[Avodah] kol hamoseif gorea

[Micha Berger]

On Wed, Feb 24, 2010 at 12:57:02PM +0200, Eli Turkel wrote:
: I assume that even in the charedi world the claim is that statements
: in the gemara are true even when they contradict modern science.
: Is there anyone who claims that chazal knew modern physics, could have
: constructed airplanes or PCs or heart transplants had they wanted to?.

You didn't ask for my opinion, but I'd like to remind people of my post
(which Google isn't helping me find) which shows that Chazal's astronomy
roughly kept pace with Roman scientific consensus.

It's not that there was a machloqes as to whether the raqi'ah was a dome
that the sun went behind at night, or the sun went under a flat earth at
night (and thus heated up the water on the ground, since it was closer),
or the earth was round. It's that the tanna or amora who said each lived
at a time when that was what the current theory was. Even down to finding
that the amoraim of EY, directly under Roman rule, was about a generation
ahead of their Bavli contemporaries. (Which didn't always mean they were
more accurate. The Romans stepped back from the Ptolmeic model sometime
close to Rebbe's lifetime.)

Which is why the Y-mi speaks about dawn and dusk in terms of the sun's
travel across the thickness of the raqi'ah. The discussion, at less
length but still involving R' Yehudah, R' Chanina and other Israeli
amoraim, is also in the Bavli.

That in turn gives me a chance to share a more recent thought. In Y-mi
Berakhos 1:1 (3b-4a), R' Yosi bei R' Bon raises the following topic: If
it is day when the sun is traveling up and around the inside the raqi'a,
and nighttime when it is above the raqi'ah (going over the dome back
from west to east), what is it when the sun is traveling the thickness
of the raqi'ah from one side to the other?

If both are night, then the night and the day are not equal on the
equinox, and yet we learn that tequfos Tishrei and Nissan are defined
when laylah veyom shavin! R' Huna learns it out from the respect
accorded to a king. We say the king is here when the first of his
entourage arrives, we say he is gone when the last departs. Even though
it is assymetric, maximizing the time we say the king is present.
Similarly, it is day while the sun is 

How do we understand R' Huna pragmatically, as a pesaq halakhah?

R' Goren's commentary on the Y-mi (which was cited by RYGB in
<http://yerushalmionline.org/audio/002.mp3>) suggests that Chazal were
talking about refraction.

What's refraction? It's why light bends when it enters a lens, or why a
spoon that sticks out of a glass of water looks broken at the point
where the spoon breaks the surface. It's the bending effect of entering
something denser (loosely speaking: the real term is "higher refractive
index") at an angle. It goes slower through glass or water than through
air, so one side is slowed before the other, causing bensing.

Light also goes slower through air than through a vacuum. So there is a
bending when sunlight enters the atmosphere.

We therefore see the sun slightly before it is physically at the
horizon. We also see it slightly after.

Second, because of this bending, the image of the sun when it's near
the horizon is going noticably slower than during most of the day.

I would therefore suggest something less radical than RSG's suggestion
that Chazal figured out about the top of the atmophere and refraction.
Let's take them at face value, they were discussing things in terms of
a pre-Ptolmeic astronomy. But what they were describing was something
very real.

They saw that at dawn and dusk, the sun doesn't move up or down much.
And as RYbRB says, even on the equinox, the day is assymetric -- we see
the sun for more than half this trek.

So they saw something real, that we would attribute to refraction,
but they attributed it to the width of the raqi'ah. This would allow us
(meaning: people who know the refractive index, how high the atmosphere
goes, etc...) to define R' Huna's implications about zemanim into our
own terms, and (if we hold like him) come up with a formula for the time
between alos and haneitz, and between sheqi'ah and tzeis.

Tir'u baTov!
-Micha

-- 
Micha Berger             One who kills his inclination is as though he
micha at aishdas.org        brought an offering. But to bring an offering,
http://www.aishdas.org   you must know where to slaughter and what
Fax: (270) 514-1507      parts to offer.        - R' Simcha Zissel Ziv