Joe Schoemann wrote:
Concerning the flood of Noah, Rashi writes (Bereishit 8:4) it took four days to reduce one cubit of water because it took sixty days to go down 15 cubits. My question is, scientifically, this is not necessarily true, being that that the earth is round, so as you get higher off the ground the circumference gets larger - i.e., more water per cubit of height. So how can Rashi assume that it took each cubit the same amount of time?
Dear Joe Schoemann,
You're absolutely right! If a ball of water loses water at a constant rate, so the smaller it gets, the faster the outside layer shrinks. Why, then, does Rashi seem to assume that the water's height went down at a constant rate? That's a good question.
But let's think. How big would a ball of water be if it circled the earth? Well, it's about 4,000 miles from the earth's center to the mountaintops, so a ball of water around the earth would be 4,000 miles from center to surface.
Now, Rashi bases his calculations on a period when the water went down 15 cubits - approximately 23 feet. Compared to 4000 miles, 23 feet is about one ten-thousandth of one percent!
So, you're absolutely right! Rashi's figures are not exact. They are off by a miniscule amount. Now, this rough approximation could be made less rough by calculating the relative volume of water absorbed during each four-day period during the sixty days, using the formula V=4/3 pi r3.
By the way:
Q: Where was Noach when the lights went out?
A: In d'ark.