For the past few weeks, some may have reported a bright “UFO” hovering just above the western horizon right after the sun has gone down. But by the time the sky becomes dark enough for other stars to appear, this bright “mystery object” has vanished.
This strange bright point of light appearing in the evening twilight glow is Venus, known colloquially as “The Evening Star.”
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The moon that starts the month
Many calendars, including the Jewish and Muslim, have lunar months defined as beginning with the evenings when the moon first appears. As an example, the “Knife of Time,” a poetic term for an extremely thin crescent moon, will hang low in the western twilight on the evening of March 19, heralding the opening of Nisan, the first month of the year A.M. 5786. (A.M. is for Anno Mundi, which dates from the traditional year of the Creation, 3761 B.C.). This expressive description of a young crescent moon appears in the Icelandic Edda of almost 8 centuries ago and accordingly characterizes how the moon’s cycles cut the year into convenient segments.
When I used to answer astronomy questions by phone on behalf of New York’s Hayden Planetarium, we would get a surge of inquiries as we neared the Islamic month of Ramadan, from people wanting to know “… when would the moon be born again?”
To people who follow a lunar calendar, the first sight of a waxing crescent is of everyday practical importance. Indeed, tracking the moon’s phases was probably the basis for the earliest calendars. However, it is difficult to do this accurately because the average phase cycle — called a synodic month — is about 29.53 days, and any synodic cycle can be as much as 6 hours longer or shorter. Given the added uncertainty of the weather, it is easy to understand how confusion could arise over when a new month begins.
Quite often, when people see a narrow crescent, they refer to it as a “new” moon. But properly defined, a new moon is the invisible instant when it’s in conjunction with the sun, whereas when sighting a delicate crescent moon, we should refer to it as “young” for an indefinite but short span beginning at the first visibility.
For most of us, the first sighting of a waxing crescent moon in the evening sky usually comes two or three days after the new phase, when the sunlit portion of the moon is roughly equal, respectively, to about 5 to 12%. A crescent moon against the deepening twilight sky is a lovely sight to behold and has become important for some religions.
Venus points the way
As it turns out, on Thursday evening (March 19), Venus can serve as a very useful benchmark to make a sighting of an exceedingly narrow, hairline crescent moon less than 24 hours after the new moon phase. In fact, we might go so far as to say that Venus may be the linchpin to making a successful sighting. Without her, the chances of finding the moon would likely be considerably more difficult.
All you’ll need is good vision, a suitably clear and transparent sky and a flat, unobstructed western horizon. Observers should be outside from roughly half an hour to one hour after sunset. This is critical, since the moon will be best visible when it is roughly 2 degrees to 5 degrees above the actual (true) horizon. First, find Venus, which should be rather obvious against the bright twilight sky. Once found, make a fist and hold it out at arm’s length. The width of your fist measures roughly 10 degrees. From Venus, your quarry, an exceedingly young crescent moon, will be situated 7 degrees to Venus’s lower right.
First try to find it with just your bare eyes, but if unsuccessful, then use binoculars. If you find the crescent first with binoculars, slowly lower them while focusing your gaze at the moon’s location to see if you can perceive it without optical aid.
Why this week offers a rare viewing opportunity
Not every new moon offers such a favorable circumstance, for the moment of new moon needs to fall within certain limits of local time. For example, if a new moon were to occur during the late-morning hours, at sunset that day, the crescent moon would be an impossibly hard-to-see eight hours old and much too close to the sun in the sky. This month, the new moon occurs on Wednesday, March 18, at 9:23 p.m. EDT or 6:23 p.m. Pacific Time. With sunset occurring at mid-northern latitudes at around 7:00 p.m., this would be a good time to try and see a crescent moon on the following evening that is less than 24 hours old.
The season of the year is also important. For north temperate latitudes, the best time is around the March equinox, as then the ecliptic — the imaginary coordinate line that serves as the apparent path of the sun, moon and planets — crosses the western horizon most steeply. In such cases, the moon will appear to stand almost directly above the sun’s setting point on the horizon. In this upcoming case, the equinox occurs on the very next day!
Also, should the crescent occur close to the time of perigee (closest to the Earth), it will move up and away from the sun into the evening sky more quickly, improving its visibility. As it turns out, the moon arrives at perigee on Sunday, March 22.
Just how thin is this moon?
So, just how thin a moon are we talking about? If the moon were a perfect sphere, the illuminated crescent would arc 180 degrees around the rim of the disk. However, on this occasion, the bright arc will likely subtend only to, at most, 130 degrees and it might even appear to be broken, with bright points appearing where individual mountains are illuminated.
From Atlantic Canada, the moon will be only about 20.6 hours past new, and the width of the sunlit crescent will amount to just one percent. As one heads westward, the difference in time between the moment of new phase and the possible visibility of the moon, as well as the width of the hairline crescent, slowly increases. For those living near and along the Atlantic Seaboard, the moon will be 21.6 hours old and the sunlit crescent measures just 1.1 percent wide. Along the Pacific coast, the moon will have aged to just over 24 hours, but the crescent will still measure a mere 1.4% in width.
To see the moon when less than 24 hours old is an exhilarating experience, appearing so delicately thin that it almost looks like it might break apart! Certainly, if the weather appears favorable in your area on Thursday, you should attempt this very challenging observation.
On the following evening (Friday, March 20), the moon will be nearly two days old, and the crescent will have widened to 5% illuminated. It will also have shifted to a position some eight degrees, almost directly above Venus.
By then, it should be relatively easy for most everyone to see.
Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer’s Almanac and other publications.