VS. 6-7] ILLUMINATED PART OF THE MOON 85 360 degrees of time are equivalent to 60 ghaiis or 21600 asus, so that one degree of time is equivalent to 1/6 of aghatl or 60 asus. Thus 12 degrees of time are equivalent to 2 ghatis. On the fifteenth lunar day of the dark half of the month, the Moon comes near the Sun from behind and is lost in his splendour. After about two days she is beyond the limit of invisibility and is again seen in the sky af .er sunset being in advance of the Sun. In order to see whether the Moon will be visible on the first or second lunar day of the light half of the month, one should calculate the (tropical) longitude of the Sun for sunset on that day and also for the same time the (tropical) longitude of the Moon as corrected for the visibility corrections. If the portion of the ecliptic lying between the Sun and the Moon thus obtained sets at the local place in two ghaiis or more, the Moon will be visible after sunset on that day, otherwise not. Similarly, in order to see whether the Moon will be visible before sunrise on the fourteenth or fifteenth lunar day of the dark half of the month, one should calculate the (tropical) longitude of the Sun for sunrise on that day and also for the same time the (tropical) longitude of the Moon as corrected for the visibility corrections. If the part of the ecliptic lying between the Sun and Moon thus obtained rises at the local place in two ghatis or more, the Moon will be visible before sunrise on that day, otherwise not. A rule relating to the determination of the measures of the illumi- nated and unilluminated parts of the Moon : 6-7. (In the light half of the month) the Rversed-sine of the difference ( between the longitudes of the Moon and the Sun) multiplied by the true diameter of the Moon and divided by 6876 gives the measure of the illuminated part (of the Moon). When the difference exceeds a quadrant, one should add the radi- us to the Rsine of the excess and from that (find) the measure of the illuminated part. In the dark half of the month, one should obtain in the same way, the unilluminated part (of the Moon) with the help of the Rversedsine (of the difference between the longitudes of the Moon and the Sun diminished by 6 signs) and from the Rsine (of the excess of that difference over a quad- rant). 1 'Gf . MBh, vi. 5(ii)-7.
