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Cosmic Challenge: Satellites of Uranus


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Cosmic Challenge:
November 2019

Satellites of Uranus

Phil Harrington

This month's suggested aperture range:

10-inch (25 cm) to 14-inch (36 cm) telescopes

 

If you were like me, then one of the first things you saw through a telescope was Jupiter and its four Galilean moons. I was absolutely amazed with the idea that I could come back after only a few hours and see that the moons had moved with respect to the planet and each other. Imagine Galileo's astonishment when he saw this wondrous sight for the first time.

The Galilean satellites can be seen in a pair of steadily held binoculars. Moving farther out to Saturn, its largest moon, Titan, can be spotted with little effort through the smallest backyard scopes. How about the moons orbiting the planet Uranus? Have you ever seen any of them? Not possible, you say?

Above: Autumn star map. Credit: Map adapted from Star Watch by Phil Harrington

Above: Finder chart for this month's Cosmic Challenge.  The chart plots the positions of Uranus on dates of opposition to the year 2028.
Click on the chart to open a printable PDF version.

 

 

Of the 27 known satellites in the Uranian family, four stand out, just as the four Galilean satellites do among the Jovian clan. William Herschel discovered the first two Uranian moons on January 11, 1787, six years after he had discovered the planet itself. The next two remained undetected until the British astronomer William Lassell (1799-1880) spotted them on October 24, 1851. It is these four that we hope to catch through our own telescopes.

The four major moons of Uranus -- Titania, Oberon, Ariel, and Umbriel -- take their names from the writings of William Shakespeare and Alexander Pope. Oberon was the King of the Fairies in A Midsummer Night's Dream, while Titania was his queen. Ariel was the leading sylph in Alexander Pope's poem The Rape of the Lock and, coincidentally, also the spirit who serves Prospero in Shakespeare's The Tempest. Finally, Umbriel was named for the "dusky melancholy sprite" in The Rape of the Lock.

All four are made of rock mixed with a frozen cocktail of ammonia, methane, and water ice. Like the major moons of Jupiter, Saturn, and Neptune, Titania, Oberon, Ariel, and Umbriel all orbit Uranus almost exactly in the plane of the planet's equator. Due to the odd, sideways tilt of Uranus's rotational axis, however, this can cause the moons to appear at odd angles with respect to the planet itself. As the planet orbits the Sun, the paths followed by the moons appear to change shape and orientation over time. Currently, they appear oriented northwest-southeast, tracing out narrow clockwise paths around the planet.

Uranus
s two largest moons, Titania and Oberon, each measure about 900 miles (1,500 km) in diameter. Photos taken during the 1986 Voyager 2 flyby show that Titania's battered surface is crisscrossed by valleys and faults that stretch for hundreds of miles. Although many impact craters are seen, there are fewer than on Oberon. Together, the fewer craters and many fault lines tell us that Titanias surface is younger than Oberons. Since both moons are the same physical age, this means that Titania must have once experienced a higher level of geologic activity than its neighbor.

Neither Titania nor Oberon ever exceeds 14th magnitude. Of the two, Oberon is easier to spot since it orbits farther from Uranus and, therefore, can appear farther away from the planet's glare. Even at greatest elongation, however, it is always less than an arc-minute away from the planet.

Ariel, measuring 720 miles (1,158 km) across, resembles Titania in Voyager photos, with an interweaving network of valleys and faults threaded across its frigid surface. These faults, as well as those on Titania, may be the result of the rapid cooling of the moon's hot core shortly after formation.

Finally, Umbriel (727 miles or 1170 km in diameter) bears a strong resemblance to Oberon. Its dark surface is scarred by many impact craters, but lacks the faults and valleys that characterize Titania and Ariel.
  Umbriel and Ariel get about as bright as Titania and Oberon, but because they orbit even closer to the planet, present an even greater challenge to observers. Ariel, the closest, is never more than 15" from Uranus.

 

Above:  The five largest moons of Uranus, as imaged by Voyager 2 in 1986.  NASA images.

 

Above:  An amateur's view of the planet Uranus and three of its major moons.

 

If spotting those four is "easy," then try your luck with a fifth moon, Miranda. Miranda's surface terrain is a real mishmash. Some regions are likely very old, based on crater counts, while other areas are comparatively new, again based on the number of craters.  There are many theories out there trying to explain its disheveled appearance. Some suggest it was shattered into pieces due to one or more collisions in its early history, but was able to reassemble itself in a haphazard, inside-out manner. Others explain the Miranda jigsaw-like surface was caused by upwelling of partially melted ices.  Miranda is, after all, about 50% ice.  So far, only Voyager 2 has flown past the Uranian system, so clearly more close-up investigation is needed.

 

Can you actually see Miranda for yourself?  Hold on tight, it averages 16th magnitude!  While that's below the cutoff for our featured aperture range this month, it just might be visible through some super-sized amateur telescope.

 

For the best chance at seeing any of these challenging targets, Uranus should near opposition, when its distance away from Earth is least. That just happened on October 28, which makes this month ideal for the hunt. But you'll need to wait patiently until each satellite is at its greatest elongation point from the planet. To help you find out when that will occur, several popular software programs, including the old school program Guide by Project Pluto, plot the location of each Uranian moon for any date and time. Sky & Telescope magazine's web site also has an excellent Javascript utility that will show each moon's position at any time.  The tool uses your system's time and date by default, but it can be set manually for any occasion between 2008 and this year.  Here is the line-up from November 1, 2019, at 00:00 Universal Time (UT).  [If need be, use this web site to convert between Universal Time, also known as Greenwich Mean Time (or GMT) and your local time.]

 

 

To learn the ins and outs of the S&T tool, read this article by Roger Sinnott and Adrian Ashford.

 

Good luck! And be sure to post your results in this column's discussion forum.

Until next month, remember that half of the fun is the thrill of the chase.  Game on!



About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  A revised, second printing of Cosmic Challenge: The Ultimate Observing List for Amateurs is now available with updated data tables and charts for finding various solar system objects, such as Pluto and Vesta, as well as improved renditions of the many eyepiece sketches that accompany each of the 187 challenges encompassing more than 500 individual objects.  The book is available from Amazon.com.  Visit www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2019 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.

 


  • okiestarman56, John O'Hara, happylimpet and 2 others like this


5 Comments

This challenge really hits home for me.  Now confident that I saw three on 10/9 and one on 10/27.  The Sky and Telescope link is awesome.  Thank you so much.   jd

    • PhilH likes this

I've seen Oberon and Titania with my 10-inch, and as I recall, I had also seen Ariel and Umbriel though a 25-inch Dob at the 2006 Okie-Tex Star Party. Those moons are a lot harder to see than Oberon and Titania of course.

    • PhilH likes this

Last month on October 25th I was fortunate enough to observe Uranus’s moons Oberon, Ariel, & Titania (identified using the S&T tool) with my 16” truss Dob telescope at  332x magnification from my somewhat suburban location east of Phoenix. It certainly was a treat to see them!

    • PhilH likes this

I've seen the four big ones but not Miranda, using a 10".

    • crsrs likes this
Photo
Dave Mitsky
Nov 10 2019 07:36 PM

I've seen the three brightest satellites of Uranus.

    • crsrs likes this


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