8. The revolution of the moon.
All the zodiacal constellations are thus included in the nakshatras except Capricorn, for which Aquila and Delphinis are substituted. These, as well as Hydra, are a considerable distance from the ecliptic, but may perhaps be nearer the moon’s path, which, as already seen, slightly diverges from it. But this point has not been ascertained by me. The moon completes the circuit of the heavens in its orbit round the earth in a little less than a lunar month or 27 days 8 hours. As twenty-seven nakshatras were demarcated, it seems clear that a nakshatra was meant to represent the distance travelled by the moon in a day. Subsequently a twenty-eighth small nakshatra was formed called Abhijit, out of Uttarāshādha and Sravana, and this may have been meant to represent the fractional part of the day. The days of the lunar month have each, as a matter of fact, a nakshatra allotted to them, which is recorded in all Hindu almanacs, and enters largely into the Joshi’s astrological calculations. It may have been the case that prior to the naming of the days of the week, the days of the lunar month were distinguished by the names of their nakshatras, but this could only have been among the learned. For though there was a nakshatra for every day of the moon’s path round the ecliptic, the same days in successive months could not have the same nakshatras on account of what is called the synodical revolution of the moon. The light of the moon comes from the sun, and we see only that part of it which is illuminated by the sun. When the moon is between the earth and the sun, the light hemisphere is invisible to us, and there is no moon. When the moon is on the opposite side of the earth to the sun we see the whole of the illuminated hemisphere, and it is full moon. Thus in the time between one new moon and the next, the moon must proceed from its position between the earth and the sun to the same position again, and to do this it has to go somewhat more than once round the ecliptic, as is shown by the following figure.[6]
Fig. 2.—Revolution of the Moon round the Earth.