bortle 7 18.6 mag/□″ | No | Challenge | Challenge | Challenge | Challenge |
| Bortle 8 18.1 mag/□″ | No | No | No | No | No |
| Bortle 9 17.8 mag/□″ | No | No | No | No | No |
At the eyepiece
Almost indistinguishable from its neighbour to the west, which is the interesting thing about it — two small globulars of nearly identical size and brightness, side by side. Look at both and try to tell them apart; it is harder than it sounds.
When, and from where
Highest in the middle of the night in July. A southern object: overhead at 32°S, and low and murky from Britain and the northern United States even at its best. North of 58°N it never rises.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | — | Never rises |
| 51°N London, Calgary | 7° | Low — in the murk |
| 40°N New York, Madrid | 18° | Low — in the murk |
| 30°N Cairo, Houston | 28° | |
| 0°N the equator | 58° | |
| 33°S Sydney, Cape Town | 89° |
Nearby in the sky
The whole catalogue →2 more of the catalogue sit within five degrees, close enough to take in on the same night without moving far. Everything below is measured from M70 itself, not from the middle of Sagittarius.
- M69 Globular cluster 2.5° apart Sagittarius · magnitude 7.6 · binoculars from Bortle 6 or darker
- M54 Globular cluster 3.1° apart Sagittarius · magnitude 7.6 · binoculars from Bortle 5 or darker
- M28 Globular cluster 8.5° apart Sagittarius · magnitude 6.8 · binoculars from Bortle 6 or darker
- M22 Globular cluster 8.5° apart Sagittarius · magnitude 5.1 · binoculars from Bortle 5 or darker
One of 110. Positions J2000; the sky is the final authority.