(1) The problem involved the advance of the perihelion of the planet Mercury.(2) He calculated to within a month the return in 1759 of Halley's comet to its perihelion (closest point to the Sun).(3) The radius vector is shortest when the planet is at perihelion and longest at aphelion.(4) today Mercury is at perihelion(5) This affects the climate because the influx of solar energy to our planet would then vary between perihelion and aphelion by a larger proportion than at present.(6) For objects orbiting Earth, periapsis and apoapsis are known as perigee and apogee; for objects orbiting the Sun, these locations are known as perihelion and aphelion.
