Both Uranus and the asteroid 8 Flora came to opposition on Halloween. Catch them both in the same corner of the sky the next clear night.
Planets are often helpful when it comes to finding other celestial objects. Jupiter pointed the way to Pluto for many of us this past summer. In the same way, Uranus shares the sky with one of the larger main-belt asteroids this month — 8 Flora. Both objects came to opposition on Halloween and remain within 11.5° degrees of each other through at least next January, making for a celestial two-for-one special.
Located far from cozy Earth at a distance of more than 1.7 billion miles (2.8 billion km), Uranus was the first planet discovered with a telescope. William Herschel found it by accident on the night of March 13, 1781, while examining double stars in search of optical pairs he hoped to use in stellar parallax measurements. Here is his account:
"On Tuesday, the 13th of March, between ten and eleven in the evening, while I was examining the small stars in the neighborhood of H Geminorum (1 Gem), I perceived one that appeared visibly larger than the rest; being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet."
Herschel first saw Uranus at 227× using his homemade 7-foot telescope, equipped with a 6.2-inch (15.7 cm) polished metal alloy mirror made of a mixture of copper and tin called speculum metal.
Like amateurs today, he knew that using higher magnifications on stars won't increase their apparent size but will expand a comet or planet. To confirm his observation, he increased the power to 460× and 932× and watched his "comet" grow, confirming that it belonged to the solar system. Subsequent study of its orbit revealed that Sir William had found something far more exciting — the first new planet since antiquity.
Although nearly four times as large as Earth, Uranus's great distance places it right at the border of visibility. If you've never seen the seventh planet without optical aid, now's the time. At magnitude 5.7, Uranus is bright enough to see with averted vision from a rural, moonless sky. It's super-easy in binoculars, where it looks like a modest star slowly inching across the sky week by week.
You'll need a 3-inch or larger telescope magnifying around 100× to discern the planet's frosty blue hue and tiny 3.8″ disk. Methane gas, which absorbs red light and reflects back the blue, is responsible for its color.
The Sun shines 390 times dimmer at Uranus than it does on Earth and looks a lot smaller, just 1.5′ across, or one-twentieth as big as "our" Sun. Through a safe solar filter, it would resemble a pinhead. Like Jupiter, Saturn, Venus, and Neptune, perpetual clouds blanket Uranus. The clouds of Venus are composed of sulfuric acid droplets while those of Jupiter and Saturn are made of ammonia ice. Uranian clouds are made of noxious hydrogen-sulfide ice and would easily win the award for stinkiest in the solar system — one whiff and you'd recoil from the smell of rotten eggs.
Uranus holes up in the constellation Aries this autumn, a little more than two fists (22°) to the left or east of Mars. It slowly crawls westward in retrograde motion until resuming direct (prograde) motion on January 14, 2021. Mars resumes direct motion on November 15th and gradually approaches Uranus until overtaking it in conjunction on January 20th.
Through the telescope and in photos taken without special filters Uranus looks like an aqua-hued cue ball, but its bland visage hides a unique personality. Winds up to 560 miles per hour (900 kph) roar through the atmosphere, where the temperature dips to a staggering 370° below zero Fahrenheit (50 K). Deep below the cloud deck, pressure and heat increase rapidly, converting water, methane, and ammonia into a hot, dense fluid that circulates above a small, rocky core. Computer models indicate that temperatures near the core reach 9,000°F (4,982°C).
An enormous impact with another planet-sized body eons ago knocked Uranus on its side, tilting its axis by 98°. Uranus has the most extreme seasons of any planet in the solar system. For one-quarter or 21 years of its 84-year orbit, the Sun shines directly over the North Pole while the South Pole experiences 21 years of winter. Half an orbit later the situation is reversed. Here on Earth, winter's my favorite season, but if I moved to Uranus I might reconsider.
Uranus has 27 known moons, two of which are visible in 10-inch and larger telescopes — Titania at magnitude 13.9 and Oberon at 14.1. Look for them around the time of maximum elongation when their separations from the planet are approximately 30″ and 40″, respectively. Both are faintly visible in my 15-inch reflector at 245× with averted vision. Ariel (magnitude 14.3), which lies just 10″ from the planet and is strongly affected by its glare, will take more firepower. I spotted it once in a friend's 24-inch Dob on a night of settled seeing. Sky & Telescope's Moons of Uranus observing tool will help you find them.
Uranus possesses 13 known faint rings composed of ice and possibly organic compounds, which are thought to account for their dark color. The planet also has a weird, lopsided magnetic field that regularly opens and shuts. When open, it can direct charged particles from the solar wind into its upper atmosphere, where they spark peculiar displays of the aurora borealis.
The next clear night when the earthly wind blows hard and cold, instead of huddling inside, uncap your binoculars and spend a few minutes shivering with the seventh planet.
Less than a dozen degrees southeast of Uranus, you'll find the main belt asteroid 8 Flora. At 81.7 million miles (131.4 million km) away, it's almost 21 times closer and completes an orbit in 3.3 years compared to the ice giant's 84 years. I saw it faintly at magnitude 8.1 on October 26th in 8×40 binoculars, even with a bright gibbous Moon in the sky. With the Moon now departing, it will be much easier to spot. Flora begins November at magnitude 8.1 and ends the month at 8.8.
Flora was discovered in 1847 and named for the Latin goddess of gardens and flowers. It's about 147 kilometers across — comparable in size to the state of Connecticut — and the largest asteroid in the inner main belt. It's also one of the brightest.
Based on minerals detected in its reflectance spectra, heat appears to have altered Flora's surface in the past. Planetary scientist Michael Gaffey thinks the asteroid "is probably the residual core of an intensely heated, thermally evolved, and magmatically differentiated planetesimal, which was subsequently disrupted." This dovetails with the fact that Flora is a member of the Flora family of asteroids which number more than 13,000. The pieces are likely the remains of a single larger object disrupted by impact.
That means it shares at least one quality with Uranus — an evolutionary path altered by a major collision. There's no escaping the chaos and violence of the early solar system. Look close enough, and it's stamped into everything.