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The Agena AstroProducts Guide to Observing Uranus and Neptune

By: Brian Ventrudo
September 25, 2017
Observing Guide - Uranus and Neptune

Figure 1 – Color-enhanced images of Uranus (left) and Neptune from the Voyager 2 spacecraft. Image credit: NASA/JPL.

1. Ice Giants

Beyond Saturn, in the cold reaches of the outer solar system, lie the giant planets Uranus and Neptune. Uranus is the third-largest planet by radius and the fourth-largest by mass in the solar system, while Neptune is a little smaller but slightly more massive than Uranus. Both planets were once considered gas giants like Jupiter and Saturn. Now astronomers understand Uranus and Neptune are quite distinct from their larger neighbors. These planets are made of just 20% hydrogen by mass compared to 90% for Jupiter and Saturn, and they have much higher bulk concentrations of heavier elements and frozen gases such as water and methane. Because of these differences, astronomers now designate Jupiter and Saturn as "gas giants", and Uranus and Neptune as "ice giants".

2. An Overview of Uranus and Neptune: A Study in Contrasts

Uranus and Neptune are alike in many ways which suggests they formed under similar conditions out of similar materials. But like Venus and Earth, two other planets that are superficially similar, Uranus and Neptune also have profound and unexplained differences.

Uranus lies about 20 astronomical units (AU) from the Sun (3 billion km) and has an orbital period of 84 years. At this distance, the Sun's light is just 1/400th times as bright as on Earth. The planet rotates once every 17 hours, which is unexceptional, but the polar axis of Uranus is tilted 98 degrees from perpendicular, as if the planet were knocked over on its side, and it rotates about its axis in a direction opposite most of the other planets in the solar system. The orbital plane of its moon and rings are also tilted. Astronomers suspect the planet was struck by a large proto-planet early in the history of the solar system and knocked over on its side. Uranus also generates less internal heat than Jupiter, Saturn, or Neptune, which again may be a result of an interior scrambled by a major impact event.

Neptune, by contrast, seems to have enjoyed a more placid youth. It has an axial tilt of 28 degrees, a rotational period of 16 hours, and a nearly perfectly circular orbit suggesting it endured no major collisions in its early history. It revolves around the Sun in 164.8 years and lies about 30 astronomical units from the Sun (4.5 billion km) which means it receives 1/900th times as much of the Sun's light as Earth and less than half the light received by Uranus. For reasons that are not completely understood, Neptune has a deeper blue color and more visibly active cloud bands and weather patterns than the paler blue-green Uranus. The cloud tops in the atmosphere of Neptune are indeed icy cold: they have a temperature of 55 K (-218 degrees Celsius). However, again for reasons that are not completely understand, the cold atmosphere of Neptune has sufficient energy to drive winds of 2,100 km/s, faster than any other winds on any planet in the solar system.

2.1 The Atmospheres of Uranus and Neptune

Through earthbound telescopes, Uranus and Neptune reveal little detail. Even in close-ups captured by Voyager 2 during its rendezvous with the planets in the 1980s, Uranus appeared nearly featureless at visible wavelengths, likely because a high photochemical haze hides the lower cloud layers. Neptune, by contrast, revealed startling detail in its cloud layers including well defined bands and a cyclonic oval "dark spot" similar to Jupiter's Red Spot. The dark spot seems to have since faded. The more pronounced weather in Neptune's cloud layers may be the result of the greater heat generated within the planet which stirs up the icy clouds.

Unlike Jupiter and Saturn, Uranus and Neptune take on a greenish-blue and bluish-green hue, respectively. The colors are a result of the absorption of red light by icy methane gas in the atmospheres. Both planets, Uranus especially, has a set of coal-black rings which cannot be observed directly with telescopes. The rings were discovered indirectly from Earth in 1977 when astronomers observed the unexpected fluctuation in brightness of a star just before and after it was occulted by Uranus.

Uranus has 27 moons in total. Five are bright enough to be visible from Earth. They are named after characters from plays by Shakespeare and a poem by Alexander Pope: Titania, Oberon, Miranda, Ariel, and Umbriel. Titania and Oberon, the largest moons each with a radius of about 1600 km, were discovered by William Herschel himself with painstaking observations made in 1787.

