Molad

How is the molad calculated?

Traditionally each month starts off before the new month with us blessing the upcoming new month in the synagogue on the Shabbat before Rosh Chodesh.  This is known as Shabbat Mevarchim and it is customary to announce the time of the molad (birth of the new moon) in accordance with Jerusalem time, so that it can be kept in mind while saying the blessing.

What is Jerusalem time?

To explain what Jerusalem time means, one must first understand the difference between civil time and solar time.

In civil time, noon is 12:00 p.m.

In solar time, noon is the time when the sun reaches its highest position in the sky, and is called high noon (or Chatzos in halachic terms).

Sometimes there can be a significant disparity between the civil noon and solar noon. As an example, in the summer, the two times may be an hour apart.

Jerusalem time is based on solar time. So 1:00 p.m. in Jerusalem time is about one hour after high noon give or take, which may be 1:30 p.m or even 2:00 p.m in civil time. So when the molad is announced, it is always the time of the molad in Jerusalem based on solar time. Traditionally, the time is announced in hours, minutes and chalakim,

"The molad will occur Yom Rishon (Sunday) evening, eight minutes and seven chalakim after seven o'clock."

All times are Jerusalem times, and are so announced in the synagogue regardless of location. The traditional announcement of the molad in hours and minutes (e.g., 7:08 PM), and the number of chalakim as well as a chelek which is one 1080th part of an hour, or 3.33 seconds., for use in the traditional announcement.

The Jewish calendar is lunar based, with each month consisting of one lunar cycle of 29 days, 12 hours, 44 minutes and 3 1/3 or 3.33 seconds, and it takes this time for the moon to complete one circuit / orbit around the earth after the previous molad.

The molad is the time of the moon's “birth”, and is the point in the moon's orbit in which it is positioned directly between the earth and the sun. It is at this point the sun shines only on the back of the moon thus making the moon invisible to observation by anyone standing on the earth's surface.

The moon continues it’s rotation around the earth until it becomes visible once again, and the cycle repeats.

The length of the moon’s cycle varies slightly, this is because the earth’s position in orbit changes as well.  The average length of the cycle is 29 days, 12 hours and 793 chalakim (*).

(*) (approximately, 44 minutes, as a chelek is a halachic designation of 3 1/3 or 3.33 seconds.)

It is also important to note that Chazal established the molad on the basis of the average lunar cycle, thus the molad is permanently set at 29 days, 12 hours and 793 chalakim after the previous molad.

Lastly because a month has to be made up of whole days, the Jewish month alternates between 29 and 30 days. This is why Rosh Chodesh will often occur a day or more after the average molad.