Beginning amateur astronomers soon encounter the term “Messier objects.” They learn that this is a list of objects outside our Solar System that are visible through small telescopes. This list was originally compiled by Charles Messier, in the 18th century, from his observations and those of contributors. The catalog has been updated over time, as recently as the mid-20th century, to 110 objects in total.
By the 1970s amateurs had noticed that all 110 Messier objects could be observed at low northern latitudes over the course of a night in mid-to-late March.
You’ve probably heard of Charles Messier’s catalog of celestial deep-sky objects for 18th century comet hunters. When they stumbled upon an unknown faint fuzzy object, they’d consult this list to see if it was a known object. It was first published in 1774 and expanded with help from fellow observers in that century. Astronomers and amateurs in the early 20th Century rediscovered it, added a few new objects, and made corrections. There are now 110 “M” objects in the modern catalog.
Every sky chart labels the location of these objects with the letter “M” and a number. For instance, the brightest Messier is M45 — the famous naked-eye Pleiades open star cluster. A third of the way around the sky you’ll find the dimmest: M95 — a galaxy of magnitude 10 or 11 (depending on the source) in Leo.