Chazal and NASA

Did Chazal (the Sages of the Talmud) have supernatural knowledge of the natural world? Do their statements prove Torah MiSinai? One alleged "proof" to this effect is regarding the length of the lunar month.

The argument goes as follows: In the Gemara, Rabban Gamliel says that the average length of the lunar month is 29.530594 days. NASA, with years of research using satellites, telescopes, laser beams and supercomputers, calculated the mean length of the lunar month as 29.530588 days. The difference between this figure and that used by the Sages is only 0.000006! How could Chazal have calculated it to such accuracy? They must have had supernatural sources of knowledge.

This sounds amazing!

But is it true?

I've spent 25 years researching such alleged proofs based on incredible statements by Chazal. I approached them initially with a very trusting (and naive) perspective that since they are proposed by dynamic and brilliant rabbis, they must be true. Much to my dismay, every time I looked into such arguments, I found that they fell into one of three categories:

A) They were ambiguous statements that could be read in all kinds of different ways

B) They were things that non-Jews also knew, and do not require supernatural sources of information

C) They were things that are not actually true.

The lunar cycle argument is no different.

First, let us consider whether the claim is, on the outset, reasonable. Recall that Chazal did not know where the sun goes at night. Accordingly, it does not seem that they had supernatural sources of knowledge for astronomy.

Still, one could argue that perhaps the supernatural knowledge was only for particular things. So let's look at what Rabban Gamliel actually said, and the context in which he said it:

"The Sages taught in a baraita: Once the sky was covered with clouds, and the form of the moon was visible on the twenty-ninth of the month. The people thought to say that the day was the New Moon, and the court sought to sanctify it. Rabban Gamliel said to them: This is the tradition that I received from the house of my grandfather: The monthly cycle of the renewal of the moon takes no less than twenty-nine and a half days, plus two-thirds of an hour, plus seventy-three of the 1,080 subsections of an hour." (Rosh HaShana 25a)

Looking at the Gemara, we see that it is being misrepresented. Rabban Gamliel was not giving the average length of the lunar month. Instead, he was giving the minimum length of the lunar month. He explicitly says this ("takes no less than") and it's also the context, in which he was responding to a question about whether it could be Rosh Chodesh already. The lunar months actually vary in length by several hours, because the speed of the moon’s rotation around the earth is not uniform. And as a minimum length of the month, Rabban Gamliel's figure is several hours off.

But in any case, not only do we no longer have Rabban Gamliel's statement being scientifically accurate, we have an innate problem. The duration that he gives as being the minimum length of the month is actually the mean length of a lunar month, as used in the Jewish calendar! Several Acharonim already recognized this problem. It is therefore presumed that what we have here is a textual interpolation (see Sacha Stern, Calendar and Community. A history of the Jewish calendar second century BCE – tenth century CE, Oxford, 2001, p. 201). What Rabban Gamliel originally said - which makes sense as a response to the situation discussed in the Gemara - was that the monthly cycle is no less than 29 days. Much later, someone added the additional units of time to the text, mistakenly thinking that it should be "corrected" to the mean value.

There is no way to know when this textual interpolation took place. But what we do know is that, whenever after Rabban Gamliel the average length of the lunar month was known, the exact same figure was known in antiquity by other peoples - and even earlier.

The exact duration of the month found in the Gemara was already given by Ptolemy, in base-sixty notation, in the 2nd century CE. He in turn was quoting Hipparchus, from the second century BCE - nearly two hundred years before Rabban Gamliel. And Hipparchus in turn had received this value from the ancient Babylonians! There have even been discoveries of Babylonian cuneiform tablets which contain this number.

In summary: Rabban Gamliel's statement, as the minimum length of the month, is not actually correct. As a later interpolation of the mean length of the month, which was employed in the rabbinic calendar, it is correct - but the same value was already known much earlier by the Greeks and even earlier by the Babylonians.

So not only is this not a proof of Torah MiSinai - it's actually a worse problem. The people using this as a "proof of Torah" argue that such knowledge can only come from a Divine source! So, according to them, this is proof that the Babylonians had supernatural knowledge of the natural world, and evidence that their religion was the One True Religion! 

Fortunately for the rest of us, they are mistaken. You don't actually need supernatural sources of knowledge to come up with this figure. The Babylonians were simply very meticulous astronomers. By measuring the duration between eclipses, one can figure out the mean length of the lunar month. (See here for an extremely technical discussion.)

Now, if you're a person that likes these kinds of proofs, and you're thinking that I'm just a bitter, cynical person, consider this: I used to be just as starry-eyed a believer in these proofs as you. The reason why I became cynical is precisely because I was taken in by such proofs which turned out to be fundamentally flawed. As Rambam says, the problem with using flawed proofs is that when people discover the flaws, they lose confidence in everything you present. If you use such "proofs" with other people, you'll just be turning others into cynics.

Don't try to "prove" Judaism with gimmicks. It doesn't work and it can backfire badly. Instead, present the extraordinary history of the Jewish People, culminating in our miraculous return to our homeland, along with the incredible meaningfulness and value of a Torah-observant lifestyle. 

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