Uranus is one of the two “ice giant” planets, edging out Neptune as the third largest planet. Its atmosphere is mostly hydrogen and helium with enough methane to tint the planet blue. Deep inside, Uranus has a rocky core surrounded by a thick rind or slushy ice. Uranus is “only” 1.77 billion miles from Earth this October. Credit: NASA/ESA/L. Sromovsky
This week, Thursday to be exact, the planet Uranus and Earth will be at their closest for the year. This special day is called opposition and refers to Uranus appearing opposite the sun in the sky. When the sun sets, the 7th planet will crest the eastern horizon, arc across the sky the entire night and then set at sunrise.
When planets are at opposition they’re not only closest but brightest. Bright of course is a relative term. Mars at opposition can become much brighter than Sirius, the brightest star. Uranus shines meekly at magnitude 5.7 slightly above the naked eye limit. On dark nights if you know exactly where to look, you can glimpse the planet without optical aid. I’ve seen it many times; my hope is that with the help of these maps, you will too.
Engraving of 18th century astronomer William Herschel in his element.
English astronomer John Flamsteed was the first to see Uranus in 1690 but thought it was just another star in the constellation Taurus. He even gave it a name: 34 Tauri. Despite running across it at least six times he never realized he was looking at a planet and not a star.
Enter William Herschel, a German transplant living in Bath, England and passionate sky watcher who never missed a clear night. From his garden on 19 New King Street, he stumbled across what he thought was a new comet in Taurus in his homemade 6-inch reflecting telescope on Tuesday night March 13, 1781. Closer examination of the object at high magnification showed it as a fuzzy disk, distinct from the point-like stars.
Extract from William Herschel’s journal entry made on March 13, 1781. The second paragraph reads: “In the quartile near Zeta Tauri the lowest of 2 is a curious either nebulous star or perhaps a Comet.” Source: Journal of the Royal Astronomical Society
Like any good comet hunter then or now, Herschel returned to his “comet” several nights later on the 17th to see if it had moved. For all practical purposes stars are fixed, but planets and comets inch across the sky as they orbit the sun. They can’t help but give themselves away.
Herschel next reported his find to England’s Royal Astronomical Society so others could see and track the new object. Very soon, astronomers realized this was no comet but a brand new planet, the first discovered since antiquity.
Because its axis is tipped 98 degrees, Uranus orbits the sun spinning on its side. The poles alternately face the sun for 42 years at a time. Credit: Nature of the Universe
And what an odd planet! We now know that Uranus is not only the coldest planet in the solar system – even chillier than more distant Neptune – but the only one that spins on its side. Like Earth, most of the planets spin like tops with their rotation axes tilted a little this way or that, but Uranus takes it to the extreme with a tip of 98 degrees. Seen from afar, it looks like a ball rolling on its side, and that has some curious consequences when it comes to seasons.
Since Uranus takes 84 years to make one spin around the sun, each of the planet’s poles gets 42 years of continuous sunlight during the summer season followed by 42 years of winter darkness. The northern hemisphere currently faces the sun with the first day of Uranian summer due in 2028.
Because of the planet’s extreme axial tilt, we see Uranus from different perspectives over its 84-year orbit. Right now the view is “opening up” again after a near “edge-on” view in 2006. Credit: NASA/ESA/G. Bacon – STScI
“Day” is something of a misnomer. A day on the planet lasts a little more than 17 hours, but as seen from the poles, a day in the traditional sense lasts 42 years. If you could hang out at one of the poles, the sun would rise on the first day of spring, creep slowly across the sky and finally set on the first day of fall 42 years later.
Uranus comes up into view around 9:30 p.m. local time in the southeastern sky below the Great Square of Pegasus and to the right of the small constellation Aries. It’s not far from the star Delta in Pisces. Once you find Delta with eye and binoculars, you’re ready to use the detailed map below. All maps: Stellarium
Uranus is the third largest planet, edging out distant Neptune by just 85 miles. All the large outer solar system planets radiate heat from their cores which are still contracting under the force of gravity since their formation 4.5 billion years ago. Contraction creates heat which escapes the core and spills into space.
Delta heads up a little row of stars that points toward the planet Uranus. If you place Delta to one side of your binocular view, you’ll see Uranus near the other side. The planet is the same brightness as the 5.7 mag. star. Positions are shown every 5 days from Sept. 30 to Oct. 15.
Oddball Uranus gives off little internal heat and no one’s sure why. Temperatures as low as -371 degrees F (-224 C) have been recorded in its lower atmospheric layers. Scientists hypothesize that there might be some sort of barrier deep below the atmosphere that blocks heat coming from the core. That or the planet was struck by another developing planet long ago, causing it to expel its internal heat as well as knocking the axis kittywampus.
Uranus and a few of its moons as seen in the Hubble Space Telescope. The planets also has at least 13 very narrow rings composed of water ice coated by dark, organic materials. Credit: NASA/ESA
While Uranus may be challenging to see with the naked eye, it’s easily visible in binoculars even in less than ideal skies. That’s the good news. The bad news is that the planet isn’t near any bright stars that would give a beginning observer a leg up in finding it. No problem. We’ll get you there anyway.
Through a telescope magnifying about 75x you can tell you’re looking at a planet and not a star. Stars are tiny and possess a fiery radiance; Uranus looks slightly larger and “dull” in comparison. Once you crank up the power to 150x and higher, you can’t miss its tiny disk, tinted blue from atmospheric methane gas. Amateur astronomers using scopes in the 12-inch to 16-inch range can even spot two of the planets 27 known moons, Titania and Oberon.
Uranus moves slowly westward through the constellation Pisces during October. You can watch its progress in binoculars as it passes among several “fixed stars” of similar brightness. Face southeast starting around 9:30 – 10 p.m. when the planet has cleared the trees and houses and begin by locating the Great Square of Pegasus, a big, vacant square of sky with a star marking each of the four corners. Drop down one outstretched fist below the Square to find the dimmer star called Delta in the constellation Pisces. Now center your binoculars on Delta and use the map below to hone in on the planet’s location.
I wish you well in discovering the 7th planet all by yourself just as Herschel once did.