Papers of John Guy Porter

Two copies of Porter's script for 'The Night Sky in September' on the lunar eclipse of 5 September and the partial solar eclipse of 2 September, both unseen from England; Mars rising before midnight, between Aldebaran and Capella, and growing brighter; the mystery of the moons of Mars; Jupiter setting before midnight, the eclipse sequence of its moon Callisto commencing once more, and the four large moons of Jupiter discovered by Galileo in 1610, their eclipses visible in a small telescope; Callisto being able to miss the shadow of Jupiter, but now entering a new eclipse cycle; the appearance of the five visible moons from Jupiter's surface; the size of the four large moons, the distance and period of Io and the dependence of this period on the mass of Jupiter; the brightness of Jupiter's moons being much less than our Moon due to their greater distance from the Sun, the first three satellites never having a full moon phase as they are always eclipsed, and Callisto (the fourth satellite) now entering a three-year period when it will be eclipsed every 16 days; the predictions of these phenomena found in the 'Nautical Almanac' (now the 'Astronomical Ephemeris') since 1767, with diagrams or graphs, and the international cooperation involved in this work; the fifth moon (the closest to Jupiter) being the fastest moving satellite in the solar system; the discovery of seven other moons by photography and their arrangement in groups, with the outer four in retrograde motion; the effect of the Sun on the satellites, their highly complex orbits just now being calculated from theory; and the return to fashion of satellite astronomy.