Dance of the Planets May 2011
June 07, 2011
This composite time-lapse photograph of the positions of Jupiter and Venus, as viewed from east of Salt Lake City, Utah on May 12 through May 15, illustrates the complexity of the motions of the planets, Sun and Moon. Unraveling the true motion of the planets, illustrated in the schematic below the photo, took astronomers 1800 years to achieve. The photo was prepared by setting up a digital camera on a fixed tripod, pointing it at the same point on the horizon, and taking images on successive days between 5:25 a.m. and 5:45 a.m. The planets' positions were superimposed on a base image taken May 1 -- this image includes the waning crescent Moon. A photo of the Sun, taken through a solar filter on May 15, was composed into the image as it would have appeared on May 1, and so the location of the Sun and Moon roughly marks the ecliptic on May 1. Image scale is provided by the Moon and Sun, both of which are approximately one-half degree in diameter.
Several planetary motions are illustrated. After a May 9 conjunction, Venus appears to travel north along the east local horizon and toward the rising Sun. Jupiter grows more distant from Venus each day and travels along a line approximately perpendicular to Venus's path. The ecliptic and the point of Sunrise shift to the north each day.
These seemingly contradictory motions can be visualized as three cars driving down a curved highway as shown in the schematic. The Earth is green, Venus gray and Jupiter orange -- distances are not to scale. Venus, on the inside of the curved track, moves the quickest, and Jupiter is slowest. As Venus rounds its orbital curve on the opposite side of the solar system from the Earth (when viewed from the Earth) it appears to travel towards the Sun. But the Earth is moving faster in its orbit than Jupiter -- about one degree per day versus Jupiter's one-twelfth of a degree. Consequently, Earth appears to overtake Jupiter on the solar system's orbital highway, and an observer on the Earth looking east at sunrise sees Jupiter rise towards the southern horizon. Note that Mars and Mercury are also within this image frame during this rare planetary grouping but were too faint against the brightening morning sky to be recorded by my camera.