Basics of Crescent Moon Sighting (image@Pixabay)The beloved month of Muslims, Ramadan, is bidding farewell soon. All eyes are once again set on the crescent moon to mark the end of the holy month, and to decide on Eid al-Fitr, one of the two celebrations in Islam. The day is observed on the first day of Shawwal, the 10th month in Arabic or the lunar calendar. The Eid al-Fitr is celebrated only when the new moon is visible. Muslims across the globe celebrate Eid over a difference of one or two days as per the visibility of the new moon. In India, the Eid is expected to fall on April 10 or 11.
Each month in Islam starts with the visibility of the crescent moon. The new crescent moon is the first time when moon can be visible after a new moon, ie; when the moon is in its invisible phase. Moon usually takes 29.5 days to go through full cycle. Since it is not possible for a month to have a half day, Islamic month either consist of 29 days or 30 days, depending on when the new crescent moon is visible.
Like the time changes for the sun set and sun rise around the world, the visibility of the crescent moon is also different in different parts of the world. this explains why Eid al Fitr or Eid al Adha is not celebrated on same day in all parts of the world. The moon is the slimmest curve in the sky with just a fraction of its side illuminated.
In places like the United Kingdom, the northern hemisphere, the crescent moon will appear as a backward C shape and in southern hemisphere, it will appear the other way around, like a regular C shape.
There are a few criteria for a crescent moon to be visible. The sun must be below the horizon, for the sky needs to be dark enough to spot the small silver line of the new crescent moon. It must be above the horizon to determine the beginning of the month. Both the moon and the sun must be far enough in the sky, which is known as Danjon Limit, which states that the sun and moon need to be separated by over 5-7 degrees, which is same as the width of first three fingers of a person held out at arms length.
(with inputs from Royal Museum Greenwich)