Uranus in Pisces

Coming back to our home solar system after journeying among the distant remnants of stellar death, there are two planets in the evening sky: Uranus and Neptune. Nearly the same size, Uranus is slightly larger at four times the size of Earth compared to Neptune’s 3.88 times. Uranus is also closer, orbiting the Sun 20 times farther than Earth, whereas Neptune is 30 times farther out. This makes Uranus visible to an unaided eye on a dark night, but Neptune requires use of a telescope.

Tonight, Neptune is in Aquarius, sets at 7:21 p.m. and is only 25 degrees above the horizon at sunset. Uranus, higher up in Pisces, is in a position to help us see it with our unaided eye, so I will focus on it.

Tonight has far too bright a moon to spot Uranus and, unfortunately, the total lunar eclipse tomorrow morning is for the other side of the world as it begins for us just as the moon sets (at 6:48 a.m., EST). So wait until Friday when the moon won’t rise until 9 p.m. to look for this distant world. As shown in Figure 1, it is nestled between the ropes tied to the tails of the two fish of Pisces. Using binoculars, find Alresha (Al-RESH-uh) and move upward to Omicron. Placing Omicron in the left side of the field of view should allow you to see Mu on the right. Uranus will be nearly centered above them at a height about half their separation distance. It will be fuzzier than the stars, whose images should remain quite sharp.

Uranus was spotted by many astronomers before it was recognized to be a planet. It may have been one of the stars recorded by Hipparchos in his star catalog of 128 BC. In the late 17th century, John Flamsteed, the then-astronomer royal, observed it at least six times and cataloged it as the star 34 Tauri. It was William Herschel who first recorded observeing it with his telescope (in Bath, England) in 1781 and was suspicious of its nature. It appeared “nebulous” to him, more like a comet than a star. He also noticed its motion between March 13 and 17, though it only moved 2 arcminutes (there are 60 arcminutes in a degree). He was a consummate observer! As a confirmation that it was not a star, he looked at it with different magnifications and saw that the image became bigger, unlike a star that always remains a pin-point. He thought he had found a comet, but also thought it might be a planet. He reported it to the Astronomer Royal Nevil Maskelyne, who thought it could be either a planet or comet. Other astronomers were more inclined to think it a planet. The first to compute its orbit was Anders Johan Lexell, a Finnish-Swedish astronomer working in Russia. Since the orbit turned out to be nearly circular, Johann Elert Bode of Berlin concluded that it was a planet beyond the orbit of Saturn. In 1783, the discovery of Uranus by Herschel was formally recognized by the Royal Astronomical Society, and King George III had him move to Windsor so the royal family could look through his telescopes and rewarded him with an annual stipend of $200.

Herschel wanted to name the new planet after King George, but Bode suggested Uranus, the Latinized name of Ouranos, the Greek god of the sky and father of Saturn, who was the father of Jupiter, making Ouranos his grandfather. Bode’s suggestion ultimately won out when the Nautical Almanac adopted the name.

The pronunciation of Uranus is always problematic. I put the emphasis on the first syllable, YOOR-uh-nus, instead of the second syllable to get closer to Ouranous. My students generally stick to emphasizing the second syllable … and giggling.

As a planet, though, Uranus is not to be giggled at! It’s the coldest world in the solar system even though it is not the most distant. Deep cloud layers are as cold as –370 degrees F! It’s not currently understood why Uranus is so cold, but it may be due to the inclusion of many volatile ices in its atmosphere which has led it and Neptune to be called Ice Giants in contrast to Jupiter and Saturn being Gas Giants.

As shown in Figure 2, Uranus and Neptune share a more complex composition than Jupiter and Saturn that are almost entirely made of hydrogen with a few ammonia compounds giving them their orange and brown colors. The blues of Uranus and Neptune are due to methane gas that absorbs red light, allowing only blues to be reflected. Their thick mantles are mixes of methane, ammonia, water, and hydrogen sulfide. It is these compounds that form much ice in the outer solar system making them Ice Giants.

There is more that’s unusual about Uranus. Most of the planets orbit the Sun with their equator within 30 degrees of their orbit (Earth’s is tilted by 23.5 degrees). Uranus, however, orbits the Sun on its side, leaning over 82 degrees! This means its seasons are extreme: Each hemisphere gets 42 years of continuous sunlight … and continuous darkness! In 2007, it was the Uranian Vernal Equinox as the sun shone directly on the equator, giving every place on the planet equal nine-hour days and nights in its 18-hour day. When the Voyager spacecraft passed by Uranus in 1986, it was in its southern summer with the south pole almost directly pointing at the Sun. The atmosphere was found to be very quiet. During the equinox, however, the atmosphere has become more active with “dark spot” storms and bright cloud patches migrating across the world.

The strangeness of this world extends to its magnetic field. The Earth’s magnetic field is very similar to that of a bar magnet centered in the planet tilted by about 11.5 degrees. The tilt for Uranus is 60 degrees, and it is displaced from the center of the planet toward the south pole by a third of Uranus’ radius, making the field much stronger on one pole than the other.

The volunteer astronomers at the Adirondack Public Observatory are eager to show you Uranus and many other wonders of the night. The Roll Off Roof Observatory is open to the public on the first and third Fridays of each month approximately one half-hour after sunset. Whether you’re an avid amateur astronomer or have never visited an observatory, come and view through our telescopes and learn about the Wilderness Above. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO at 518-359-6317 to talk with one of our astronomers.