I'm extracting from this M.Y. question:

In Talmud Bavli Rosh Hashanna 20b, R' Zeira quotes R' Nachman as saying:

כ"ד שעי מכסי סיהרא לדידן שית מעתיקא ותמני סרי מחדתא לדידהו שית מחדתא ותמני סרי מעתיקא

For 24 hours, the [moon] light is covered: For us [in Babylon] - 6 of the old [month] and 18 of the new [month]; for them [in Jerusalem] - 6 of the new and 18 of the old.

(Translation mine, with help from Rashi)

Has there been any scientific, Rabbinical or other study that agrees with this study or explains this differently?

I'm skeptical that there is actually a 6 hour difference. Here are some reasons why:

  • At the equator, the earth rotates about 1,000 miles in 1 hour. The speed is slower as you move away from the equator. But, based on this, alone. Babylon is less than 6,000 miles from Jerusalem.

  • A Google search revealed that the GPS coordinates of Babylon, Iraq. Latitude: 32.5352 Longitude: 44.4192.

The latitude of Jerusalem, Israel is 31.771959, and the longitude is 35.217018

We see that the latitude of the 2 places is about the same so the earth is rotating at about the same speed at both of these places.

I'm aware that current time zone divisions don't completely coordinate with the earth's rotational speed, but, I gather that it's pretty close. However, as the Talmudic statement was made long before a concept of "time zones" as we have it, I assume that they calculated this on observation and compared the observations in both of these places.

I'm asking how accurate they were based on current scientific calculations.

  • If I recall correctly Maharal in Beer HaGoleh discusses this gemarra.
    – robev
    Commented Jun 28, 2019 at 19:37
  • @robev I occasionally review Art Scroll RH appendix that somewhat discusses this. It is tough understanding it, but it is a critical aspect of one of the 4 Rosh Hashanna postponement rules.
    – DanF
    Commented Jun 28, 2019 at 19:39
  • Time zones aren't assigned according to latitude. The dividing lines go north/south, longitudinally. Lima, Peru and Chicago, Illinois have equivalent time zones despite being at significantly different latitudes (and consequently significantly different rotation speeds)
    – Daniel
    Commented Jun 28, 2019 at 20:37

1 Answer 1


This question is based on a misunderstanding of the gemara. The assertion here isn't that there is a 6-hour time difference between Eretz Yisrael and Bavel. Rather, the case refers back to the previous discussion in the gemara about determining whether the molad occurred before or after midday.

First, some background. As we've already learned on this daf, the moon cannot be seen 6 hours before or after the molad due to its proximity in the sky to the sun (by definition, the molad occurs when the moon is directly between the earth and sun). Furthermore, it is an astronomical fact that at the time of a new moon, moonrise occurs at the same time as sunrise and moonset occurs at the same time as sunset. Finally, the molad is an astronomical phenomenon and doesn't occur at different times for different places in the world. Everybody, regardless of where they are in the world, can count down "3. 2. 1. Happy Molad!" together simultaneously.

The case quoted in the gemara here is where the molad occurs just before midday in Eretz Yisrael. At that time, the moon will already have not been seen for 18 hours (the previous six hours because the moon was too close to the sun and the 12 hours before that because the moon was below the horizon). Those 18 hours were all of the previous month, like R' Zeira said. After the molad, the moon will remain invisible for another 6 hours, making 24 hours total: 18 of the old month and 6 of the new. Six hours after the molad, the moon will be visible very briefly at the end of the day before setting below the horizon.

Since the molad was just before midday in Israel, it occurs slightly after midday in Bavel (which is to the east of Israel). Let's call it 12:05. The moon was not visible for the previous 6 hours of the old month due to its proximity to the sun, but would have been briefly visible after rising around 6:00 until being obscured by the sun around 6:05. 6 hours after the molad, at 6:05 p.m., the moon will already have set, so it won't be visible until moonrise around 6:00 a.m. the next morning, 24 hours after it was last seen.

So the gemara isn't saying that Israel and Bavel are 6 hours apart. On the contrary, their time difference is much smaller. That's why the moon is obscured for 24 hours in both places.

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