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What’s the youngest moon you can see with your eye alone?

Thin crescent moon in sky fading from pink below to blue, above buildings and trees.

View larger at EarthSky Community Photos. | Idriss Dhoparee caught the young moon from London, England, April 24, 2020.

This month’s new moon is September 17, 2020 at 11 UTC.

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 possible moon with the eye alone, that is, to see the thinnest possible crescent moon – the soonest after new moon – in the west after sunset. In order to see a young moon with your eye, the moon must have moved some distance from the sun on the sky’s dome, in the hours following new moon. A very young moon 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 the photograph below – by Thierry Legault – shows the moon at the instant of new moon. That record can only be duplicated, not surpassed.

Thread-like crescent on blotchy blue background.

Record for youngest moon photo, taken at 07:14 UTC on July 8, 2013. A photo like this requires special equipment. The unaided eye can’t see the new moon, except during a solar eclipse. Image via Thierry Legault.

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.

4 views of thin crescent moon with earthshine.

View at EarthSky Community Photos. | Peter Lowenstein in Mutare, Zimbabwe, captured this series of photos of the young moon with earthshine visible on April 24, 2020. He wrote: “Young crescent moon setting at 6.17 pm. On Friday evening clear visibility and a cloudless sky promised to provide a perfect view of the young crescent moon (only 18 hours old) setting below the local horizon. However sunset at a quarter past five was followed by the development of a large sunset stratospheric aerosol twilight arch which was so bright that it prevented the thin young moon from becoming clearly visible to the naked eye for almost three quarters of an hour. This left only twenty minutes to watch it setting.”

What is 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).

Extremely thin waxing crescent moon against an orange background.

View at EarthSky Community Photos. | Helio C. Vital in Rio de Janeiro, Brazil, caught the very young moon – just 28 hours past new moon, only 1.7% illuminated – on July 21, 2020. Thank you, Helio!

Sunset scene with extremely thin young moon.

View larger. | Very young moon like one you may see with the eye, as captured by EarthSky Facebook friend Susan Gies Jensen on February 10, 2013, in Odessa, Washington. Beautiful job, Susan! Thank you.

Bottom line: It has long been a sport for skywatchers to spot the youngest possible crescent moon after sunset, with the eye alone. What does it take to see a very young moon? Details here.

Bruce McClure

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