Of Neptune's 14 known moons, Triton is by far the largest and most interesting. Discovered just 17 days after the planet, the moon is too small to reveal much detail in earthbound telescopes. But Voyager 2 made some astonishing images of Triton that showed uneven patches of blue and pink and gray, as well as surface geysers which blast material 10 km into the moon's thin atmosphere of nitrogen and hydrocarbon gas. Triton's low density and geyser-like activity suggest the object is not a natural rocky moon but an icy body from further out in the solar system. In some ways it resembles Pluto, which itself is a large low-density dwarf planet. In fact Triton has a radius of 1350 km, some 200 km larger than Pluto.

2.2 The Discovery of Uranus and Neptune

Uranus shines at magnitude 5.7 on average, easily bright enough to be visible without optics in very dark sky. But the planet is faint enough such that in thousands of years of observations, no stargazer ever noticed the wandering motion of the planet before the invention of the telescope. Uranus was discovered on the night of March 13, 1781 by one of history's greatest amateur astronomers, William Herschel. This unexpected discovery assured Herschel's worldwide fame and secured him a stipend from King George III which propelled Herschel into a career as a professional astronomer.

Neptune is nearly a twin of Uranus, but it's half again as far away and so appears fainter and smaller in a telescope. While Uranus was discovered by chance by the keen-eyed Herschel, 8th-magnitude Neptune was discovered in a more scientific manner. British astronomer John Couch Adams predicted the location of Neptune mathematically based on slight variations induced by the planet's gravity on the orbit of Uranus. With the help of Adams' calculations, the French astronomer Urbain-Jean-Joseph Leverrier first observed Neptune on September 24, 1846. Leverrier was not the first to see the planet. John Lamont and J.J. Lalande had observed Neptune early in the 19th century but had not noticed its planetary nature. Also without recognizing it, Galileo observed Neptune in late December 1612, just days before the planet was occulted by Jupiter, an astonishing event that no one has witnessed since.

Observing Guide - Uranus and Neptune

Figure 2 -- Among the best images ever taken by an amateur astronomer of Uranus (left) and Neptune (left). Damian Peach captures these images when both planets were near opposition in 2010 under very steady sky on the island of Barbados. Image credit: Damian Peach.

While broadband filters offer modest improvement in nebulae for visual observers, these filters can be much more useful for imaging. That's because CCD and CMOS cameras are much more sensitive to red light at the H-alpha wavelength than the human eye, so they produce brighter images of objects like stars and nebulae that emit H-alpha light.

But for visual use, which broadband filter works best? Experienced visual observers suggest that there is no one broadband filter that is optimal for every celestial object, for all telescopes and eyepieces, and in all sky conditions. What's more, the visual performance of each filter can be somewhat subjective. All filters will show some improvement for visual observation of nebulae, especially in suburban and darker skies, but most are often overwhelmed by light pollution in urban skies. More expensive broadband filters, on average, tend to have better rejection of out-of-band light, better coatings, and higher quality glass substrates.

3. Observing Uranus and Neptune

3.1 Where to Find Uranus and Neptune

Before you examine Uranus and Neptune through a telescope, you have to find them. The planets are much trickier to find than the five brighter planets visible in the sky, all of which are easy naked-eye objects. Uranus can be glimpsed without optics, which is a worthwhile project in itself, but generally you need a map or planetarium software to find these ice giants. If you don't have appropriate software, Sky and Telescope magazine publishes up-to-date maps to help you spot them. From 2017 through 2024, the planet Neptune moves generally eastward each year from the constellation Aquarius into Pisces, while Uranus moves from Pisces to Aries to Taurus.

3.2 Observing the Planetary Disks

As the Voyager 2 spacecraft discovered in 1986 and 1989, Uranus and Neptune are beautiful blue-green worlds seen close up. But they are extremely distant and tiny as seen from Earth and reveal little detail. Uranus has an apparent size of 3.3" at conjunction to 4.1" at opposition, while Neptune has an apparent size of 2.2" to 2.4". To make Uranus appear as large in a telescope as the full Moon appears to the unaided eye, a magnification of 500x is required. For Neptune, 750x is required. These high magnifications are often impractical, even in large telescopes, because of turbulence in the atmosphere.

Both planets appear star-like in binoculars and at low magnification in a telescope. At magnitude 5.7, Uranus is bright enough to be easily visible against the background stars. A 70-80 mm telescope at 100x-150x will start to show disk of the planet. If you're unsure you are seeing Uranus, just swap in eyepieces with different magnifications and look for the disk of the planet to change size. Stars will remain point-like at all magnifications. Because these two planets are extended objects, they tend to twinkle less than stars.

