Transit of Phobos from Mars north of 70.4°N; for some days in the year, its shadow misses the surface entirely and falls north or south of Mars. At any given geographical location on the surface of Mars, there are two intervals in a Martian year when the shadow of Phobos or Deimos is passing through its latitude. During each such interval, about half a dozen transits of Phobos can be seen by observers at that geographical location (compared to zero or one transits of Deimos). Transits of Phobos happen during Martian autumn and winter in the respective hemisphere; close to the equator they happen around
Moons of Mars satellites, Earth had one and Jupiter had four (known at the time), that Mars by analogy must have two. Furthermore, as they had not yet been discovered, it was reasoned that they must be small and close to Mars. This would lead Swift to making a roughly accurate estimate of their orbital distances and revolution periods. In addition Swift could have been helped in his calculations by his friend, the mathematician John Arbuthnot. Voltaire's 1752 short story "Micromégas", about an alien visitor to Earth, also refers to two moons of Mars. Voltaire was presumably influenced by Swift. In recognition of