Moore called his collection the Caldwell Catalogue to differentiate it from the one created by French astronomer Charles Messier. Moore's full name was Patrick Alfred Caldwell-Moore. Moore restricted himself to 109 entries to be consistent with Messier's list², and did not include any of Messier's objects in his own catalogue. What makes Moore's catalogue different from Messier's is that Messier listed his by discovery, they're quite haphazard around the Northern Hemisphere sky (he was viewing from France). Moore's catalogue is (mostly³) ordered by declination, with C1 at +85° in the Northern Hemisphere and C109 at −81° in the Southern Hemisphere. His choices span 51 of the 88 internationally recognised constellations.

The Caldwell Catalogue includes star clusters and nebulae in our own galaxy the Milky Way, and other galaxies many millions of light years⁴ distant. Some of the planetary nebulae⁵ included in the Caldwell Catalogue are quite spectacular and provide a fitting swansong for a dying star. They are particularly interesting because this will be the fate of our own Sun when it has used up all of its hydrogen, swelled to a red giant and finally explodes to leave a white dwarf surrounded by gas (the planetary nebula). This isn't something we personally should worry about as the Earth will be swallowed up by the expansion of the Sun and that won't happen for a few billion years yet.

All of the following appear in the Caldwell Catalogue:

Messier Marathon vs Caldwell Chase

There exists among amateur astronomers (and, no doubt, some professional ones) a challenge called the 'Messier Marathon', wherein one chooses a specific location on a particularly clear night to attempt to chalk up each of Messier's 109 'non-comets'. It is possible to do but no doubt quite stressful, as the weather can change quite rapidly and spoil the whole event. It's not possible to attempt a 'Caldwell Chase' to collect all of Moore's astronomy gems in one night unless one owns, or has access to, a private jet. An attempt to bag them all would be much better spread, say, over a few weeks to coincide with a holiday in the opposite hemisphere of the planet to where you usually reside. An idea for the Bucket List then.