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Rosh Hashanah 21

1) OPPOSITION OF THE MOON, BY LAND AND BY SEA

QUESTION: Rav Nachman gave advice to people who would be traveling at sea. Since they had no way of knowing when the new month was declared, they needed some way of ascertaining the time to get rid of their Chametz. Rav Nachman told them that when they see the full moon setting at the same time that the sun rises, that is the day to get rid of their Chametz.

The Gemara asks that we see the moon set at the time the sun rises on the *fifteenth* of the month, but Chametz must be destroyed on the *fourteenth*! The Gemara answers that at sea, the horizons are clearer, and therefore the moon is seen setting at sunrise on the fourteenth of the month, one day earlier than people on land see it setting at sunrise.

(a) Why is it that on land people see the moon set at sunrise one day later than those at sea see the same thing?

(b) What day is considered the halfway point through the month? Since the average month is a little more than 29 1/2 days, half of that is about 14 3/4 days. After 14 3/4 days have passed since the beginning of the month, the moon should be seen to be setting at the time that the sun rises. However, Rav Nachman says that the moon can be seen setting at sunrise on the *fourteenth* day. At sunrise on the fourteenth day of the month, only 13 1/2 days have passed since the new moon! How can Rav Nachman give that as the time for destroying Chametz?

ANSWERS:

(a) As we discussed earlier (see Insights to 20b), the moon travels around the earth slower than the sun. At the Molad, the moon and sun are at the same point in the sky. As the days pass by, the moon lags farther and farther behind the sun (that is, it drops another 12 degrees towards the east each day). Halfway through the month the moon has lagged so far behind the sun and it is on the other side of the earth, at the opposite horizon (or 180 degrees away from the sun). The next day it will be only 168 degrees away, having moved another 12 degrees further from to the east. On that day, when the sun rises, the moon will be higher up (12 degrees) in the western sky than it was the day before at sun rise.

At sunrise at the fourteenth day of the month, says Rav Nachman, the moon is 180 degrees away from the sun and at the opposite horizon. It sets at sunrise. This opposition, however, can only be witnessed properly by sea, when the horizon is unobstructed. On land, though, there are obstructions in both the eastern and western horizons. If the obstructions are tall enough in each horizon (about 6 degrees), one will see the moon setting at the time the sun rises when they are actually closer together by 12 degrees, or only 168 degrees apart from each other. Thus, people on land will see the moon setting at the time the sun rises one day later, on the fifteenth of the month.

(b) The answer to our question about the midpoint of the month is that in the days of Rav Nachman, they still declared the new month based on the testimony of witnesses. The new moon was visible for the first time only 6 hours after the Molad when the Molad occurred before midday, or 18 hours after the Molad when the Molad occurred after midday. Therefore, day 13 1/2 of the Hebrew month (that is, 13 1/2 days from the day that Beis Din declared to be the new month) is actually between day 14 and 14 1/2 from the time of the Molad of the new moon. Therefore, Rav Nachman could accurately say that the people at sea could determing the day of Bi'ur Chametz, the fourteenth of Nisan, by seeing the moon setting at the time of sunrise. (KUNTRAS "DI SHEMAYA")

21b