How long after new moon can you spot the thin crescent moon against the bright background of a twilight sky?
Cases of naked eye sightings of the moon when it is younger than 24 hours are not very common and sightings within 20 hours of a new phase are very rare. The best opportunities usually come in February, March and April, because during these times the lunar crescent is directly above a sun that has just set.
And on Thursday evening, April 20, everyone across North America will have the opportunity to spot an extremely young crescent moon. A very inspiring observation to be sure.
Related: Moon Facts: Fun facts about Earth’s moon
April 20 might sound a little familiar to you, since a hybrid solar eclipse is scheduled for the same day. The eclipse will be seen as full from a very small part of Western Australia, as well as part of Indonesia, where it will be midday on April 20. But on the other side of the world. for North America, the new moon responsible for that eclipse on that date will occur shortly after midnight (12:12 a.m. New York time), or during the mid-evening hours (9:12 p.m. Pacific time) on the 19th April.
Typically, on the day of a new moon, our closest neighbor in space rises around the same time as sunrise and sets around sunset. However, by the time evening falls in North America on Thursday, April 20, the moon will have moved far enough away from the sun to the east that it “probably” can be glimpsed very low in the west-northwest sky shortly after sunset.
At any age within 24 hours of new, the moon appears – to quote British astronomer Guy Ottewell – “Incredibly thin and scarcely brighter than the low, dense sky that surrounds it.” Ottewell’s “low dense sky” refers to denser air or haze that always seems to hug the horizon, sometimes down to an altitude of 5 or 6 degrees. Such a haze can dull and redden the light of even a dazzling object such as the sun.
Now imagine trying to see something as elusive as a thin sliver of the moon, illuminated only 1% (or less) by the sun, completely immersed in this horizon haze and you can understand the degree of difficulty trying to make out such an extremely thin object crescent moons.
Opening the calendar
Calendars such as the Jewish and Muslim have lunar months that begin not with the new moon in the astronomical sense, but with evening when the moon first appears. Indeed, for people who follow a lunar calendar, the first sighting of a delicately thin crescent lunar crescent is of daily practical importance.
For many years, when answering questions from the general public for the New York Hayden Planetarium Q&A line, I occasionally answered calls around certain religious holidays (such as Ramadan) asking, “On which night could I see the moon having been born?” In fact, tracking the phases of the moon was probably the basis for early calendars. The first appearance of the crescent moon in the western sky was known to some as “The Knife of Time”.
problematic criteria
Surprisingly, however, even today no one can say with absolute certainty when the very first sighting of the crescent moon will be possible. Historians have longed to have some simple criteria for deciding whether or not a razor-thin crescent could have been sighted from ancient Babylon on a specific date.
Interestingly, the first documents containing first-visibility predictions came from the Babylonians. The methodology for making such predictions was further developed by Muslim and Indian astronomers, including the Persian Omar Khayyam (1048-1131), known for his contributions to mathematics and astronomy. Some have based the earliest possible sighting of the moonshard on the difference in minutes between sunset and moonset, stating that if the moon will be visible if it sets at least 48 minutes after sunset. Still others cite that the moon will be visible if it is separated from the sun by a specific number of angular degrees.
Yet another factor is the moon’s distance from the Earth. When near perigee (that point in its orbit closest to Earth) the lunar disk appears up to 14% larger in apparent size than at apogee (when it is farthest from Earth). Also, the moon moves noticeably faster in its orbit at perigee than at apogee, thus making a moon that appears a little larger and changes its position in the sky more rapidly a little easier to perceive when so close. in the sun.
Moonwatch: April 20, 2023
So now I’m “throwing down the lunar gauntlet” and offering this observing challenge to everyone. At sunset on April 20, for those living in the Eastern Time Zone, the crescent moon will be just under 20 hours old and only 0.7% illuminated by sunlight. From the Central Time Zone it has just under 21 hours and 0.8% daylight, while along the Pacific Coast it has 23 hours and 1.1% daylight.
In doing some research, I found a number of similar cases where observers were able to spot the filament of the moon in the twilight sky just after sunset. Interestingly, 80% of these cases occurred during the month of April.
All you need to join this moon-gazing challenge is a clear, flat horizon looking west-northwest. This is crucial, as the moon will be at its best when it is 7.5 degrees above the horizon. (Your clenched fist held at arm’s length measures about 10 degrees wide.) Placing the bottom of your fist at the horizon, the top of your fist measures about 10 degrees. So for this experiment, consider looking about three-quarters of a fist away from the horizon. Take particular note of where exactly on the horizon the sun sets, because your attempt to spot the moon will be directly above this point on the horizon.
As for when you should watch, try 20 to 25 minutes after dark. During that time, the moon will be about 7.5 degrees above the horizon, while the sun will be about 4.5 degrees below the horizon. This is during civil twilight, the brightest of the three stages of twilight, when the sun is just below the horizon, so there’s generally enough natural light for most outdoor activities.
Binoculars against your unaided eyes
But with such a bright background sky, you’ll definitely need binoculars to spot the very thin sliver of moon. Slowly scan the region of the sky where it should be. Local sky conditions will be very important; a foggy sky (where the sun appears as an orange-red ball as it sets) will almost certainly make the job difficult or nearly impossible. But if you have a clear, transparent sky (the sun still looks blinding as it dips below the horizon), your chances will greatly increase.
However, to possibly contribute to knowledge of when the months of lunar calendars should begin, you should try to find the moon first with the naked eye; only use the binoculars afterwards if there really is no hope of finding it with the naked eyes.
If you don’t have binoculars or a telescope for getting up close to the moon, check out our guides to the best telescopes and best binoculars. And if you’re looking to take some great lunar photography, check out our guide on how to photograph the moon, as well as our best cameras for astrophotography and the best lenses for astrophotography.
Remember that the moon is not a smooth sphere, but has a rough surface. Since it will be less than 15 degrees from the sun, binoculars or a small telescope could show the bright arc broken up in places, with bright spots or dots appearing where individual mountains are located.
This is sure to be an inspiring and (if successful) hilarious observation. If you see it, I’d be interested in knowing all the details. Send me an email to: skywayinc@aol.com.
Good luck and clear skies!
Joe Rao serves as an instructor and guest lecturer in New York Hayden Planetarium (opens in a new tab). He writes about astronomy for Natural History Journal (opens in a new tab)THE Farmers Almanac (opens in a new tab) and other publications. Follow us on Twitter @Spacedotcom (opens in a new tab) and go Facebook (opens in a new tab).
