[Avodah] Modifying Halachos based on scientific findings?

[Galsaba at aol.com]

Do we change Halachos based on scientific findings?
The two I am referring to are Birchat Hachama. 
On this one we hold by Amora Shemuel Calendar. A year is 365.25 days, 
although scientifically we know that a year is closer to 365.24 days. We did not 
change the cycle of blessing the sun, we do it every 28 x 365.25 days.
 
On the other hands, the measure Revi'is, to those that hold the Mida Ketana 
(the smalller measurement),
we used to measure 86.4 cc, according to Harav Na'eh.
Recently, I read a few articles that shows that Harav Na'eh based his 
measurement (86.4 cc) on the coin called Dirham.  According to the Rambam, Reviis is 
27 Dirham.
The weight of a Dirham (according to Harav Na'eh) is 3.2 gram.  However, the 
new articles claims that 3.2 grams was the weight of the Dirham during the 
Ottoman regime in Eretz Israel, but in the time of the Rambam it was about 2.8 
grams, therefore Revi'is is 75 grams.
 
In this case it looks like we change Halacha based on archaeologic findings 
(that claims that Dirham in the time of the Rambam was 2.8 grams.)
 
Can we do that?
 
galsaba
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