Observers across much of the U.S. and Canada have a unique opportunity Monday night, November 20–21, to see Uranus’s brightest moon occult a star.
On Monday night, November 20–21, Uranus's brightest and largest moon Titania will occult an 11.8-magnitude star just after midnight Eastern Standard Time across a wide swath of the northern and central U.S., southern Canada, and southwestern Europe.
At first blush you might consider the event too obscure to bother with given that 14th-magnitude Titania requires a 10-inch or larger telescope to identify. But I strongly encourage you to watch this unique event even if the moon eludes your view. What you will see is the reasonably bright star TYC 1236-00841-1 disappear for up to 72.8 seconds. For that you'll only need a 6-inch instrument. Moreover, if you carefully time the disappearance and reappearance your data will help astronomers determine Titania's shape and size with great precision. Observers with larger telescopes will probably lose sight of the moon in the star's modest glare shortly before and after the occultation but see it plainly once it sits over its target.
While Titania is no more than a 13.8-magnitude pinprick of light in the telescope, it's as absorbing as any solar system body on closer inspection. The moon orbits Uranus every 8.7 days, identical to its rotation rate, which tells us it's tidally locked to Uranus just like our own Moon is to Earth. Its high density suggests a mixed composition of rock and ice. Infrared spectroscopic observations in the early 2000s revealed crystalline water ice on Titania's surface. While the moon's abundant craters attest to ancient bombardment the presence of enormous crustal rifts points to more recent resurfacing events.
To prepare for the event first check David Dunham's Time and Circumstances file, which lists cities and the Universal Times of the middle of the occultation along with the altitudes of Uranus and the Sun. Dunham is the founder of the International Occultation Timing Association (IOTA) and encourages observers to share exact times of the star's disappearance and reappearance. Each time interval — different for every location — defines a chord across Titania that in toto precisely defines its size and shape, among other details. For more information, please consult the organization's Observing Basics website as well as Dunham's Titania Occultation page.
You'll be using higher magnifications for the best view so be sure you let your scope cool to air temperature. Whether watching for science or fun (or both) this promises to be an exciting event. When was the last time you stood in Titania's shadow?