Watch Uranus's Moon Titania Cover a Star Monday Night

By: Bob King November 20, 2023 0

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.

Titania from Voyager 2 — Voyager 2 took this composite image of Neptune's largest moon Titania during its flyby of the Uranian system on January 24, 1986. Titania measures about 1,580 kilometers (980 miles) across and displays abundant impact craters and prominent fault valleys that extend across much of its diameter. The large impact crater Gertrude (upper right), named after the mother of Hamlet in William Shakespeare's play *Hamlet*, is about 326 kilometers (203 miles) wide.
*NASA / JPL*

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.

Titania occultation path — Observers situated in the bolded, ladder-like path will see Titania occult the 11.8-magnitude star TYC 1236-00841-1 on November 21st between 5:13 and 5:24 UT (12:13 to 12:24 a.m. EST). The 2.2-magnitude dip at the moment of disappearance (and at the star's return) will make for an arresting sight! Also check out this interactive map.
*Occult 4.0*

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.

Uranus and moons — In this simulation, Uranus, its three brightest moons, and TYC 1236-00841-1 (the occulted star) are shown shortly before the star of the occultation. The moon, located about 30″ east of the planet, will move west to west-southwest during the event.
*Stellarium*

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.

Uranus locator — Uranus lies in the wilds of Aries between Jupiter and the Pleiades just south of a small triangle of 4th- and 5th-magnitude stars.
*Stellarium*

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?