Starhopping to Uranus

Most of the planets in the solar system hover close to the sun, which illuminates them and makes them among the brightest objects in the sky. The outer planets, Uranus and Neptune, are far from the sun and catch few of its rays, making them dim and hard to find.

The planet Uranus is currently located in the dim constellation of Pisces. Although just visible with the unaided eye, Uranus is brighter than any of the stars in this area, so is relatively easy to locate with binoculars. Credit: Starry Night Software

Uranus, for example, reaches opposition with the sun on Thursday October 3. Directly opposite the sun in our sky, it is at its closest and brightest, yet it is only just barely visible to the unaided eye in a dark moonless sky. For most people, a binocular will be an essential tool for spotting Uranus.

This is done through a technique popular with amateur astronomers called “starhopping.” We use the brighter stars as guideposts to locating faint or distant objects. In this case, we start with the most prominent group of stars in the autumn sky: the four stars that form the Square of Pegasus. Although not among the brightest stars, these are all good second magnitude stars, about as bright as the stars in the Big Dipper.

Many people who go looking for the Square of Pegasus miss it because it is so large. The four sides of the Square are roughly 15 degrees long, about the length of the handle of the Big Dipper. Look for them in the east as the sky gets dark around 9 p.m. At that time the Square is rising, so is tilted over, making it more of a diamond shape than a square.

Now we begin our “starhop.” We start with the two stars that form the bottom of the square (if you’re in the northern hemisphere). Make that one side of a south-pointing equilateral triangle. The southern point of the triangle marks the “Circlet,” part of the dim constellation of Pisces. This is an oval of dim stars, more easily seen in binoculars than with the unaided eye.

To the left of the Circlet is a chain of stars, part of the chain that binds the two fish of Pisces together. The first two stars in this chain are a bit brighter than the rest. About half way between these two stars, and a bit to the south, you should find Uranus.

How will you know you have found it? First of all, although dim, Uranus is brighter than any of the stars in this area. Secondly, it has a distinctive blue-green color, quite unlike any star. Finally, if you plot its position relative to the stars, and check again in a few days, you will find that it has moved to the right.

Of course, the acid test is to point a telescope at it. Most newcomers to astronomy are surprised at how small any of the planets appear in a telescope, but Uranus is smaller still, a mere 3.7 arc seconds in diameter, about a tenth the size of Jupiter. Unless you have a very large telescope, it will appear as nothing more than a tiny blue-green pinhead.