A thin young crescent Moon can be spotted from North America this evening, Tuesday October 16th. This may be a chance to set your young-Moon record, though it's far from the best time of year.A pleasant pursuit for generations of skywatchers has been determining how young a Moon you can see. That is, how soon after the moment of new Moon can you definitely detect a hairline crescent low above the sunset horizon?
This evening, October 16th (2012), offers such a possibility. The Moon was exactly new (passing closest to the line from Earth to Sun) at 8:03 a.m. Eastern Daylight Time October 15th. That means the crescent will be about 35 hours old shortly after sunset in the eastern time zone, and about 38 hours old after sunset for the West Coast.
A 35-hour sighting might seem like old hat for veteran young-Moon spotters. The 24-hour mark is often taken as the dividing line between wannabes and the inner sanctum of this pursuit. But everyone has to start somewhere, and opportunities don't come often. New Moon must fall at just the right time in your time zone for you to really test your limit.
And, ideally, it should happen at the right time of year. Today's sighting will be a tougher challenge than you might think, because early autumn is the poorest season to try. "The ecliptic hugs the evening horizon at sunset," notes S&T senior contributing editor Roger Sinnott, our longtime young-Moon expert. "The Moon will be a rather fat crescent a few days from now before it is very easily seen. Early spring is much better."
That's when the ecliptic line, near which the Moon travels, extends straightest up from the horizon after sunset.
But have a try anyway. Scan just above the west-southwestern horizon in the fading of sunset. Binoculars help. Moonwatchers distinguish between their binocular record and naked-eye record.
As a bonus, Mercury is close to the Moon and fairly bright at magnitude –0.2.
If you succeed, note the time to the minute. Find the time difference since new Moon noted above, put this in your observing log, and you'll have a personal record to shoot for.
Much work has gone into the young-Moon visibility question, both among Western astronomers and in the Islamic world for determining the start of months in the lunar calendar. Read more: Seeking Thin Crescent Moons.