This past December, the skies graced us with a vibrant conjunction of Saturn and Jupiter — just in time for the holiday celebrations. Since that time, Saturn and Jupiter have been creeping ever-so-slowly toward the evening’s west horizon. Still beautiful, and still bright and still easily visible. But, if you thought two planets in conjunction were cool, imagine a three planetary conjunction. Between April 8 and 11, sky watchers can catch a glimpse of a low-horizon planetary trio in the western sky, best viewed about 30 minutes after sunset. Jupiter, Saturn and our solar system’s smallest planet, Mercury, will be in conjunction. It’s a sight not seen in almost six years and a great encore to December’s planetary display.
Mercury, well-known to astronomers for thousands of years and named after the Roman messenger god, is the smallest planet in our solar system and the closest to the sun. It is a rocky dense planet with a disproportionally large metallic core measuring 2,400 miles out of its 3,032-mile diameter. Only slightly larger than Earth’s moon, it is similar in color, with tremendously large craters and basins. For example, Caloris Basin is 960 miles wide and is one of the largest in our solar system. And if you thought the Grand Canyon was large, the largest cliffs on Mercury are hundreds of miles long and a mile high, whereas the lowest points on this small planet are found in the Rachmaninoff Basin — 3.34 miles below the average landscape. Did you catch that name? Yes, many of the features on Mercury are named after famous deceased authors, musicians and artists. Even Dr. Seuss has a named crater.
Mercury has a wildly eccentric orbit. At perihelion (when it is closest to the sun) it is at approximately 36 million miles from the sun. At aphelion (when it is farthest from the sun) it is 43,380,000 miles away. And it is fast. It travels through space at 29 miles a second (112,000 mph) and takes only 88 days for one revolution around the sun. That means your age on Mercury is nearly three times that on earth. Mercury also has a slow rotation making a day on planet Mercury 59 days long. Yet, a full day-night cycle (solar day) is 176 days. Confused? Think about like this: As the planet rotates (ever so slowly), and speeds swiftly around the sun, the time that a certain place on Mercury experiences day, night and day again would take 176 days. Each Mercury day lasts for about two Mercury years. Now, that’s a long day at the office!
There are two other interesting facts about Mercury. If you thought our summer and winter temperature changes were amazing here in the Midwest, consider this: Mercury’s daily temperature fluctuates from 800 degrees Fahrenheit during the day to minus-290 degrees Fahrenheit by night. That’s a 1,100-degree swing. The other interesting feature of Mercury is its magnetic field. The magnetic field is about 1.1% as strong as Earth's. And although that doesn’t seem like much, this small magnetic field sometimes interacts with the solar wind — those tiny particles and plasma which continue to stream from the sun into space. When this interaction occurs magnetic tornadoes of plasma are formed that then reach Mercury’s surface. Heat, frigid cold, and plasma tornadoes — Earth is sounding better all the time.
Take a look on Jan. 9 or 10 in the western sky just after dusk. Locate a trio of bright lights. Jupiter will be the brightest, followed by Saturn, and then Mercury. If you have a pair of binoculars, all three planets should be in your field of vision at once. And if you missed the triple conjunction, don’t worry. Mercury will get higher in the sky later in the evening throughout January peaking in brightness between January 24 during its greatest eastern elongation. In the meantime, consider naming the craters after your favorite artists, musicians or authors. What might you call them?