Neptune takes a little more effort to see than Uranus. It averages a brightness of magnitude 7.8, more than bright enough to be seen in a pair of binoculars, but its small apparent diameter makes it a challenge todetect in a small telescope. It reveals a disk at 200x in a 6-inch or larger scope. Smaller scopes have a harder time discering such a small disk.

As with observing any planet, the best telescopes for observing Uranus and Neptune are long focal-length and large aperture scopes, especially Schmidt-Cassegrains such at Celestron NexStar and CPC scopes and Meade LX200and LX90 systems.

The long focal ratio of Maksutov-Cassegrain telescopes also helps with higher magnification, and the Sky-Watcher 180mm Mak-Cass has long been a favorite of lunar and planetary observers because it packs a long 2700mm focal length in a tube just 22" long.

Observing Guide - Uranus and Neptune

Figure 3 - Long focal-length telescopes are critical for giving large images of Uranus and Neptune.

Cranking up the magnification is also critical when observing the disks of Uranus and Neptune, so eyepieces with short focal length are required. While the featureless disks of Uranus and Neptune begin to reveal themselves at 100x to 200x, respectively, more is better. Since little detail is visible, relatively simple and inexpensive short-focal-length eyepieces such as Plossls from GSO, Tele Vue Optics, as well as Agena's Starguider eyepieces can work well, as can long-eye-relief Tele Vue DeLites. The optimum magnification can change from night to night with atmospheric conditions, so keep eyepieces with a range of focal lengths at hand. A good barlow lens or focal extender also helps you obtain higher magnification using eyepieces with slightly longer focal lengths and better eye relief.

Observing Guide - Uranus and Neptune

Figure 4 - A Barlow lens or focal extender gives higher magnification with eyepieces with moderate focal length and good eye relief. Ideal for high-power planetary observation.

Uranus reveals a slightly blue-green color in a telescope of 6 inches to 8 inches aperture. Fainter Neptune reveals even less color. Larger scopes reveal a more bluish color to the eye. A color filter can help you see some color in both planets with 12-inch or larger telescopes. Good filter choices include #12 Yellow, #58 Green, and #38A Dark Blue filters.

While seeing detail visually on the disks of Uranus and Neptune is a tall order with any telescope, experienced imagers have teased out some detail on these planets. The expert planetary imager Damian Peach, for example, has captured some crisp and impressive images of these planets using a Celestron 14 telescope, various planetary imaging cameras, and IR filters. Figure 2 shows an example of his work and you can see more of his extraordinary images of Uranus and Neptune at his website.

3.3 Finding the Moons of Uranus and Neptune

When it comes to seeing the moons of Uranus and Neptune, there is no substitute for large aperture.With 10" or larger telescope and much patience, you can see two of the moons of Uranus. Titania (mag 14.0) and Oberon (mag 14.1) have orbits that put them as far as 33" and 44" from Uranus,approximately, at opposition, which is far enough to escape the glare of the planet's disk. Ariel (mag 14.3) and Umbriel (mag 15.0) are much harder because they extend just 10" to 15" from the relatively bright planetary disk. A 12" or larger scope is required to see these icy worlds. It helps to put the planet out of the field of view and use clean high-contrast eyepieces with few optical elements such as Plossls. The moon Miranda (mag 15.8) is only visible in very large telescopes.

Observing Guide - Uranus and Neptune

Figure 5 - A Stellarium simulation of the position of the five largest moons of Uranus. Credit: Stellarium.org.

In the Neptunian system, only the large moon Triton is bright enough to see in an amateur telescope. The Pluto-like Triton is probably a dwarf planet from the Kuiper belt that was captured by Neptune. At opposition, Triton shines at magnitude 13.6 and its orbit takes it up to 17" from Neptune as seen from Earth. Compared to the moons of Uranus, Triton is easier to see because dimmer Neptune produces much less glare.

To find the position of the moons of Uranus and Neptune on a particular night, use your favorite planetarium software or these Javascript tool from Sky and Telescope:

http://www.skyandtelescope.com/wp-content/observing-tools/uranus_moons/uranian.html
http://www.skyandtelescope.com/wp-content/observing-tools/neptune_moons/neptune.html

4.0 Summary

While they don't attract the interest and attention of the brighter planets of our solar system, Uranus and Neptune are still accessible with modest telescopes and should make the must-see list of every amateur astronomer. These distant ice giants give up little detail in a telescope, but their subtle colors and intriguing natural satellites present excellent challenges to visual observers and imagers equipped with telescopes and accessories that give high magnification and good resolution.