This month shall be to you the head of the months; to you it shall be the first of the months of the year. (Exodus 12:2)
In simple terms, the verse above indicates that Nissan should be counted as the first month in the cycle of the festivals. In addition, Rashi (d. 1105) describes how God explained to Moses how each month should begin at the renewal of the moon. The word hazeh – this, implies that God waited until it was dark to teach Moses precisely how to calculate the time of the New Moon from the moon’s appearance (Rashi ibid.).
The Talmud (Sanhedrin 42a) records that our verse is also the source for the ceremony of Kiddush levanah – blessing of the moon, which usually takes place after Shabbat following Rosh Chodesh (see Chief Rabbi’s siddur page 602). Maimonides (d. 1204) combined his knowledge of astronomy with Halacha (Jewish law) to record lengthy and complicated calculations which determine the visibility of the new moon (Kiddush HaChodesh chapters 11 to 19).
Although the Hebrew calendar is now fixed there has been a long tradition of Jewish astronomers who made important contributions to the field. The Talmud states that our patriarch Abraham was an expert in astronomy who was sought after by kings from the east and west to advise them on astronomical matters (Bava Batra 16b). Rebbi Yehoshua ben Chanania knew of a particular star that would appear once in seventy years and would confuse the sailors (Horayot 10a) while the Amora Shmuel boasted that the paths of the heavens were as clear to him as the roads of his own city (Berachot 58b).
Other great Jewish astronomers included Ibn Ezra (d. 1164) and Ralbag (d. 1344) who invented an astronomical viewing device called Mateh Yaakov (Jacob’s staff), which measured angles between heavenly bodies and was used by sailors for navigation. Rabbi Avraham Zakut (d. 1510) calculated astronomical almanacs which the explorer Christopher Columbus used during his travels.
Whether it is cosmological calculations, medical knowledge or advances in technology, scientific understanding is critical for the application of modern Jewish law.