Over the past few weeks, some may have reported a bright “UFO” hovering just above the western horizon just after the sun set. But when the sky darkens enough for other stars to appear, this bright “mystery object” has disappeared.
This strange bright point of light appearing in the glow of the evening twilight is Venusknown colloquially as “The evening star“.
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The moon that starts the month.
Many calendars, including Jewish and Muslim, define lunar months 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 remain 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 World Yeardating from the traditional year of Creation, 3761 BC). This expressive description of a young crescent moon appears in the Icelandic language. edda dating back almost 8 centuries and, consequently, characterizes how the cycles of the moon divide the year into convenient segments.
When I used to answer questions about astronomy over the phone on behalf of the Hayden Planetarium in New York, we received a flurry of inquiries as we approached the Islamic month of Ramadanof people who want to know “…when the moon born again?”
For people who follow a lunar calendar, the first sight of a crescent moon is of practical everyday importance. In fact, monitoring the moon phases It was probably the basis of the first calendars. However, it is difficult to do this accurately because the average phase cycle, called synodal month – is approximately 29.53 days, and any synodic cycle can last up to 6 hours or so. Given the added uncertainty of the weather, it’s easy to understand how confusion could arise over when a new month begins.
Very 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 is in conjunction with the sun, while when we see a delicate crescent moon, we must refer to it as “young” for an indefinite but short period beginning at first visibility.
For most of us, the first sighting of a crescent moon in the night sky usually occurs two or three days after the new phase, when the portion of the moon illuminated by the sun is approximately equal to, respectively, about 5 to 12%. A crescent moon against the deepening twilight sky is a lovely sight to behold and has become important to some religions.
Venus points the way
It turns out that on Thursday night (March 19), Venus can serve as a very useful landmark for spotting an extremely narrow and thin crescent moon less than 24 hours into the new moon phase. In fact, we could go so far as to say that Venus may be the lynchpin for a successful sighting. Without it, the chances of finding the moon would probably be considerably more difficult.
All you will need is good vision, a suitably clear and transparent sky, and a flat, unobstructed western horizon. Observers should be outside approximately half an hour to an hour after sunset. This is essential, as the moon will be best visible when it is approximately 2 to 5 degrees above the actual (true) horizon. First, find Venus, which should be pretty obvious against the bright twilight sky. Once found, make a fist and hold it at arm’s length. The width of your fist measures approximately 10 degrees. From Venus, its prey, an extremely young crescent moon, will be located 7 degrees to the lower right of Venus.
First try to find it with your eyes alone, but if you can’t, use binoculars. If you first find the crescent with binoculars, lower them slowly while focusing your gaze on the moon’s location to see if you can perceive it without optical aid.
Why this week offers a rare viewing opportunity
Not all new moons offer such favorable circumstances, as the timing of the new moon must fall within certain boundaries of local time. For example, if a new moon occurred during the late morning hours at sunset that day, the crescent moon would be incredibly difficult to see for about eight hours and would be too close to the sun in the sky. This month the new moon occurs. on Wednesday, March 18 at 9:23 pm EDT or 6:23 pm Pacific Time. Since sunset occurs in mid-northern latitudes around 7:00 pm, this would be a good time to try to see a crescent moon the following night that is less than 24 hours old.
The season of the year is also important. For northern temperate latitudes, the best time is around march equinoxlike then ecliptic (the imaginary coordinate line that serves as the apparent path of the Sun, Moon, and planets) crosses the western horizon more sharply. In such cases, the moon will appear to be almost directly above the sunset point on the horizon. In this next case, the equinox occurs the next day!
Additionally, if the crescent moon occurs near the time of perigee (closer to the Land), will rise and move away from the sun into the night sky more quickly, improving its visibility. It turns out that the moon reaches perigee on Sunday, March 22.
How thin is this moon?
So what thin moon are we talking about? If the Moon were a perfect sphere, the illuminated crescent would form a 180-degree arc around the edge of the disk. However, on this occasion, the bright arc will likely subtend only, at most, 130 degrees and may even appear broken, with bright spots appearing where individual mountains are illuminated.
From Atlantic Canada, the moon will be just 20.6 hours from the new moon, and the width of the sunlit crescent will amount to just one percent. As one heads west, the time difference between the time of the new phase and the possible visibility of the Moon, as well as the width of the thin crescent, slowly increases. For those living near and along the Atlantic coast, the moon will be 21.6 hours long and the sunlit crescent will measure 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 just 1.4% across.
Viewing the Moon when it is less than 24 hours old is an exhilarating experience, as it appears so delicately thin that it almost seems like it is going to break. Certainly, if the weather looks favorable in your area on Thursday, you should try this very challenging watch.
The following night (Friday, March 20), the moon will be almost two days old and the crescent moon will have widened to 5% illumination. It will also have moved to a position of about eight degrees, almost directly above Venus.
By then, it should be relatively easy for most people to see.
Joe Rao serves as an instructor and guest lecturer at New York University. Hayden Planetarium. Write about astronomy during natural history magazine, sky and telescope, The Old Farmer’s Almanac and other publications.
