The Motion of Mercury

But of all celestial motions, the most marvelous is that of Mercury, which follows paths so nearly untraceable that they are not easily investigated.

Adding to the difficulty is the fact that Mercury is mostly invisible, as it moves within the rays of the Sun, appearing only for a very few days before disappearing again. Nevertheless, it too can be understood—provided that one applies a sufficiently keen intellect.

Like Venus, Mercury also has two epicycles that revolve within its orbit. The larger epicycle moves in step with the orbit, much like in Venus, positioning its apse 14.5 degrees beyond the bright star Spica in Virgo. The smaller epicycle, however, rotates in the opposite manner, completing two revolutions for every one of the larger epicycle, so that in any position of the Earth, when it aligns with or directly opposes Mercury’s apse, the planet is at its farthest distance from the center of the larger epicycle, while in the quadrants, it is at its nearest point.

We have stated that Mercury’s orbit returns in three months, or 88 days. Its semidiameter measures 9 and 2/5 parts, where the semidiameter of the great orbit is taken as 25 parts. From this, the first epicycle measures 1 part and 41 minutes, while the second epicycle measures about 34 minutes, or one-third of the first.

However, in the case of Mercury, this system of circles is insufficient, unlike in other planets. When the Earth moves through its apse alignments, Mercury appears to move within a much smaller orbit than what the previous explanation of circles would imply; yet, at the quadratures, it appears to move in a much larger orbit.

Since no other irregularities in its longitude are observed, it follows that these variations must be due to an oscillating approach and retreat of the orbit’s center along a straight line. This requires two additional small orbits, both having axes parallel to the axis of the main orbit. The center of the larger epicycle remains at a constant distance from the center of the immediately enclosing orbit, just as this orbit’s center is equally distanced from the outermost orbit’s center.

This displacement has been determined to measure 14 minutes and 30 seconds of a single 25-part division of the entire system. The motion of the outermost small orbit completes two full revolutions per year, while the inner orbit, moving in a reflexive manner, completes four such returns.

This composite motion causes the center of the larger epicycle to shift back and forth in a straight line, much like the oscillations in latitude we discussed before.

Thus, when the Earth is at the apse of Mercury’s orbit, the center of the larger epicycle is closest to the center of the main orbit; but at quadrature, it is at its farthest distance. At midway points (45-degree separations), the center of the larger epicycle aligns with the center of the outer orbit, and the two coincide. The magnitude of this approach and retreat amounts to 29 minutes of the same 25-part measurement system.

Thus far, this describes the longitudinal motion of Mercury.

Mercury’s Latitude

Mercury’s latitudinal motion follows a pattern similar to Venus, but always in the opposite direction. When Venus moves northward, Mercury moves southward. Mercury’s orbit is inclined to the ecliptic by seven degrees. However, its maximum deviation—which remains entirely southward—never exceeds three-quarters of a degree. Otherwise, everything previously stated about Venus’ latitude also applies to Mercury and need not be repeated.

The Number of Planetary Circles

Thus, Mercury moves in a system of seven circles, Venus in five, the Earth in three, and the Moon in four around it. Mars, Jupiter, and Saturn each have five circles.

In total, 34 circles suffice to explain the entire structure of the cosmos and the entire dance of the stars.