Just how many planets are visible without a telescope? Not including our own planet, most people will answer “five” (Mercury, Venus, Mars, Jupiter and Saturn).
Those are the five brightest planets, but in reality, there is a sixth planet that can be glimpsed without the aid of either a telescope or binoculars.
That sixth planet is the planet Uranus. This week will be a fine time to try and seek it out, especially since it is now favorably placed for viewing in our late-evening sky and the bright moon is out of the way.
Of course, you’ll have to know exactly where to look for it. Astronomers measure the brightness of objects in the night sky as magnitude. Smaller numbers indicate brighter objects, with negative numbers denoting exceptionally bright objects. But Uranus is currently shining at magnitude +5.7, relatively dim on the scale; barely visible by a keen naked eye on very dark, clear nights.
It is currently located within the constellation of Aries, the Ram, about a dozen degrees to the east (left) of the brilliant planet Mars. It’s already one-third up from the eastern horizon by 11:30 p.m. local daylight time and will reach its highest point — more than two thirds up from the southern horizon — just before 4 a.m.
It is best to study the accompanying chart first, then scan that region with binoculars. Using a magnification of 150-power with a telescope of at least three-inch aperture, you should be able to resolve it into a tiny, blue-green featureless disk.
An icy, cold world
This week Uranus is about 1.771 billion miles (2.851 billion kilometers) from Earth (only Neptune is farther away). It takes 84.4 years to orbit the sun. The planet has a diameter of about 31,518 miles (50,724 km), making it the third-largest planet, and according to flyby magnetic data from Voyager 2 in 1986, has a rotation period of 17.23 hours.
At last count, Uranus has 27 moons, all in orbits lying in the planet’s equator in which there is also a complex of nine narrow, nearly opaque rings, which were discovered in 1978.
Uranus likely has an icy, rocky core, surrounded by a liquid mantle of water, methane and ammonia, encased in an atmosphere of hydrogen and helium. In fact, Uranus has the coldest atmosphere of any planet in the solar system with a minimum temperature
of -371 degrees Fahrenheit (minus 224 degrees Celsius).
A freakish tilt
A bizarre feature is how far over Uranus is tipped. The other planets are tilted somewhere between 3 degrees and 29 degrees, but Uranus’ north pole lies 98 degrees from being directly up and down to its orbit plane.
From our point of view, this means that sometimes we see Uranus with its north pole pointing at us. At other times we see it with its equatorial belt oriented vertically instead of horizontally. From the point of view of a hypothetical astronaut visiting Uranus, daylight and darkness would be nothing short of extraordinary. Its seasons are extreme: when the sun rises (as an example) at the north pole, it stays up for 42 Earth years; then it sets and the north pole is in darkness for 42 Earth years.
In the late winter of 1781, British astronomer Sir William Herschel had just finished building a new 6.3-inch (16 centimeters) reflecting telescope and began to study the stars through it. On the night of March 13, he had his telescope turned on the constellation of Gemini, the twins. There, to his great surprise, he came across an extra star that was not plotted on any of his star charts. An accomplished astronomer, Herschel was quick to realize that what he found could not possibly be a star, for it appeared in his telescope as a glowing disk as opposed to a twinkling speck of light.
Continuing his observations of this unusual object night after night, Herschel soon perceived movement; it was slowly shifting its position among the background stars of Gemini. Finally, he decided that he had discovered a new comet and he wrote up a detailed report of his observations, which were published on April 26.
The report of a new comet excited astronomers all over Europe, and they all eagerly trained their telescopes on Herschel’s discovery. King George III, who loved the sciences, had the astronomer brought to him and presented him with a life pension and a residence at Slough, in the neighborhood of Windsor Castle.
Soon, enough observations were made to calculate an orbit for Herschel’s “comet.” That’s when an increasing number of astronomers began to doubt that what they were looking at was really a comet. For one thing, it seemed to be following a nearly circular orbit…