Categories: Astronomy

Astro-Challenge: Spotting Slender Moons

Up for a challenge? Some of the toughest targets for a backyard observer involve little or no equipment at all. Northern hemisphere Spring brings with it one of our favorite astronomical pursuits: the first sighting of the extremely thin, waxing crescent Moon. This unique feat of visual athletics may be fairly straight forward, requiring nothing more than a working pair of Mk-1 eyeballs… but it’s tougher than you think. The angle of the evening ecliptic in the Spring is still fairly high for mid-northern latitudes, taking the Moon up and out of the weeds when it reaches waxing crescent phase.

This challenge has a fairly dedicated group of followers worldwide, seeking to spot the razor thin crescent on each successive evening after New phase. And in this instance, solar eclipses — which, by definition, must occur during a New Moon
— don’t count as a first Moon sighting, though it’s amusing to hear flat-earthers raise the objection that the incoming occluding body can’t be seen during an eclipse.

The Moon becomes generally visible around 20 hours after New, an easy to spy crescent hanging against the deep cerulean sky during the ‘Blue Hour” of twilight. Crystal clear skies are key to this challenge, As the thinnest scud of low-hanging clouds can hide the Moon… plus, the thin silver thread of the Moon will vanish from view against a low contrast background sky with the slightest bit of haze.

When can you first see the crescent Moon with binoculars? The naked eye? Against the daytime sky?

Be careful, in the case of scouting for the Moon while the Sun is still above the horizon, to physically block it from view behind a hill or building. You don’t want to accidentally get sight of the blinding Sun through binoculars while hunting for the crescent Moon, even for a second. And yes, you can carry out the challenge in reverse and try to nab the extremely slender Old crescent Moon at dusk.

Can you see it? The amazingly thin Moon captured by Rob Sparks and Mike Weasner.

Be forewarned: old hands at the moonspotting challenge may snicker at your 20-plus old crescent Moon record. The real difficulty lies when the Moon is 12-20 hours old… though this is not impossible. Robert Sparks and Mike Weasner managed to spy the Moon using binoculars at just 13 hours and 48 minutes past New from Oracle, Arizona on New Year’s Day, 2014. The verified record for a naked eye sighting stands at 15 hours and 32 minutes past New by Steven O’Meara in May 1990, and the record binocular sighting is an amazing 11 hours and 40 minutes past New, accomplished by Mohsen Mirsaeed in September, 2002.

Enter the Danjon Limit

There’s also a diminishing point at which sighting the slim crescent Moon is no longer possible. This is referred to as the Danjon Limit, an minimum angle between the Moon and the Sun at which the length of the arc of the crescent diminishes to zero. This was first proposed by astronomer André-Louis Danjon in 1931. A keen lunar observer, Danjon also lends his name to the color scale for total lunar eclipses, known as the Danjon number. The Danjon Limit stems mainly from the fact that the Moon has rough edges, which shadow and dissolve the thin crescent. Danjon estimated the limit at about seven degrees elongation from the Sun which converts to roughly 11-12 hours past New. A recent 2012 study however, gives a more optimistic value of 5 degrees, or 9-10 hours past New. Keep in mind, the Moon’s orbit is inclined about five degrees relative to the ecliptic, meaning it can pass anywhere from directly in front of the Sun (as it does during a total solar eclipse), and up to five degrees to the north or the south of it.

Astrophotographer Thierry Legault actually nabbed the ultra-thin Moon at New (!) just 4.4 degrees from the Sun on July 8th, 2018, though he used specialized equipment to do it.

An amazing capture. Image credit and copyright: Thierry Legault.

As mentioned, the challenge evolves throughout the year, owing mainly to the angle of the ecliptic. Several sites, including Moonwatch and the United States Naval Observatory’s page provide information and maps for spotting the crescent Moon worldwide for each upcoming lunation.

Seeing floaters yet? Spotting the thin Moon takes patience. It’s also true that youngsters have better eyes for such a feat than us old timers, as the lens of our eyes yellow and fog over through the years, mainly due to a half century-plus of general wear and ultraviolet sunlight exposure. I like to start off sweeping the suspect area with binoculars. Seeing the slim crescent Moon can seem like magic at first, an improbable symmetric object that shouldn’t be there. As the Moon fattens over successive evenings, Earthshine (also known as Ashen Light, or, in the words of my Farmer’s Almanac-quoting Grandpappy, the “Old Moon in the New Moon’s Arms) will become apparent. This is due to sunlight, reflected back off of the Earth on to the nighttime side of the Moon. Standing on the Moon looking back at the Earth, you’d see an opposite gibbous phase, when the Moon is a crescent.

What’s more, Earthshine can appear more pronounced, from one lunation to the next. This is due to the amount of highly reflective cloud and snow cover on the Moonward facing side of the Earth. This value is not only seasonal, but is changing due to anthropogenic climate change increasing cloud cover, versus an increasing albedo (change in brightness and reflectivity) due to a phenomenon known as global dimming. The Project Earthshine Study carried out from 1998 to 2008 from the Big Bear Solar Observatory chronicled this effect.

Why spot slender Moons? It’s true that this arcane superpower won’t save you during a zombie apocalypse. Still, It’s fun to keep a personal record to challenge. Pushing your visual skills to the limit also allows you to see the subtle in nature, and perhaps see things we generally miss in a cursory view. This is also a testament to the physiological limits of the human eye, a pair of optical sensors designed by millions of years of evolution, making due with just water and jelly.

Looking west on the evening of May 5th. Credit: Stellarium.

Sighting the next crescent Moon is also special, as it marks the start of the month of Ramadan on the Muslim calendar. The Islamic calendar is solely based on the sighting of the crescent Moon and its 29.5 synodic period, meaning that it moves up approximately 11 days versus the Gregorian calendar. The Moon reaches New next on May 4th at 22:47 Universal Time (UT)/6:47 PM EDT, and first visibility favors southern Africa on the evening of May 5th. The Moon should become generally visible on the evening of May 6th. Note that this can sometimes lead to a discrepancy worldwide as to the exact start and end of Ramadan.

Here are first sighting ops for the Moon worldwide for the for 2019. The Moon is generally visible westward of the sites marked. Credit: Dave Dickinson

Good luck on spotting the crescent Moon this next time around!

David Dickinson

David Dickinson is an Earth science teacher, freelance science writer, retired USAF veteran & backyard astronomer. He currently writes and ponders the universe as he travels the world with his wife.

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