It has become a sport for amateur astronomers to spot the youngest moon. That is a very slim crescent moon, seen low in the western sky for a short time after sunset. A longstanding, though somewhat doubtful record was held by two British housemaids, said to have seen the moon 14 and three-quarter hours after new moon – in the year 1916.
A more reliable record belongs to Stephen James O’Meara who saw the young crescent with the unaided eye 15 hours and 32 minutes after new moon in May 1990. The record with an optical aid presently belongs to Mohsen Mirsaeed, who saw the moon 11 hours and 40 minutes after new moon in 2002.
The moon passes more or less between the Earth and sun once each month at new moon. Then you can’t see the moon – it crosses the sky with the sun during the day. But about a day after new moon, you’ll see a very thin waxing crescent moon setting shortly after the sun. And that’s a young moon – a lighted crescent in the twilight sky.
How young a moon can be seen depends on the time of year and on sky conditions. It’s possible to see the youngest moons – the thinnest crescents, nearest the sunset – around the spring equinox. That would be March for the Northern Hemisphere or September for the Southern Hemisphere.
Bottom line: What’s the youngest moon you can see? In general, it’s really tough to spot a moon less than about 20 hours old – 20 hours after the moon passed more or less between the Earth and sun. But try for your own personal best when the opportunity arises. Remember, you must have absolutely perfect sky conditions!