Shalom Rav, I love your site and am so happy I can learn so much Torah from it and ask Shayla’s when I need to here. So I have 2 questions
Q1) How does the Molad line up in terms with Rosh Chodesh, like what is the calculation for the phase of the moon in terms of knowing when Rosh Chodesh is?
Q2) Why do we sometimes have a 2 day Rosh Chodesh and some months do not
Thank you for your kind words, they warm my heart.
Regarding your questions,
- Calculating when Rosh Chodesh is quite complicated, and understand the orbit of the moon, sun and the stars, is quite a science. In fact, chazal were experts at it, and it caused the nations to revere the wisdom of Klal Yisroel. If you would like to see more about this, read the Rambam in Hilchos Kiddush Hachodesh where he discusses these calculations at length. The molad is usually 29 days 12 hours and 793 chalakim after the previous one, which is a little more than 29 and a half days. There are times when the actual molad will not be on the day of Rosh Chodesh, but even a day or two before it.
- Nowadays being that we don’t have a Bais Din that can rule when the new Chodesh will, and we are doing it through basic calculations, for most months, we have one month with 29 days and the next one with 30 days. (This basically balances out the 29 and a half days of the month’s lunar cycle, but there are exceptions.) When the month is a “regular” month Rosh Chodesh will be on the 30th day from the previous Rosh Chodesh, which is four weeks and one day after the previous months Rosh Chodesh. Therefore, that day is always Rosh Chodesh, whether it is the 30th of the previous month or the first of the next month. Then, if the month is a large month, we will also have Rosh Chodesh on the next day, which is the first of the next month, which is really the correct day for Rosh Chodesh. Therefore, some months have one day Rosh Chodesh, which will be on the first of the new month, or two days, the 30th of the first month, and the 1st of the new month.