What is the youngest moon you can see with your eye alone? It has long been a sport for amateur astronomers to spot the youngest moon with the eye alone. In order to see the young moon with your eye, the moon must have moved some distance from the sun on the sky’s dome. It always appears as a very slim waxing crescent moon, seen low in the western sky for a short time after sunset. A longstanding, though somewhat doubtful record for youngest moon seen with the eye 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 was achieved by Stephen James O’Meara in May 1990; he saw the young crescent with the unaided eye 15 hours and 32 minutes after new moon. The record for youngest moon spotted with the eye using an optical aid passed to Mohsen Mirsaeed in 2002, who saw the moon 11 hours and 40 minutes after new moon. But Legault’s photograph at the instant of new moon? That record can only be duplicated, not surpassed.
The moon passes more or less between the Earth and sun once each month at new moon. Then you will not see the moon because it is crossing the sky with the sun during the day. About a day after new moon, you might see a very thin waxing crescent moon setting shortly after the sun. The young moon appears as lighted crescent in the twilight sky, often with the darkened portion of the moon glowing dimly with earthshine.
How young a moon you can expect to see with your eye depends on the time of year and on sky conditions. It might be possible to see the youngest moons – the thinnest crescents, nearest the new moon – around the spring equinox. That would be March for the Northern Hemisphere or September for the Southern Hemisphere. But it’d also help if, in addition, the moon were near lunar perigee – closest point to Earth in its orbit – and maximally north of the ecliptic if you live in the Northern Hemisphere (or maximally south of the ecliptic if you live in the Southern Hemisphere).
Bottom line: What’s the youngest moon you can see with your eye? In general, it’s really tough to spot a moon less than about 20 hours old – that’s 20 hours after new moon, when the passed more or less between the Earth and sun for that monthly orbit.