Exploring the Ice Giants: Neptune and Uranus at Opposition for 2018


Have you seen the outer ice giant planets for yourself?

This week is a good time to check the most difficult of the major planets off of your life list, as Neptune reaches opposition for 2018 on Friday, September 7th at at ~18:00 Universal Time (UT)/2:00 PM EDT. And while it may not look like much more than a gray-blue dot at the eyepiece, the outermost ice giant world has a fascinating tale to tell. Continue reading “Exploring the Ice Giants: Neptune and Uranus at Opposition for 2018”

Ice Giants at Opposition

It seems as if the planets are fleeing the evening sky, just as the Fall school star party season is getting underway. Venus and Mars have entered the morning sky, and Jupiter reaches solar conjunction this week. Even glorious Saturn has passed eastern quadrature, and will soon depart evening skies.

Enter the ice giants, Uranus and Neptune. Both reach opposition for 2015 over the next two months, and the time to cross these two out solar system planets off your life list is now.

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Looking east at dusk in late August, as Uranus and Neptune rise. Image credit: Stellarium

First up, the planet Neptune reaches opposition next week in the constellation Aquarius on the night of August 31st/September 1st. Shining at magnitude +7.8, Neptune spends the remainder of 2015 about three degrees southwest of the +3.7 magnitude star Lambda Aquarii.  It’s possible to spot Neptune using binoculars, and about x100 magnification in a telescope eyepiece will just resolve the blue-grey 2.3 arc second disc of the planet. Though Neptune has 14 known moons, just one, Triton, is within reach of a backyard telescope. Triton shines at magnitude +13.5 (comparable to Pluto), and orbits Neptune in a retrograde path once every 6 days, getting a maximum of 15” from the disk of the planet.

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The path of Neptune from late August through early November 2015. Inset: the position of Neptune’s moon Triton on the evening of August 31st: Image credit: Starry Night Education software

Uranus reaches opposition on October 11th in the adjacent constellation Pisces.  Keep an eye on Uranus, as it nears the bright +5.2 magnitude star Zeta Piscium towards the end on 2015. Shining at magnitude +5.7 with a 3.6 arc second disk, Uranus hovers just on the edge of naked eye visibility from a dark sky site.

Uranus, left of the eclipsed Moon last October. Image credit and copyright: A Nartist

It’ll be worth hunting for Uranus on the night of September 27th/28th, when it sits 15 degrees east of the eclipsed Moon. Uranus turned up in many images of last Fall’s total lunar eclipse.  This will be the final total lunar eclipse of the current tetrad, and the Moon will occult Uranus the evening after for the South Atlantic. This is part of a series of 19 ongoing occultations of Uranus by the Moon worldwide, which started in August 2014, and end on December 20th, 2015. After that, the Moon will move on and begin occulting Neptune next year in June through the end of 2017.

The visibility footprint of the September 29th occultation of Uranus by the Moon. Image credit: Occult 4.0.

Uranus has 27 known moons, four of which (Oberon, Ariel, Umbriel and Titania) are visible in a large backyard telescope. See our extensive article on hunting the moons of the solar system for more info, and the JPL/PDS rings node for corkscrew finder charts.

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The path of Uranus, from late August through early December 2015. Inset: the position of the moons of Uranus on the evening of October 12th. Image credit: Starry Night Education software

The two outermost worlds have a fascinating entwined history. William Herschel discovered Uranus on the night of March 13th, 1781. We can be thankful that the proposed name ‘George’ after William’s benefactor King George the III didn’t stick. Herschel initially thought he’d discovered a comet, until he followed the slow motion of Uranus over several nights and realized that it had to be something large orbiting at a great distance from the Sun. Keep in mind, Uranus and Neptune both crept onto star charts unnoticed pre-1781. Galileo even famously sketched Neptune near Jupiter in 1612!  Early astronomers simply considered the classical solar system out to Saturn as complete, end of story.

A classic 7″ Merz refractor at the Quito observatory, nearly identical to the instrument that first spied Neptune. Image Credit: Dave Dickinson

And the hunt was on. Astronomers soon realized that Uranus wasn’t staying put: something farther still from the Sun was tugging at its orbit. Mathematician Urbain Le Verrier predicted the position of the unseen planet, and on and on the night of September 23rd, 1846, astronomers at the Berlin observatory spied Neptune.

In a way, those early 19th century astronomers were lucky. Neptune and Uranus had just passed each other during a close encounter in 1821. Otherwise, Neptune might’ve remained hidden for several more decades. The synodic period of the two planets—that is, the time it takes the planets to return to opposition—differ by about 2-3 days. The very first documented conjunction of Neptune and Uranus occurred back in 1993, and won’t occur again until 2164. Heck, In 2010, Neptune completed its first orbit since discovery!

To date, only one mission, Voyager 2, has given us a close-up look at Uranus and Neptune during brief flybys. The final planetary encounter for Voyager 2 occurred in late August in 1989, when the spacecraft passed 4,800 kilometres (3,000 miles) above the north pole of Neptune.

All thoughts to ponder as you hunt for the outer ice giants. Sure, they’re tiny dots, but as with many nighttime treats, the ‘wow’ factor comes with just what you’re seeing, and the amazing story behind it.

