Trick or Treat! Uranus at Opposition on Halloween

Uranus is the third largest planet after Jupiter and Saturn. Its atmosphere is mostly hydrogen and helium, but a small amount of methane present tints the globe a pretty shade of aqua. NASA images

We used Mars last week to point to the pulsating, red giant Mira in the constellation Cetus. Today we'll use it again, this time to direct us to Uranus. At Uranus we find ourselves in the cold, abysmal realm of the outer solar system far from Saturn's familiar rings and Jupiter's belted globe. The seventh planet lies more than 1.7 billion miles (2.8 billion km) from the cozy Earth this month. 

Although nearly four times as large as our planet, Uranus's great distance makes it too faint for most of us to see without optical aid. But if you're going to spot it, now's the time to look — Uranus comes closest to the Earth at opposition which occurs on Halloween this year.

This diagram shows the apparent size of the sun in fractions of a degree as seen from the eight planets and Pluto. At Uranus, the sun is only 1/20th as big as we see it from Earth. Stellarium with additions by the author

The sun shines 390 times dimmer at Uranus than it does on Earth. It's also a lot smaller, just 1.5 arc minutes across or one-twentieth the size we see it. Viewed through a solar filter the sun would look like a pinhead. That's exactly what the planet itself looks like through a small telescope with a magnification of 75x-100x. A blue pinhead, that is. Methane gas in its atmosphere absorbs red light and reflects back blue.

Like Jupiter, Saturn, Venus and Neptune, perpetual clouds blanket Uranus. The clouds of Venus are composed of sulfuric acid droplets while those at Jupiter and Saturn are made of ammonia ice. Uranian clouds are made of noxious hydrogen-sulfide ice and win the award for stinkiest in the solar system. If you could take a quick whiff you'd quickly recoil from the intense smell of rotten eggs. 

This is a replica of the 7-foot telescope that William Herschel used to discover Uranus. It had a 6.2-inch (15.7 cm) polished metal alloy mirror made of a mixture of copper and tin. Nowadays, mirrors are made of glass coated with a thin layer of highly reflective aluminum. Alun Salt / CC BY-SA 2.0

Uranus was the first planet discovered with a telescope, a feat accomplished by English astronomer William Herschel in 1781. He certainly wasn't expecting to discover a new planet when he happened upon Uranus on March 13th that year. To make sense of what he saw in the eyepiece Herschel described the curious sight as a compact, star-like comet. Further observations soon revealed it was something more extraordinary — the first new planet found since antiquity!

Uranus is in the constellation Aries two fists to the east of Mars. It reaches opposition on Halloween (Oct. 31), when it's closest to the Earth for the year. Use this map to get oriented and then refer to the detailed map below to track the planet down. Stellarium

You can see it, too. Uranus holes up in the constellation Aries this fall two fists to the left or east of Mars. Under a moonless, rural sky it's faintly visible as a magnitude 5.7 "star" with the naked eye. If you're fortunate enough to have access to a dark place I encourage you to try for it. Otherwise any pair of binoculars will show the planet. To detect its color and shape however, you'll need a small telescope.

Uranus moves very slowly to the west across Aries this month and next. I've marked two positions a month apart. The white circle is 5.5° across, a typical binocular field of view. The green dashed lines are suggested star alignments you can use to identify and track the planet. Stellarium with additions by the author

Every planet has a personality including Uranus. In close-up photos it's a bland, aqua-hued cue ball, but look closer and you'll see just how unique it really is. Winds up to 560 miles per hour (900 kph) roar through its atmosphere where the temperature hovers around 370° below zero Fahrenheit. Imagine the wind chill factor! 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. Models indicate that temperatures near the core reach 9,000° F (4,982° C).

Uranus rotates on its side with its poles pointed toward the sun and possesses a faint ring system. Six of the planet's 27 moons are seen. NASA / ESA and Erich Karkoschka, Univ. of Arizona

Did I mention that the planet's axis is tilted 98°? Uranus rotates on its side like a kicked-over top that keeps on spinning. This gives it the most extreme seasons of any planet in the solar system. For one-quarter (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. I love winter but 21 years?

This is a portrait of Uranus and seven of its 27 moons. Titania and Oberon are faintly visible in 10-inch and larger telescopes. ESO

Uranus has 27 known moons and 11 faint, spindly rings made partially of ice and other materials — possibly organic compounds — that give them a dark color. Like Earth the planet has a magnetic field, which can direct charged particles from the solar wind into its upper atmosphere to spark displays of the aurora borealis.

Welcome to one of the coldest, windiest, weirdest places in the solar system. The next clear night when the earthly wind blows hard and cold, instead of huddling inside, uncap your binoculars and spend a few minutes relating to the seventh planet.