As Venus has been lingering over the western horizon in the evenings, Jupiter has
been rising in the east.
Tonight it will rise at 6:30 p.m., 25 minutes before Venus sets. Normally, the planets move eastward with respect to the stars due to their and Earth's orbital motion about the sun. However, Earth moves faster in its orbit than the outer planets so as we catch up and pass them, they appear to move backwards (westward) with respect to the stars. It was this looping motion that fascinated ancient astronomers, leading Ptolemy to create his complicated system of geocentric epicycles to predict their positions, while Aristarchus (c. 270 B.C) and other Greeks proposed that all the planets orbited the sun.
Nicolaus Copernicus cited Aristarchus in an early draft of his De revolutionibus orbium coelestium that brought the geocentric system back into western thought. Galileo's and Tycho Brahe's careful observations led the mathematician Johannes Kepler to develop his three laws of planetary motion describing orbits as ellipses in which each planet's orbital speed varies with its distance from the sun and those farther from the sun move more slowly.
Earth's orbital distance varies by about 3 percent between perihelion at 91.4 million miles to aphelion at 94.5 million miles. Our speed thus varies between 67,760 mph and 65,500 mph. The next perihelion will occur On January 4, 2014, at 6:59 am EST. Thus we will be closest to the sun and moving fastest in our orbit. This speed results in fewer days of fall and winter, 178 days between the Autumnal Equinox (last on Sept. 22, 2013) and Vernal Equinox (next on March 20, 2014), than of spring and summer, 186 days between the Vernal Equinox and Autumnal Equinox (next on Sept. 22, 2014). This is why February is a short month.
Some people are surprised to discover that we are closest to the sun when we in the north are experiencing winter. It seems they must have missed school on the days when they were taught that the seasons are due to the changing angle of the sun as Earth, with its axis tilted 23.5 degrees from its orbital plane moves around the sun. In Tupper Lake, the sun's highest noontime angle above the horizon, 69 degrees occurs on the summer solstice (next on June 21, 2014) while the lowest is a mere 22 degrees (next on Dec. 21, 2013). This is why it seems like late afternoon even at noon during these December days.
Venus, on its faster, inner orbit, zips along at an average of 78 thousand MPH and Mercury at a blazing 109 thousand mph. Given their shorter paths around the sun their years are correspondingly short at about 7.5 and three months, respectively.
The outer planets move much more slowly and have longer years. It takes Jupiter slightly less than 12 years to orbit the sun as it lumbers along at only 30 thousand mph on its orbit five times larger than Earth's. Hence we are again in the process of passing Jupiter. On Nov. 7, Jupiter changed from prograde motion to retrograde motion westward and will loop through Gemini during this winter. On the diagram which shows the orientation of the sky at 4 tomorrow morning (turn it 90 degrees to the left to match the early evening sky) Jupiter is shown just northwest of Wasat (Wah-SAAT, meaning "middle").
From there, it will move between the cosmic twins of Castor and Pollux toward the brighter star Mebsuta (Meb-SUIT-uh, the outstretched lion's paw).
Jan. 5 is the date we actually pass Jupiter, putting it exactly opposite the sun in our sky. This is also when it's closest to us at 391 million miles. On Jan. 7, 1610, Jupiter was also at opposition, near the waxing gibbous moon (between 1st quarter and full) so Galileo swung his telescope to observe this world. He saw three bright spots in a line to either side of the planet and assumed they were stars. However, on the next evening all three were to the west of Jupiter. He knew that Jupiter was moving westward and had assumed the three stars would be to the east of it on that second night. Puzzled, he continued to watch Jupiter. Eventually he saw a fourth body and figured out that they were moons orbiting the giant planet. They are the largest of Jupiter's 67 known moons.
In order outward from Jupiter, they are Io (EYE-oh, usually, though EEE-oh is also heard) covered with active volcanoes and Europa with a water ice surface underlain by a liquid ocean, both about the size of Luna, Earth's moon. Ganymede (GAN-uh-mead) that is 8 percent (383 km) larger than Mercury and Callisto is 1 percent (58 km) smaller than Mercury.
With our roll-off roof facility up and running, astronomers of the Adirondack Public Observatory are eager to dazzle you with telescopic views of the cosmos every other Friday night (next on Dec. 20).
Go to the APO web site at apobservatory.org and click on "events" for more information and directions to our site above Little Wolf Pond in Tupper Lake. Listen for me on North Country Public Radio about once a month during "The Eight O'Clock Hour" or email me with any questions at firstname.lastname@example.org.