In recent weeks, some may have reported a bright “UFO” hovering just above the western horizon just after the sun has set. But when the sky gets dark enough for other stars to appear, this bright “mystery object” has disappeared.
This strange bright point of light appears in the evening twilight glow is Venusknown colloquially as “The Evening star.”
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The moon that starts the month
Many calendars, including the Jewish and Muslim ones, 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 Nisanthe first month of the year AM 5786. (AM is for Anno Mundiwhich derives from the traditional year of creation, 3761 BC). This expressive description of a young crescent appears in Icelandic Edda nearly 8 centuries ago and consequently characterizes how the lunar cycles cut the year into practical segments.
When I used to answer astronomy questions on the phone on behalf of New York’s Hayden Planetarium, we would get a flurry of inquiries as we approached the Islamic month Ramadanfrom people wanting to know “… when would the moon be born again?”
For people who follow a lunar calendar, the first sight of a waxing crescent moon is of daily practical importance. Actual tracking of phases of the moon 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’s easy to understand how confusion can arise when a new month begins.
Quite often, when people see a narrow crescent moon, they refer to it as a “new” moon. But properly defined, a new moon is the invisible moment when it is in conjunction with the sun, whereas when we see a delicate crescent, we should refer to it as “young” for an indefinite but brief span beginning at 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 part of the moon is roughly equal to about 5 to 12%, respectively. A crescent moon against the deepening twilight sky is a beautiful sight to behold and has become important to some religions.
Venus shows the way
As it turns out, on Thursday night (March 19), Venus can serve as a very useful benchmark for observing an extremely narrow crescent less than 24 hours after new moon phase. In fact, we might go so far as to say that Venus may be the key to making a successful observation. Without her, the chances of finding the moon would probably be significantly more difficult.
All you need is good vision, a reasonably clear and transparent sky and a flat, unobstructed western horizon. Observers should be outside from about half an hour to an hour after sunset. This is critical, as the moon will be best visible when it is about 2 degrees to 5 degrees above the actual (true) horizon. First find Venus, which should be quite clear against the bright dusk sky. Once found, make a fist and hold it out at arm’s length. The width of the fist measures approximately 10 degrees. From Venus, your break, a very young crescent, will be 7 degrees to Venus in the lower right.
First try to find it with just your eyes, but if that doesn’t work, then use binoculars. If you find the crescent moon first with binoculars, lower it slowly while focusing your gaze on the moon’s position to see if you can perceive it without optical aid.
Why this week offers a rare viewing opportunity
Not every new moon offers such favorable conditions, for the moment of new moon must 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 incredibly difficult to see at eight hours old and far too close to the sun in the sky. This month the new moon occurs Wednesday 18 March at 9:23 PM EDT or 6:23 PM Pacific Time. With sunset at mid-northern latitudes around 7pm, this would be a good time to try to see a crescent moon the next evening that is less than 24 hours old.
This year’s season is also important. For northern temperate latitudes, the best time is around march evelike then 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 be almost directly above the sun’s setting point on the horizon. In this upcoming event, the equinox occurs the very next day!
Should the crescent also occur near the time of perigee (closer to Earth), it will move up and away from the sun into the evening sky faster, improving visibility. It turns out that the moon arrives at perigee on Sunday, March 22.
How thin is this moon?
So how thin a moon are we talking about? If the moon were a perfect sphere, the illuminated crescent would curve 180 degrees around the edge of the disc. But on this occasion, the bright arc is likely to only extend to, at most, 130 degrees, and it may even appear to be broken, with bright spots appearing where individual mountains are illuminated.
From Atlantic Canada, the moon will only be about 20.6 hours above new, and the width of the sunlit crescent will be only one percent. As one moves westward, the difference in time between the moment of new phase and the possible visibility of the moon increases, as well as the width of the crescent moon. For those living near and along the Atlantic coast, the moon will be 21.6 hours old and the sunlit crescent is only 1.1 percent wide. Along the Pacific coast, the moon will have aged to just over 24 hours, but the crescent will still measure only 1.4% in width.
Seeing the moon when it is less than 24 hours old is an exciting experience, and looks so delicate it almost looks like it could break! Certainly, if the weather looks favorable in your area on Thursday, you should try this very challenging sighting.
By the following evening (Friday, March 20), the Moon will be nearly two days old and the crescent will have expanded to 5% illuminated. It will also have shifted to a position around eight degrees, almost directly above Venus.
Then it should be relatively easy for most people 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.
