The evening sky this month offers a great opportunity to see the Solar System’s two planetary extremes among the five bright naked-eye planets in terms of their orbital motion—sprinting Mercury and sluggish Saturn!
The word “planet” means “wanderers.” And the five planets known since antiquity have certainly earned that name, continually moving around the sky as they do among the fixed stars of the zodiac. The fastest and innermost one is Mercury, which takes just 88 days to orbit our Daytime Star while the outermost is Saturn requiring a leisurely 30 years to do so. (We see this same thing in relation to the artificial satellites circling the Earth—the lower they are in orbit the faster they move.)
Around mid-month in January, Mercury begins to appear out of the twilight glow after sunset. By the 24th, it will stand 19 degrees above the western horizon and be obvious to the unaided eye. Within just a few weeks it will have disappeared below the horizon and become a pre-dawn object. So fast does it move and brief are its appearances in the sky that some early astronomers thought that this was actually two different planets—an evening one and a morning one! Looking at the planetary table on the back of the Star and Planet Locator, Mercury’s rapid movement makes it impossible to list what constellation it will be in each month. As a result, its position is simply listed as being in either the evening (EVE) or morning (MOR) sky.
In marked contrast is the beautiful ringed planet Saturn, which moves so slowly that it often stays in a given constellation for a year or more. On the 15th of December, it crossed the border from the constellation Capricornus into Sagittarius—where it was joined with Jupiter in last month’s spectacular double conjunction on the 21st (highlighted in the December Sky Talk column). While the two worlds are still near each other, they are setting below the southwestern horizon where they will slowly move into the eastern morning sky. It’s not surprising that keeping track of the orbital antics of these and the other planets took early astronomers centuries to finally unravel.
Before closing, mention should be made that the annual Quadrantid Meteor Shower will happen on the night of January 2nd to 3rd. Normally, as many as “40 shooting stars” an hour might be seen radiating at its peak from the constellation Bootes, but a gibbous Moon will hide all but the brightest of them.
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of 10 books on stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.