Midrash for Shabbat 149:3
מגושתא רב ושמואל חד אמר חרשי וחד אמר גדופי תסתיים דרב דאמר גדופי דאמר רב זוטרא בר טוביה אמר רב הלומד דבר אחד מן המגוש חייב מיתה דאי ס"ד חרשי הכתיב (דברים יח, ט) לא תלמד לעשות אבל אתה למד להבין ולהורות תסתיים
and he who is able to calculate the cycles<span class="x" onmousemove="('comment',' Sc. of the seasons. ');"><sup>4</sup></span> and planetary courses but does not, one may hold no conversation with him.<span class="x" onmousemove="('comment',' The science of astronomy was necessary for the fixing of the calendar, upon which Jewish Festivals depended. In early times this was done by observation, but gradually calculation took its place. Hence Rab's indignation at one who fails to employ such knowledge. ');"><sup>5</sup></span>
Sifrei Devarim
"you shall not learn to do as the abominations of those nations": I might think that you are forbidden to understand and teach (the falsity of their ways); it is, therefore, written "to do." You may not learn them to do them, but you may learn them to each and to understand.
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