Uranus: A Guide to its 2013 Opposition

Up for a challenge? Got a big 12” light bucket of a Dobsonian telescope and looking for something new to point it at? This week, as the Moon reaches New phase on October 4th and stays safely out of the late evening sky, why not check out Uranus and its retinue of moons. And yes, we’ve heard just about ALL the Uranus jokes —its pronounced yer-in-us, thank you very much — but feel free to attempt to pen an original if you must.

Now, back to astronomy. Uranus reaches opposition for 2013  on Thursday, October 3rd at 14:00 Universal Time. Opposition is the point in time that an outer planet rises as the Sun sets. In the case of Uranus, its opposition dates advance forward by about 4-5 days each year.

The current location of Uranus in Pisces. Created by the author using Stellarium. (click to enlarge).

This also marks the start of the best time to hunt for the planet among the star fields of the constellation Pisces. Uranus will reach its maximum elevation above the southern horizon for northern hemisphere viewers for early October around local midnight. For observers south of the equator, Uranus will transit to the north. Incidentally, Uranus also currently sits near the equinoctial point occupied by the Sun during the March equinox, making viewing opportunities nearly equal for both hemispheres.

Uranus is 19.04 astronomical units distant during opposition 2013, or about 158 light minutes away. Shining at magnitude +5.8, Uranus presents a tiny blue-green disk just under 4” across at opposition.

Uranus currently lies six degrees SW of the +4.4 magnitude star Delta Piscium, on the border of the constellations Pisces and Cetus. Uranus will actually be crossing once again into the non-zodiacal constellation of Cetus later this year.

Discovered in 1781 by Sir William Herschel, Uranus has only completed 2 full orbits (2.75 to be precise) in its 84.3 year trips about the Sun. We can be thankful that William’s proposal to name the planet Geogium Sidus after his benefactor King George the III didn’t stick!

Starry Night Education
The path of Uranus into Cetus. Created by the author using Starry Night Education software.

At opposition, Uranus will be located at;

Right Ascension: 0h 40’

Declination: +3° 25’

Five of the 27 known moons of Uranus are also within the grasp of a moderate-sized backyard scope as well. The trick is to catch ‘em near greatest elongation, when they appear farthest from the “glare of Uranus” (hey, there’s a freebie for a snicker or two). An eyepiece equipped with an occulting bar, or simply nudging Uranus out of the field of view can also help.

With magnitudes ranging from +13 to +16, the moons of Uranus are similar in brightness to Neptune’s large moon Triton or the tiny world Pluto.

The five brightest moons of Uranus and their respective maximum elongations are:

Chart constructed by author.

And here’s a handy finder chart for the coming month, showing maximum elongations for each:

A corkscrew graph featuring the greatest elongations for the five brightest moons of Uranus. (Created by Ed Kotapish using PDS Rings Node).
A corkscrew graph featuring the greatest elongations for the five brightest moons of Uranus through October. (Created by Ed Kotapish using PDS Rings Node).

The first two moons were named Titania and Oberon by William’s son John after characters from William Shakespeare’s A Mid-Summer Night’s Dream. William discovered the first two moons of Uranus on the night of January 11th, 1787 using his 49.5” reflector. His scopes were so advanced for his day, that it wasn’t until over a half a century later that William Lassell discovered Umbriel and Ariel using the Liverpool Observatory’s 24” reflector in 1851.

Gerard Kuiper would later add tiny Miranda to the list, nabbing it with the McDonald Observatory’s 82” Otto Struve Telescope in 1948. We would then have to wait until Voyager 2’s 1986 flyby of Uranus in 1986 to add more. To date, Voyager 2 remains the only spacecraft to visit Uranus and Neptune.

The current convention established by the International Astronomical Union is to name the moons of Uranus after characters from the plays of Shakespeare or Alexander Pope’s Rape of the Lock.

There’s still a wide range of names in said literature to choose from!

It’s interesting to note that the orbits of the moons of Uranus are also currently tipped open about 25 degrees to our line of sight and widening. They were edge on in December 2007, and will be perpendicular to our Earthly view come 2029, after which they’ll head back to edge on in 2049. This is because Uranus and the orbits of its moons are tipped at a 97 degree angle relative to the planet’s orbit. This is why elongations for its moons are often quoted it terms of “north and south” of the planet, rather than the familiar east and west. Shadow transits of the moons can occur with about a year and a half during plane-crossing seasons, but they’re ~42 years apart and tough to spot on the tiny disk of Uranus!

October 4th Starry Night Education
An example of the orientation of Uranus’s moons on October 4th, with Oberon at greatest elongation. Note that Miranda is the tiny unlabeled moon with the interior orbit. (Created by the author using Starry Night Education software).

Uranus also reached aphelion in 2009 at 20.099 AU from the Sun —we’re still at the farther end of the spectrum, as oppositions of Uranus can range from 19.09 to 17.28 AU distant.

Uranus will rise earlier on each successive evening until it reaches quadrature at the end of the year on December 30th. At this point, it’ll be roughly due south at local sunset. Keep in mind, there’s also another ice giant worth hunting for in the adjacent constellation of Aquarius named Neptune.

So ignore those bad puns, and be sure to take out that 10” (scope, that is) and point it at Uranus!