![]() The orbit of the moon makes it so that it is not always the same distance from Earth – as shown in this exaggerated image. In addition, the moon moves notably faster in its orbit at perigee compared to apogee, thus making a moon that appears somewhat bigger and shifts its place in the sky more rapidly a bit easier to perceive when so near to the sun. When near perigee (that point in its orbit nearest to Earth) the lunar disk appears up to 14% larger in apparent size compared to apogee (when it's farthest from the Earth). Still another factor is the moon's distance from the Earth. Still others cite that the moon will be visible if it is separated from the sun by a specific number of angular degrees. The methodology for making such forecasts were further developed by Muslim and Indian astronomers, including Persian Omar Khayyam (1048-1131) who was well known for his contributions to mathematics and astronomy. Some based the first possible sighting of the lunar sliver on the difference in minutes between the setting of the sun and the moon, stating that if the moon will be visible if it sets at least 48 minutes after sunset. Interestingly, the earliest records containing predictions of first visibility came from the Babylonians. Historians have yearned to have some simple criterion for deciding whether or not a razor-thin crescent might have been sighted from say, ancient Babylon on a specific date. Surprisingly, however, even to this day, no one can say with absolute certainty when the very first sighting of the waxing crescent moon becomes possible. (Image credit: ASIF HASSAN/AFP via Getty Images) Problematic criteria The first appearance of the crescent moon in the western sky was known to some as "The Knife of Time."Ī member of the Pakistan Ramadan moon-sighting committee looks through a telescope for the new moon that signals the start of the Muslim fasting month of Ramadan in Karachi on April 23, 2020. Indeed, to people who follow a lunar calendar, the first sighting of a delicately thin waxing lunar crescent is of everyday practical importance.įor many years, when I was answering inquiries from the general public for the New York Hayden Planetarium's question and answer line, I would occasionally field calls around the time of certain religious holidays (like Ramadan) asking, "What night might I see the moon being born?" Indeed, tracking the moon's phases was probably the basis for the earliest calendars. Calendrical openerĬalendars such as the Jewish and Muslim have lunar months beginning not with new moon in the astronomical sense but with the evening when the moon first appears. Now imagine trying to see something as elusive as a thin sliver of moon, only 1% (or less) illuminated by the sun, fully immersed in this horizon haze and you can understand the degree of difficulty in trying to catch sight of such exceedingly thin crescent moons. Such haze can attenuate and redden the light of even a dazzlingly bright object as the sun. Nonetheless, when evening comes to North America on Thursday, April 20, the moon will have moved far enough away from the sun to the east, to allow it to "possibly" be glimpsed very low in the west-northwest sky shortly after sunset.Īt any age under 24 hours from new, the moon appears - to quote British astronomer, Guy Ottewell - "Breathtakingly thin and barely brighter than the low dense sky around it." Ottewell's "low dense sky" makes reference to the thicker air or haze that always seems to hug the horizon, sometimes up to an altitude of 5 or 6 degrees. Typically, on the day of a new moon, our nearest neighbor in space rises about the same time as sunrise and sets at about sunset.
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