Ice Giants: Neptune and Uranus

Uranus and Neptune, the so-called ice giants, are the only major planets in our solar system that aren't easily visible to the unaided eye. If you already know what these planets look like and just need charts to find them, skip to the bottom of this article on finding these ice giants. But if you've never seen Uranus or Neptune before, it's helpful to know how they were discovered in the first place. Every time you set out to find a new celestial object, you are in some sense reliving the original discovery.

The Discovery of Uranus

In 1779, an obscure amateur astronomer named William Herschel decided to view all the bright stars in the sky at high magnification to see if they were double stars. Two years into this project, on March 13, 1781, he noticed a "star" in Taurus that looked quite different from all other 6th-magnitude stars when viewed at 227× in his homemade 6.2-inch reflecting telescope. When he observed it again four nights later, it had moved with respect to the background stars, proving that it was actually an object inside our solar system. At first, he assumed that it was a comet.

William Herschel, the amateur astronomer who discovered the ice giants, Neptune and Uranus.
William Herschel, the discoverer of Uranus
Wikimedia Commons / PD

When professional astronomers viewed Herschel's "comet," they saw only a garden-variety star. That's because — unknown to him or anyone else — Herschel's homemade reflector was far superior to most professional scopes. But it was easy to watch Herschel's object moving from one night to the next, and that allowed mathematicians to compute its orbit. It turned out to take a nearly circular path around the Sun, just like all the known planets, and very unlike the elongated orbits of comets. And the new object was much farther from the Sun than any solar-system body had ever been seen before. Considering how bright it appeared, it must be many times bigger than Earth.

Herschel had, in fact, stumbled upon the discovery Uranus — the first new planet discovered throughout all of human history and one of the now-known ice giants. Locating this ice giant was the most revolutionary discovery since Galileo spotted the moons of Jupiter 170 years earlier. Herschel became an instant celebrity and won a stipend from the King of England that allowed him to become a full-time astronomer.

How to Observe Uranus

As the story of its discovery indicates, Uranus is easy to see, but not so easy to recognize as a planet. If you're willing to use our sky charts to identify the planet — taking it on faith that we're telling the truth — then you won't need any tools besides binoculars. In fact, you might be able to see Uranus with just your unaided eyes if your sky is very dark.

Uranus: One of two Ice Giants discovered and observed.
Uranus, imaged at high power on August 28, 2007, with a 12.5-inch reflector using the stacked-video technique. Visually Uranus appears much more white than blue-green.
S&T: Sean Walker

But you'll need to examine the planet quite carefully with a telescope at 100× or higher to see that it's actually a tiny disk rather than a pinpoint of light like a star. That means that you need to pinpoint its location precisely. Being in the right general vicinity isn't good enough. It's easy to scan right over Uranus without noticing that it's anything but a regular star. Remember — many generations of highly skilled observers before Herschel did precisely that.

Neptune and Uranus are in Aquarius and Pisces, respectively, throughout this period. Uranus is 9° north of the celestial equator, and Neptune will be 8° south of the center line at the end of 2017 — neither gets very high in the sky for people at mid-northern latitudes. So it's important to make the best of the relatively short window of opportunity for viewing them. Neptune rises first, but because it's so much fainter, it won't actually be visible much earlier. Sometime in June or July, both planets become high enough for decent telescopic viewing in the predawn sky.

The first step is to make sure that the planet is above the horizon — and preferably fairly high in the sky — when you plan to look for it. From January through May, Uranus is so close to the Sun that it's difficult or impossible to spot. You can see Uranus as early in the year as June if you're willing to get up before dawn, and this year, you’ll get an assist from Venus. On June 3, 2017, Uranus will be less than 2° from that bright planet. Shining at magnitude –4.4, Venus will serve as a brilliant signpost to the dimmer planet.

The best time to view Uranus, however, is when it's in the evening sky from September through December. January's not impossible, but you'll have to start right after sunset. Uranus is easy to locate if you have a telescope with an accurate Go To mount; otherwise, you'll need the charts at the bottom of this article. And if you need to brush-up your chart-reading skills, you might want to look at our online article Using a Map at the Telescope.

You may be able to recognize Uranus just by its hue, which most people find faintly blue or green. Contributing Editor Tony Flanders can see the color even with his10×50 binoculars. Through a telescope, even at magnifications too low to see that planet's disk, you may notice that it shines with a steadier light than other similarly bright stars. And at 120× in a 70-mm telescope, Tony can quite clearly make out a tiny disk or dot — about the size of the period at the end of this sentence. Don't expect to see any features on the ice giant planet, though. Even giant professional telescopes can barely do that.

Uranus begins retrograde (westward) motion against the background sky on August 2nd and reaches solar opposition on October 19th, when it will shine at magnitude +5.7. On the date of opposition, the planet’s angular diameter will be 3.7″ (arcseconds).

October is a good time to hunt for the moons of Uranus. Use the S&T javascript utility Moons of Uranus to predict their positions. At the date of Uranus’s opposition, Titania will be magnitude +14.0 and Oberon will be +14.1. Ariel and Umbriel are closer to Uranus, so it will be difficult to see them, but if you have/make an occulting bar or eyepiece, go for it!

