Bharatiya Jyotihshastra (History of Indian Astronomy), translated by R. V. Vaidya v2.c12.s9.5

v2.c12.s9.5

When planets and stars are in the proximity of the sun, they are not visible before sunrise and after sunset, even though they are themselves on the horizon and the sun has not appeared above the horizon. The planets and stars thus continue to remain invisible for a week, a fortnight or for some months. After the stars and planets first become visible, their distance from the sun gradually diminishes till at last a day comes when they become invisible. They are then said to have ‘set’ on that day. Again, after they first become invisible being near to the sun, their distance from the sun begins to increase gradually, till a day comes when they emerge from its light and become visible. This phenomenon is termed the ‘rise’. The diurnal phenomenon of the appearance and disappearance above and below the horizon of the stars and planets is called the ‘udayāsta’ (i.e. rise and set) ; and their disappearance due to their proximity with the sun and reappearance,—these phenomena also are termed ‘astodaya’ (i.e. set and rise). It will thus be seen that the words ‘rise and set’ are used in two senses. It is desirable that the two phenomena should be known by different names. These are distinct terms in the case of the moon. During the dark half of the month as the moon gradually approaches the sun, her distance diminishes, till on the new moon day she disappears and then makes her first appearance in the west on the first or second lunar day after the new moon day. The phenomenon is then called Candra-darśana (first appearance of the moon). They do not characterize it as the rising of the moon. It is similarly desirable to speak of the first appearance of stars and planets after they have disappeared owing to their proximity with the sun, as their ‘darśana’ (emersion) and their first disappearance as the ‘adarśana’ (immersion). But most of our astronomers have indiscriminately used the terms ‘Udayāsta’ (rise and set) even for the phenomena of appearance and disappearance due to their nearness to the sun and the same terms are in use at present. People give much consideration to the daily rising and setting of the moon and it is very necessary for them. Similarly people use to notice its first appearance when it is very near the sun; hence in her case they naturally found it necessary to use different terms for the two phenomena. But no one ever cares much to think about the daily rising and setting of planets and stars; and hence it appears that they began to apply the term “rise and set” to their appearance and disappearance depending upon their proximity to the sun.