September 18, 2019

Astro-Challenge: Uranian Moon Spotting.

A Uranian family montage. (Credit: NASA/JPL/Voyager 2)

First, let’s clear up one thing right out the gate. This week’s astronomical challenge will have me saying the name Uranus, a lot. And I can already hear the Beavis and Butt-head snickers from the back row, as I use sentences like “Just point your 10” Dob at Uranus…” You’re going after low hanging fruit, trust me. Astronomers have heard it all before. Besides, there are much better astronomical terms with scatological connotations, not that we would dream of unleashing them on this, a family astro-blog. Let’s just agree that the proper pronunciation is Ur-IN-us, and we’ll let a snicker or three slide in the name of science.

On September 25th, the planet Uranus reaches opposition, as it does roughly every Earth year plus 4 days. Uranus itself is in an 84.3 year orbit, and it currently residing in the constellation Pisces very close to the vernal equinoctial point;

So sure, you’ve seen Uranus shining as a blue-green disk thru a telescope; at magnitude +5.5 to +6, it may even be just possible to nab it with the naked eye under absolutely dark skies. But have you even caught sight of its moons?

Of the 27 known moons of Uranus, 5 are in range of backyard scopes; a quick rundown reads as follows;

 

Moon Name

Max. Elongation

Magnitude

Orbital Period

Miranda

9”

+16

1d 09h 56m

Ariel

13”

+14

2d 12h 29m

Umbriel

18”

+15

4d 03h 27m

Titania

31”

+13.5

8d 16h 56m

Oberon

42”

+14

13d 11h 7m

The outer two, Titania and Oberon,  where discovered by William Herschel using his 49.5” diameter speculum metal mirror telescope on the night of January 11th, 1787; so advanced was Herschel’s scope for its day that no other instrument was capable of spotting the Uranian moons for a near half decade. That drought was broken by William Lassell, who discovered Ariel and Umbriel using a 24” reflector from his Liverpool observatory in 1851. Miranda was added to the list in 1948 when it was discovered by planetary scientist Gerard Kuiper using the 82” inch Otto Struve Telescope at the West Texas McDonald Observatory facility. Initially only given discovery numbers (Uranus I, Uranus II, etc) William Herschel’s son John named Oberon and Titania after characters from William Shakespeare’s A Mid-Summer Night’s Dream and Alexander Pope’s The Rape of the Lock, and the tradition took. Textbooks had Uranus down for five moons right up until the Voyager 2 flyby in 1986; in fact, I remember memorizing “Uranus has 5 moons” as a kid.

Spotting the tiny moons is tough but not impossible; I’ve caught ‘em via the 61” Kuiper Telescope atop Mt. Bigelow, Arizona on open house, and some illustrious web-cam imagers have snagged the Uranian moon family.  True, you may not have access to the 40-foot behemoth that Herschel did, but your modern day backyard telescope has superior optics and reflectivity compared to those old polished metal mirrors. Dark skies and good seeing are key. Now around opposition is a good time to try and cross the Uranian moons off of your astro-life list. (hey, if birders can do it, why can’t we?) Titania and Oberon are the easiest to nab; Lassell’s pair of Ariel and Umbriel are a tad more difficult being closer in but not impossible. I’ve never heard of an amateur catching the elusive Miranda, although this feat of astro-athletics should be possible. An occulting bar eye-piece to cover the “glare” of Uranus and high magnification will help with the hunt. Greatest elongation the planet’s 4” arc second diameter disk is the best time to search; Uranus during opposition rises in the east at sunset and rides high across the meridian around midnite to 1AM local time. An ephemeris of Uranian moon elongations as calculated by our ace astro-mancer Edward Kotapish centered on opposition as follows;

ELONGATIONS OF THE URANIAN MOONS 2011

DATES AND TIMES IN UT

10 SEP

ARIEL 1503 S

TITANIA 1533 N

11 SEP

UMBRIEL 1038 N

ARIEL 2118 N

12 SEP

OBERON 0543 N

13 SEP

ARIEL 0333 S

UMBRIEL 1223 S

14 SEP

ARIEL 0948 N

15 SEP

TITANIA 0003 S

UMBRIEL 1408 N

ARIEL 1603 S

16 SEP

ARIEL 2218 N

17 SEP

UMBRIEL 1553 S

 

 

 

Uranus “spiral-ephemeris” (Created by Ed Kotapish).

