Now Is a Great Time to Look at Uranus

Bad Astronomy
The entire universe in blog form
Oct. 8 2013 8:00 AM

Uranus Hangs a Moon. Five, in Fact.

Now, fair Hippolyta, our nuptial hour
Draws on apace; four happy days bring in
Another moon…
Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

— William Shakespeare, "A Midsummer Night's Dream", which has characters with the same names as the moons of Uranus
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Uranus gets short shrift in our solar system. Really, the only time people hear about it is when it’s the butt of a joke (of which there is exactly one that is funny) —and for the record, I pronounce it “YER-uh-niss”, which, if taken as an adjective, is to be honest not much better than how most people pronounce it.

But it’s actually a pretty cool planet. Although it wasn’t officially discovered until 1781, it’s just barely within naked-eye visibility, and is an easy object in binoculars. Through even a small telescope it shows a definite disk, even from a distance of 3 billion kilometers (about 2 billion miles), and it has a distinctly greenish cast, due to methane in its upper atmosphere— methane absorbs red light, letting only green and blue through.

Amazingly, several of its moons are also visible through a relatively small telescope, as astrophotographer Mike Carter proved just a few days ago!

Uranus
Uranus hanging five moons. Click to encaelusenate.

Photo by Mike Carter, used by permission.

The moons are labeled. These are the largest moons of Uranus, though they’re still pretty faint. The dimmest, Miranda (don’t tell River!), is about one ten-thousandth as bright as the dimmest star you can see with your naked eye. Still, Carter was able to get it in his shot, which consists of 100 15-second exposures added together, taken over the course of an hour on the evening of October 3rd/4th from his home in upstate New York.

Carter had another advantage: That was the night Uranus was at opposition. This is the term used by astronomers when a planet is directly opposite the Sun in the sky. That means the planet rises when the Sun sets, and is up all night. But it also means the planet is as close to the Earth as it will get all orbit, since the Sun, Earth, and the planet lie on a line, as seen in this mighty demonstration of my Photoshop skills:

Uranus at opposition
Uranus at opposition, when it lies on a line with Earth and the Sun. This drawing is not to scale!

Drawing by Phil Plait

If Uranus were anywhere else in its orbit, it would be farther from Earth. This means it’s the best time of the year to view the planet! It’s up all night, it’s closest to Earth (2.8 billion km, or 1.8 billion miles), it appears biggest, and it’s also at its brightest. Universe Today has a guide to observing it, as does Sky and Telescope magazine.

A couple of days ago I went out with my 10x50 binoculars and spotted it easily, once I knew the star patterns (you can download planetarium software to show you; I like SkySafari). If you have a good pair of binocs, I’d suggest giving Uranus a try. And if you spot it, think on this: The light entering your eyes left the planet nearly three hours ago! Astronomy lets you not just travel in space, but it takes you back in time, too. Quite a bargain for the price of simply going outside and looking up.

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