Special for Rosh Hashana (The Jewish New Year): Which Moon will we see on the eve of Rosh Hashana, and why does it change every day of the month?

By: Dr. Ronnie Mu'alem
by:By: Dr. Ronnie Mu'alem

We all know the phases of the Moon.

Sometimes it looks like a crescent, sometimes it’s full and sometimes it doesn’t appear at all. If we followed it closely, night after night, we would see that in every day that passes the Moon changes a little bit: First it looks like a thin crescent in the West[HB1] , close to the Sun up in the sky, right after the Sunset. The next day it grows a little bigger and moves a little further away from the Sun, and the next day the crescent grows a bit more and there’s more distance between it and the Sun. A week later, the Moon looks like a perfect half-circle, and is located in the West[HB2]  of the sky in the Sunset.

The Moon continues to wax, or “grow”, and move away from the Sun until two weeks after it first appeared as a thin crescent, it’s completely full, a circle of light with darker spots in it, and it appears in the East [HB3] when the Sun sets. At this stage, the Moon is shining in its full capacity and follows us throughout the night. After that, the Moon changes direction and begins to wane – get smaller – each night, and shines in later and later hours, until three weeks after it started to appear as a thin crescent (the new Moon), again it looks like a half-circle but is seen only in the wee hours of the night. Later it will keep waning until it once again becomes a thin crescent and will be seen only before the Sunrise. After than it disappears completely for a day or two, and then born again as a thin crescent and starts the cycle all over again.

The Jewish month is determined by the phases of the Moon. The beginning of the Jewish month is set to the first appearance of that thin crescent (in Judaism, this is called the “Birth of the Moon”). The middle of the month occurs when the Moon is full, and that’s why in Judaism most events that take place at night are set for the middle of the month (events such as Tu BiShvat, the Jewish holiday celebrated on the 15th day of the Hebrew month Shevat, also called the “New Year of the Trees.” In contemporary Israel, the day is celebrated as an ecological awareness day, and trees are planted in celebration; and Tu B’Av, the fifteenth of the month Av, which is celebrated as a holiday of love similar to Valentine’s Day). The end of the Hebrew month occurs on the 29th or 30th of the month – when the Moon is not visible at all. It is also called an astronomical new Moon (unlike the Jewish new Moon when, like we said, the smallest crescent appears, marking the first day of the Hebrew month).

To understand why there are different phases of the Moon, we need to understand a few basic principles in astronomy. Before we explain these, please watch this video with Dr. Polishuk, that will make it easier to understand:

Video courtesy of CET – Center for Educational Technology

The changing phases of the Moon during the month

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Source: Shutterstock

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Courtesy of Wikipedia

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