18 SEP

ARIEL 0433 S

OBERON 2318 S

19 SEP

TITANIA 0833 N

ARIEL 1048 N

UMBRIEL 1733 N

20 SEP

ARIEL 1703 S

 

 

 

Wide view of the current field of Uranus. (Created by the author using Starry Night).

21 SEP

UMBRIEL 1918 S

ARIEL 2313 N

23 SEP

ARIEL 0533 S

TITANIA 1658 S

UMBRIEL 2103 N

24 SEP

ARIEL 1143 N

25 SEP

OBERON 1653 N

ARIEL 1803 S

UMBRIEL 2248 S

 

 

 

Simulation of the Uranian Moon system for the night of Sept 18th (Created by the author using Starry Night).


27 SEP

ARIEL 0013 N

28 SEP

UMBRIEL 0028 N

TITANIA 0128 N

ARIEL 0628 S

29 SEP

ARIEL 1243 N

30 SEP

UMBRIEL 0213 S

ARIEL 1858 S

02 OCT

ARIEL 0113 N

UMBRIEL 0358 N

TITANIA 0958 S

OBERON 1033 S

 

Oberon as seen from Voyager 2.

03 OCT

ARIEL 0728 S

04 OCT

UMBRIEL 0543 S

ARIEL 1343 N

05 OCT

ARIEL 1958 S

06 OCT

UMBRIEL 0728 N

TITANIA 1828 N

07 OCT

ARIEL 0213 N

08 OCT

ARIEL 0828 S

UMBRIEL 0913 S

09 OCT

OBERON 0403 N

ARIEL 1443 N

 

 

Ariel as seen from Voyager 2.

Note that these are given as “North” and “South” elongations rather than the usual east and west as Uranus is tipped over on its axis (and its moons orbits along with it) almost 97° degrees.

Currently, the orbital plane of the moons are tipped open about 17° degrees to our line of sight and slowly widening. They were edge on, and the last series of transits occurred when we crossed the moons’ orbital plane in 2007; they will be fully open at 90° degrees to our Earthly vantage point in 2029, and will be edge-on again in 2049.

So damn the sophomoric jokes; pull out that 12” monster and aim it for the moons of Uranus!

This week’s astro-term is Ice Giant. Strict classification of planets has become a hoary affair; as more and more worlds blur the line in each direction, a strict etymology has become difficult. In our own solar system, Jupiter, Saturn, Uranus and Neptune have been termed as gas giants, a term first attributed to James Blish in his 1952 short story Solar Plexus. But over the years, a semi-unofficial term for Neptune and Uranus has arisen, that of the Ice Giant. This is because while large gas giants like Jupiter and Saturn are mostly hydrogen and helium, Uranus and Neptune are composed 80-85% of ammonia, methane, and water ice. The exact mass limit for the formation of such a world is uncertain, but may lie in the realm of 5-10 Earth masses. (Uranus sits at about 14.5 Earth masses) Will we ever have a taxonomy for solar system objects that’s not just a cultural one? Will we someday be able to pull our star cruiser up to a new world and state; “Captian, we’ve entered the orbit of a Class M planet?” It’s a brave new universe out there, and Ice Giants or the occassional “exo-Uranus” will definitely be a part of it!

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  1. [...] -      29th: The planet Uranus reaches opposition, a good time to try your hand at Uranian moon spotting.  [...]

  2. [...] reaches opposition on September 29th and around such time it may just be possible to spot its elusive moons (See the addendum at the end of this post). 44 Piscium can also occasionally be occulted by the [...]

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