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**Article Four: Mercury**
[This article] describes the speed of the star Mercury, its straight motion, slowing down and stopping, its return movement and appearance of instability, its connection and proximity to the Sun.
Beloved! Know that astronomers have said: In Mercury’s course, it sometimes moves swiftly, sometimes directly, sometimes slows, sometimes stops, sometimes returns, and sometimes appears unstable in its path. The explanation for this phenomenon being observed in the movements of the star is as follows:
When the star is located at the upper part of the Epicycle, and its own center’s movement aligns with the movement of the Epicycle's center relative to the rising of the Signs, it appears to move swiftly. If it is inclined slightly towards the Epicycle, then it appears to move directly [506]. As it descends into the lower part of the Epicycle and is in a central position, it seems to move slowly, because the star’s own center is descending, so its movement is not visible, only the movement of the Epicycle's center is seen. And when the star approaches the lowest point of the Epicycle, its own center’s movement, which does not align with the rising of the Signs, coincides with the movement of the Epicycle’s center relative to the movement of the carrier sphere and the rising of the Signs; these two movements being opposite and counteracting each other, it appears as if the star is stopping. Finally, when the star reaches the lowest point of the Epicycle, its own center’s movement exceeds that of the Epicycle's center, so it appears to be returning. Once the star’s return movement is complete and the two movements equalize, it appears to stop again. Then it appears to stop again, and then seems to move slowly again. Because at this time, the star’s own center, as it crosses over the horizon of the Epicycle, makes its movement invisible; only the movement of the Epicycle's center is seen. After slowing down, it moves directly again, then accelerates. However, the star completes its revolution within its own epicycle in an orderly manner. For all spheres and those contained within them move simply, similarly, and linearly relative to their spherical belt. The position before the star’s return to its previous standstill is called the first station (makãm), and the position after it is called the second station.
The duration of Mercury's retrograde period is twenty-one days. The duration of its straight course is three months and five days. When the star is inclined three and four-tenths degrees north and south from the Signs’ belt on its carrier sphere, and when the Epicycle also has its Apparent Apogee and Apparent Perihelion inclined from its apparent inclined sphere, sometimes towards the north and sometimes towards the south, with a difference of approximately six degrees in latitude between them. While continuing its movements in this manner, this star appears unstable in its course and is named “hesitant” (mütehayyir).
Regarding the explanation for the connection and proximity that befalls the star Mercury relative to the Sun: Because the movement of Mercury’s carrier sphere is equal to the movement of the Sun's eccentric sphere, the center of its Epicycle has always been equidistant from the center of the Sun. In this situation, as the star Mercury revolves around the Sun with the movement of its Epicycle, it remains at a distance equal to the radius of the Epicycle from the Sun and thus orbits the Sun. Then, upon reaching the average apogees, it sometimes appears like a torch in the morning and sometimes in the evening, and sometimes is illuminated by the Sun. Mercury, being close to the solar course like the planet Venus, approaches the Sun twice in each revolution: once during half of its swift movement, that is, when it is at the Epicycle’s Apparent Perihelion, it always appears in a state of conjunction and proximity; and once during half of its return movement, that is, when it is at the Epicycle's Apparent Apogee, it is always found in a state of conjunction and proximity with the Sun. The time elapsed between these two approaches is two months.