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.

Uranus and rings
In 2005 astronomers used the Hubble Space Telescope to photograph Uranus's delicate ring system, a southern collar of clouds and a bright, discrete cloud in the northern hemisphere.
NASA / ESA / M. Showalter (SETI Institute)

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.

Herschel's 7-foot telescope
This is a replica of the 7-foot, 6.2-inch reflector that William Herschel used to discover Uranus. 
Alun Salt / CC BY-SA 2.0

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.

Path of Uranus 2020-2021
Uranus spends 2020 in southern Aries above the head of Cetus. Click on the image for a larger version. Use the detailed map below to track it.

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.

Path of Uranus 2020-2021
Uranus is currently moving slowly westward in retrograde motion. Weekly observations in binoculars will show its changing position. Click on the image for a larger version of the chart and here for a black-and-white PDF.

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).

Uranus's orbit
Our view of Uranus's rings depends on the planet's location in orbit. At the equinoxes the rings appear edge-on, while during the solstices they're face-on.
NASA / ESA / M. Showalter (SETI Institute)

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 moon finder
The Uranus moon finder will help you find a quintet of Uranian moons. Click on the image to access.

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, Earth, and Flora
Uranus, the third largest planet in the solar system, is four times larger than Earth and 346 times the size of Flora. Neptune, though smaller, is more massive than Uranus.
Earth and Neptune / NASA. Flora approximation provided by the author

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.

Uranus and Flora occupy the same section of sky this season. Flora's position is marked every 5 days (dates are for evening hours over the Americas) with stars to about magnitude 8.8.

Asteroid Flora

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.

Asteroid Flora
This is a shape model of Flora computed using light curve inversion techniques.
J. Hanuš, J. Ďurech, M. Brož, B. D. Warner (June 2011)

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.



Image of TomR


November 4, 2020 at 1:02 pm

Hello Bob,
when I read your articles, I always feel like a king! 🙂
I also saw Flora in my 7x42 binoculars during the last days of October. Its brightness seems to vary a lot from opposition to opposition due to its elliptical path.
Thomas, Austria, 49° N

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Bob King

November 14, 2020 at 12:11 pm

Hi Tom,
Sorry for the late reply! I'm delighted you feel like a king. Remember to rule with wisdom and a big heart 🙂

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November 29, 2020 at 10:58 pm

Hello Bob,

I have read some of your stuff and am always amazed with your knowledge and willingness to share. One question I have of late is about the planes (sorry about the terminology but I have no formal education in astronomy) of Jupiter and Saturn I have observed the last few weeks.

Saturn is tilted parallel to us showing a nice view of the rings and equal from side to side, I know that the planets in the solar system rotate around the sun on the same plane (about 17 deg. if you include Pluto right? ) but do the moons and rings of a given planet rotate on the plane plane around their host as the host's do around the sun?

It seams that the moons of Jupiter are "flat" in their orbit, were as the rings of Saturn seam tilted at a 45 deg. towards us.

Thanks Kevin

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Bob King

November 30, 2020 at 12:13 am

Hi Kevin,

Thanks for writing! Yes, the planets revolve around the sun in approximately the same plane as the Earth does, but there are variations (Mercury's orbit for instance is inclined 7°; Venus 3° and so on). The large moons of both Jupiter and Saturn orbit very close to the equatorial plane of their parent planet, not in the plane of Earth's orbit.

Because Jupiter's orbit is tilted just 1.3° relative to Earth's orbit AND Jupiter's axis is inclined just 3°, the moons swing from side to side, crossing in front and back of the planet from our perspective. That's why they often appear in a row.

Saturn is different. Its large moons and rings orbit close to the planet's equatorial plane, but because Saturn's axis is inclined nearly 27°, the moons can appear above, below and off to either side of the planet. For the same reason, the tilt of the rings varies from edge-on to a maximum of 27°.

I hope this helps. If not, feel free to ask further questions.

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December 1, 2020 at 8:59 pm

Hello again and thanks for the last reply, With that type of information and a real nice telescope the moons of Uranus (with its drastic axial tilt ) would give of quite a show depending on the time of year and its relationship to Earth, with the equator sometimes facing us and sometimes one of its poles, Correct?

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