सिद्धान्तशिरोमणि: गणिताध्याय (भास्कराचार्य - ग्रहगणित, मध्यमाधिकार व स्पष्टाधिकार सटीक)

474 the Sun will have to overtake Jupiter and not vice versa. When it is the Sun that is to overtake, taking a position of Jupiter near the western horizon gradually the planet gets fainter and fainter as the Sun approaches him from west to east. Ultimately one evening the planet ceases to be perceptible within a particular distance of the Sun. This we call heliacal setting of the planet. Having thus set in the west, after a few days the planet rises in the east. This is so because, after the planet's heliacal setting, the Sun gradually approaches the planet from west to east; at a particular moment will have a longitude equal to that of the planet and then gradually gains in longitude over the planet. After a few days, in the eastern horizon, the planet will emerge from the rays of the Sun before Sun- rise; it is not the planet moving west from east but it is the Sun moving from west to east so that the Sun recedes away from the planet towards east. The planet also will be having a motion from west to east but it being slower than that of the Sun, the latter gains over the former in its motion from west to east. In the case of the inferior planets, say for example, Mercury, the velocity of Mercury is greater than that of the Sun; as such, Mercury will be overtaking the Sun and not vice versa. Thus in the eastern horizon, it is Mercury that enters the rays of the Sun, going from west to east, so that he sets in the east. After a few days of this heliacal setting, Mercury acquires a longitude equal to that of the Sun, and while overtaking the Sun from west to east, he emerges out of the rays of the Sun in the west, which is therefore heliacal rising. But, when Mercury is retrograde, the case is different. Some days after heliacal rising in the west, Mercury attains his maximum elonga- tion and then begins to retrograde. The elongation then gradually decreases, and he will set again in the rays of the Sun in the west itself. A few days thereafter, still continuing retrograde, he emerges out of the Sun's rays