Halakhah sobre Exodo 12:2

We find that the light of the sun and of all the stars is eternal, of itself it does not increase nor decrease. However, the light appears to increase or decrease depending on whether the object being viewed is far or near. Also, the light varies due to changes in the atmosphere at the beginning or the middle of the day. Only the moon has a renewal of its light.¹ Ibn Ezra believes that the sun, all planets and all stars, generate their own light, and only the moon reflects light of another body (the sun). Therefore, the Hebrew term for month, “hodesh,” can truly apply only to a lunar month.² In Hebrew, a month is called “hodesh,” which is derived from the root “hidesh” meaning “renew.” Since only the moon has a renewal of its light, “hodesh” can only refer to a lunar month. We examine when such a month begins.

One revolution, which includes all the spheres, is from east to west. The twelve zodiacal constellations¹³ The order of the zodiacal constellations and zodiacal signs is: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. complete a revolution in twenty-four hours,¹⁴ This is known as a “sidereal day,” which is the interval between two successive passes of the vernal equinox point over the meridian. A sidereal day is slightly less (by about four minutes) than a “solar day,” the interval of time between two successive passes of the sun across the meridian. Apparently Ibn Ezra used sidereal time rather than solar time. and the seven planets¹⁵ The seven planets known at that time are: Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn. also finish their revolutions in approximately the same amount of time. The second revolution is from west to east. It also includes all of the spheres, for the poles of the spheres of the planets are similar to the poles of the zodiacal sphere.¹⁶ All of the lower eight spheres that contain the planets and the fixed stars rotate at various rates around the earth from west to east. Only the sun maintains the path of the ecliptic, not deviating south nor north. It traverses the complete zodiac in 365 days, five hours, and fractions of an hour. This is a solar year and the true year, for the days return a second time to what they were in the preceding year. For this reason a year is called “shana” (repetition).

All astronomers agree that the lunar month begins at the moment when the moon and the sun are in conjunction in the same longitude. This is what our Rabbis called the “molad ” (birth, i.e., new moon). Our scholars calculated it for the mean orbit,³ This figure is given in the Talmud: 29 days, 12 hours, and 793 halaqim (Rosh haShana 25a), or 29 days, 12 hours, 44 minutes, and 3 and ⅓ seconds, approximately 29.5306 days. Such a month is known as a “synodic month” and is defined as the mean time between new moons. The current approximation for a synodic month is 29 days, 12 hours, 44 minutes, and 2.7 seconds, or 29.5306 days. and all astronomers did the same. Then they adjusted it.⁴ “Mean conjunction” is conjunction of the sun and the moon relative to their individual spheres. However, since the moon’s sphere, the sun’s sphere, and the earth are not concentric, conjunction relative to the zodiacal sphere may differ from conjunction relative to the individual spheres. Conjunction relative to the zodiacal sphere is called “true conjunction.” Thus our Rabbis, their memory should be a blessing, said: “Sometimes it comes by a long path and sometimes it comes by a short path” (Rosh haShanah 25a). Since conjunction involves two bodies, we must know with regard to each one when it is a long path or a short path. Sometimes both are long, or both short, or the sun long and the moon short, or vice versa. Also, sometimes the length or shortness is small, sometimes large, to the extent that the length or the shortness may be as much as thirteen hours. Thus at times there is a difference between our calculation of conjunction and true conjunction of these many hours, either earlier or later.