A substantial portion of Mesechta Rosh Hashana deals with fixing the Jewish calendar and a key part of that is determining the start of the month. The Chumash sets the new moon as the start of the Jewish month (see Exodus 12:2). The Mishna greatly emphasizes the importance of seeing the new moon. The new moon occurs when it crosses the line between the earth and the sun. Consequently it will be a moonless night. Adding to the difficulties, at the end and beginning of the lunar month, the moon is narrow, low, and dips below the horizon in a short time.
Observation can be difficult and if this is the method used sometimes there will be a doubt whether a month has 29 or thirty days. Nevertheless, until some time after the destruction of the Second Temple, observation of the new moon was considered an essential part of determining the start of the new month. One result of this doctrine is that Rosh Hashana lasts two days even though the Chumash specifies only one. The first day however is deemed as the primary day and therefore is more strict than the second day which has minor halachic leniencies. This is despite the fact there is a doubt as to which day really is the start of the new year. A variant of this is keeping a second day of Yom Tov outside of Israel. Again the second day is deemed rabbinic with some leniencies. Yom Kippur, because it is a fast only lasts one day.
In all cases the astronomical reality, whatever it may be, is not reckoned to be of paramount importance nor is the actual legal declaration that was made at the Temple. Social factors also play a role, as well as reasoning whose logic may be elusive or actually flawed. Curiously the Reform movement says that only one day of Yom Tov should be observed in all places. Perhaps on some level it is atonement for the like minded souls that ages ago through their mischief helped cause the current situation. Even so that is not their conscious motivation.
The Talmud states the amount of time it takes for the moon to orbit the Earth. It also discusses the exact calculated time of the birth of the new moon (in Hebrew molad מוֹלָד) and based on it which day the first crescent will be visible. This knowledge is much older and may even be mentioned in the Book of Raziel, which is attributed to Adam the first man.
Dissemination of this information was slow. Early Rome used a ten month 300 day calendar with winter months ignored. Julius Caesar, a contemporary of Hillel the elder, introduced a calendar that was accurate for centuries. By 1582 it was clearly not aligned with seasons so Pope Gregory made the needed adjustments. Even so the United States Institute of Standards and Technology from time to time inserts fractional leap seconds, based on observation, so as to keep GPS navigational systems correct. The Arabs still use a lunar calendar based on observation. The result is uncertainty when months begin and end and inconsistencies between nations based on east west longitude.
If the molad falls close to the transition point, there is room for debate if a person could actually spot it. An actual sighting would resolve the debate. Failure to see it though may be the result of other factors. Shortly before leaving Egypt, Israel was given the mitzvah to fix the calendar, making this one of the first national endeavors. It would be difficult to consistently align the first of the month with the new moon in the early stages without physical observation. Calculations though would become increasingly preferable as the people became more skilled in their use. The Torah praises the trait of wisdom therefore there is an evil inclination to reject it, creating another obstacle.
The last two Mishnas of the second chapter in Rosh Hashana capture the time when the nation could appreciate the issues around fixing the calendar, as opposed to it being an esoteric issue among scholars. It is amazing how similar this story is to debates among astronomers today. The basic story is that Rabban Gamliel, who was head of the Sanhedrin, was relying purely on calculation. Any observation that supported his calculation would be accepted even if it was nonsensical. He ran into a serious problem when there was a debate about the validity of his calculation also. The conclusion of the Mishna is that the Sanhedrin has the right to fix the start of the new month not only if it is inconsistent with astronomical reality but the testimony concerning its observation is obviously erroneous as well.
The reaction of Rabbi Dosa ben Hyrcanus seems more extreme. He states, "If we call into question the court of Rabban Gamaliel we must call into question the decisions of every court which has existed since the days of Moses until now". The implication is that there are errors in halacha going back to Biblical times. There are echoes of this in Rabbi Moshe Feinstein's introduction to his magnum opus, Egros Moshe. He basically said the he's not sure that the halacha in heaven is the way that he decided it here on Earth and entertains the possibility that his information may be lacking. Nevertheless, based on the information he does have, this is the decision.
The Jewish calendar really is remarkable. It successfully integrates the lunar month with the seasons. There is some loss of precision but not enough to be significant. While the leap days and leap months require some attention, it is stable and simple enough to be a usable tool. Finally it takes into account the needs of the community. I have not seen another calendar that has these virtues.
It is burdened with the idea that the day starts when the stars come out. Consequently invitations to affairs held in the evening like weddings and Bar Mitzvahs often include the Christian date to avoid confusion even in Meah Sharim. People tell me it's because the Chumash says, "and it was evening, and it was morning, one day", (Genesis 1:5). I never caught that logic. On the contrary it sounds to me like the day starts in the morning, which is when people get up. It also gives me an explanation to verses in the Chumash that inconsistently say that Yom Kippur is on the ninth of Tishre and Passover is on the fourteenth of Nissan. To me it sounds like these holidays start on the evening of those days. The Talmud does start numbering the hours of the day at midnight.
Astronomy is often used as a metaphor for religion, perhaps because its subtleties mimic many religious processes. From time to time some of our ideas and practices, including ones that go back many, many years require investigation. They in fact do become more refined and change. Most likely the perfection of halacha is the perfection of man.
לע"נ האמא מלכה בת חיים ז"ל נלב"ע טז ניסן תשנ"ח