The Discovery of Neptune

Urbain Jean Joseph Le Verrier accurately calculated the position of an unseen planet based on the motion of Uranus, the second planet known as one of the ice giants.
Urbain Jean Joseph Le Verrier (1811–77) accurately calculated the position of an unseen planet based on the motion of Uranus. In his day, he was famous for both his brilliance and arrogance.

Once Herschel had overturned the millennia-old wisdom that there were exactly five planets besides Earth, astronomers started actively searching for new ones. And indeed, four new planets were discovered between 1801 and 1807, all orbiting between Mars and Jupiter. But these were tiny compared to Earth, let alone Uranus — too small to show as extended disks through most telescopes. Herschel, by then the grand old man of astronomy, called them asteroids because they look just like stars (Latin astra). Asteroids' rapid motion with respect to the "fixed" stars makes them great targets for backyard telescopes.

It wasn't until 1846 that another really large planet was found. And Neptune, as the new planet came to be called, was found in much the same way that you're going to find it. J. G. Galle and H. L. d'Arrest, staff astronomers at the Berlin Observatory, looked where the new planet was predicted to be, compared what they saw with a star chart, found an uncharted star, and then verified that it was in fact a planet.

But credit for the discovery goes not to the astronomers who first saw Neptune but to Urban Jean Joseph Le Verrier, who predicted where it would be found. It had been known for some time that Uranus didn't move exactly as it should, taking the gravitational attraction of the Sun and the known planets into account. Le Verrier analyzed the discrepancy, concluded that it must be due to the pull of a large planet well outside Uranus's orbit, and predicted the new planet's location with an error of just one degree. It was a stunning triumph for theoretical astronomy.

How to Observe Neptune and Uranus

Neptune and Uranus are near-twins in actual size, but Neptune is about 50% more distant, which makes it surprisingly much harder to find. But if you can find Uranus, you can find Neptune too, with the aid of the charts below. It just requires using the same techniques more carefully.

Neptune varies from magnitude 7.8 to 8.0, about two magnitudes fainter than Uranus. It's visible in steadily-supported binoculars, but only if you look quite carefully. And while Uranus is frequently brighter than any other star visible in the same binocular or finderscope field, the sky is crowded full of stars as bright as Neptune. So you have to be careful when you match up your charts with what you see through the eyepiece.

Having said all that, it's worth remembering that even a very small telescope can easily show stars down to eighth or ninth magnitude. So Neptune is not faint by telescopic standards. In fact, it's bright enough to stimulate color vision through any telescope with 4 inches (100 mm) of aperture. Look for a hue quite similar to Uranus's, though somewhat bluer.

Neptune's disk is plainly visible at 200× through a 6-inch telescope on a night of steady seeing. But it may be quite hard to see the disk if conditions are bad or your telescope is improperly collimated. Tony's 70-mm refractor is a little too small to resolve Neptune properly, but when he examines the planet carefully at 120× it looks clearly different from a star of similar brightness. Neptune's light is distinctly steadier, and it appears more solid. Not exactly a disk, but a fat pinpoint. It's a little more apparent in Observing Editor JR Johnson-Roehr's 90-mm refractor, but don't take our word for it; see for yourself what Neptune looks like!

Neptune begins retrograde motion on June 15th, when it will be 2.5° east of Lambda (λ) Aquarii. It will move westward over the summer. Neptune reaches opposition on September 4th, when it will shine at magnitude 7.8. Its disk will be 2.4″ on that date. Can you see Triton? On June 15th, it will be a tiny, 13.6-magnitude speck. Neptune begins prograde (eastward) motion on November 22nd, heading back toward Lambda Aqr.

Charts and Tools for Observing the Ice Giants

Because Uranus and Neptune are so far from the Sun, these ice giants move very slowly across the celestial sphere. They appeared side-by-side in 1993, and they've only drifted about 43° apart since then. So they're still visible at more or less the same time of year and/or night.

Neptune reaches opposition to the Sun on September 4th (5:13 UT), and Uranus on October 19th (17:21 UT). These are the dates when the planets rise around sunset, set around sunrise, and reach their highest in the sky in the middle of the night. Neptune is reasonably well placed in the evening sky from August through December, and Uranus from September through February 2018. They're detectable after that, but too low for high-power telescopic observing.

Get charts for the June 2017 — March 2018 season.

2 thoughts on “Ice Giants: Neptune and Uranus

  1. AlphaCentauri

    I’m going to give it a shot at seeing Uranus and Neptune. I have 2 telescopes, one is a 70mm refractor, the other is a 130mm reflector. I’m learning to collimate it, so i can get the best views.I have bought some better eyepieces than the one’s that came with these scopes. So I hope to see them sometime late this summer or early fall.

    1. JRJR

      Good luck! I recommend starting with Uranus, which you can see even with small-ish binoculars (although it looks just like a star with small binos). I also recommend dark skies (because Pisces is dim), patient star-hopping, and making a sketch of all the field stars. Come back to the same field in a week or two, make another sketch, and you’ll see how far Uranus has moved.

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