Few celestial sights are as arresting and beautiful as a slender crescent Moon in the western evening sky, the day after it passes the Sun each month. A good-luck token for some, a bad-weather omen for others, it is also a sight that regulates the lunar calendar of the Muslim world.

For amateur astronomers and photographers alike, each young Moon makes an inviting target. The sunlit part encircles the Moon’s night side, which is not fully dark but is bathed in sunlight reflected back toward the Moon from Earth. This earthshine theoretically gets brighter the closer the Moon is to new, except that it can easily be washed out by twilight. On rare occasions it has even been seen during a total solar eclipse.

When it comes to sighting the youngest possible crescent in the evening sky — or, for that matter, the thinnest waning crescent in the predawn sky — we are no longer talking about a bewitching sight that you might notice casually. Spotting such a Moon is no accident — you have to plan for it very carefully and pay close attention to the astronomical conditions under which a record sighting is even possible.

The instant of new Moon occurs when our satellite’s celestial longitude (along the ecliptic) matches that of the Sun. Some 16 to 24 hours later, if the Sun has just set at your location, you might catch sight of a very slender crescent low in the western sky.

The record for the youngest Moon ever seen with optical aid, 11h 40m past new, goes to Mohsen G. Mirsaeed of Tehran who saw it on September 7, 2002. The youngest crescent ever seen by the naked eye, 15h 32m, is still that observed in May 1990 by Sky & Telescope contributing editor Stephen James O’Meara.

Any given year there are only a handful of specific dates and locations on Earth where the thinnest crescent sightings are possible. To see why, we need to explore the interplay of astronomical factors affecting visibility.