Giuseppe Piazzi at the Palermo (Italy) Observatory discovered Ceres on Jan. 1, 1801. At the time, it was dubbed a new planet. But when dozens more of these tiny bodies turned up over the coming years astronomers faced a dilemma. All the objects orbited between Mars and Jupiter and were tiny compared to the classical planets. Were they really planets or something else?
William Herschel, discoverer of the planet Uranus, suggested calling them "asteroids," a word that means "star-like," after aster, the Latin word for star. Through telescopes of the era, Ceres and its ilk showed no discernable disks but instead looked exactly like the pinpoint stars. Only in recent decades have we built instruments big and sensitive enough to make out their shapes and gross details. Asteroids are also called minor planets.
Serendipitously, Ceres was not only the first asteroid discovered but also the largest, with a diameter of 588 miles (946 km), or about one-third the diameter of the moon. It possesses enough mass for self-gravity to crush it into a nearly spherical shape, which elevates it to dwarf planet status, along with Pluto and the remote bodies Eris, Haumea and Makemake. Most asteroids have irregular shapes because they're not massive enough for self-gravity to mold them into balls.
Ceres orbits within the main asteroid belt at an average distance of 257 million miles (413 million km) and takes 4.6 years to circle the sun. Every 15 to 16 months it reaches opposition, when our planet and Ceres line up on the same side of the sun and come closest together. The next opposition occurs on Nov. 27, when it will peak in brightness at magnitude 7.0. While that's too faint to see with the naked eye, a pair of 50mm binoculars or a small telescope (or spotting scope) will nab it.
At the moment, Ceres glows at magnitude 7.8, not quite a magnitude fainter than it will become at opposition. While not ideal it's still bright enough to easily spot with the instruments described above. If you seek it now rather than wait, you can take advantage of a unique opportunity. The asteroid happens to be crossing the bright, easy-to-find Hyades cluster in Taurus during the next few weeks. To find Ceres you just need to locate the Hyades. Easy!
Face east around 10:30 p.m. in late October (9 p.m. after Nov. 7th, when we drop daylight time) and look about half-way up the sky for the Pleiades star cluster, also known as the Seven Sisters Cluster. This gorgeous twinkly mass resembles a bunch of grapes or a little dipper. Now, make a fist, extend your arm and look one fist below the cluster. You should see a bright, reddish-orange star. That's Aldebaran. Extending to the right of Aldebaran are fainter stars that outline the letter V tipped on its side. You've arrived! Those are the Hyades.
Next, aim your binoculars at Aldebaran and focus the star until it's a nice, sharp point. Then, with map in hand (on on-phone), pinpoint Ceres' location on the map for the correct date. If you see a "star" at that spot in your binoculars, congratulations! You've just spotted your first dwarf planet.
Ceres will slowly cross over the Hyades in the coming weeks. What's really cool is that it passes VERY close to Aldebaran from Halloween through Nov. 3. Closest approach occurs on Nov. 2 when just 1/4 of a full-moon-diameter will separate them. If you're into astrophotography, you can make a wonderful sequence of night-to-night photos of Ceres sliding past Aldebaran during this time.
NASA's Dawn Mission paid the dwarf planet a lengthy visit in 2015, so we have lots of data from and close-up photos of its rugged, cratered surface. The mission officially ended in 2017, and the spacecraft ran out of fuel in 2018, but it will continue to orbit Ceres for about the next 50 years. Thanks to Dawn we learned that Ceres is much icier than we thought, comprised of about 25 percent water. Most of it appears to be tucked away below the surface in the mantle.
The asteroid's surface was likely once riddled with cryovolcanoes — volcanoes that erupted water and other liquids like ammonia or methane instead of lava. Once expelled, the material froze and gradually built up towering volcanic peaks.
Made of ice, cryovolcanoes often relax and flow back onto the surface over geological time. On Ceres, one particularly young and fresh example survives to this day — Ahuna Mons. Although it's currently not active, Ahuna Mons makes an amazing sight, rising 2.5 miles (4 km) to its peak. Scientists also found ammonia-rich clays on Ceres and the remains of briny water seeps that welled up from below the surface. All these signs of good, old H2O make this unassuming little orb a potential habitat for microbial life.
You can think about all that while hot on the trail of Ceres this fall. Good luck! And please share your observations on my Astronomy For Everyone Facebook page.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.