This article includes a JavaScript utility to help you identify the moons of Uranus in your telescope's eyepiece. You'll find instructions for its use below.

Four of Uranus's 27 known satellites can be seen in moderate to large amateur telescopes. Both Titania and Oberon have been glimpsed with apertures as small as 20 centimeters (8 inches). Umbriel and Ariel, however, because they lie much closer to the planet's glare, are normally difficult with even twice that aperture.

How can you tell which moon is which? That's where Sky & Telescope's Moons of Uranus JavaScript utility comes in! For any date and time, our interactive tool shows the positions of Ariel, Umbriel, Titania, Oberon, and — for those seeking a special challenge — Miranda. Moreover, the utility can match the view in your telescope's optical system, whether it shows a correct or mirror-reversed image.

The display has several parts. At top is a diagram showing the positions of Miranda (M), Ariel (A), Umbriel (U), Titania (T), and Oberon (O) with respect to Uranus. Below the graphic are three buttons you can use to change the orientation of the diagram to match the view in your telescope. "Direct view" puts celestial north up and celestial east to the left; the routine opens in this orientation, which is the one used in most star atlases. "Inverted view" puts south up and west to the left; that is, upside down but not mirror-imaged (matching the view in a Newtonian reflector in the Northern Hemisphere). "Mirror reversed" puts north up and west to the left, matching the view in most catadioptric (mirror-lens) and refractor telescopes used with a star diagonal in the Northern Hemisphere.

Next comes the date and time. When the routine opens, it is initialized to the present (as determined from your computer's clock). Change the date and time by entering new values in the corresponding boxes and clicking the dark gray "Recalculate" button on the next row. Or click on the adjacent buttons to step backward or forward in increments of 1 day or 1 hour.

Our Moons of Uranus JavaScript uses Universal Time (UT, the same as Greenwich Mean Time), and underneath the day- and hour-increment buttons it shows what we think is the offset between UT and your local civil time, based on your computer's current settings. When changing the time manually using the Time input box, enter the Universal Time that corresponds to the local time when you will be observing.

Launch S&T''s Moons of Uranus JavaScript

Few backyard skygazers can say they've ever seen the moons of Uranus in a telescope. Aided by our handy JavaScript utility, perhaps you'll join that exclusive club. Good luck!