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Azure Passages - Uranus & Neptune

by Perry Pezzolanella, MVAS

It has been a great year for observing the two largest giant planets; Jupiter and Saturn. In the wake of the two worlds as they slip into the glare of the evening twilight are two other gas giants that are often overlooked; Uranus and Neptune. They may seem challenging to find, but all that is required are the finder charts that accompany this article and a clear moonless night, preferably away from city lights.

Uranus and Neptune are dim because they orbit the Sun at a distance of 1.8 and 2.8 billion miles, respectively. In the dim depths of the outer Solar System the daytime Sun is no brighter than a clear evening on Earth shortly after sunset. Both planets are about four times larger than Earth, slightly over 30,000 miles in diameter, and have thick atmospheres that are completely cloudy. The small amount of methane (2%) in Uranus� atmosphere absorbs the red component of sunlight and scatters the blue creating a turquoise hue. Neptune appears even bluer since it is not as hazy and has more methane (3%). These colors are dramatic whenever they are near stars of contrasting colors.

Uranus spends all of 2006 in Aquarius while Neptune is in Capricornus. Neptune is at opposition on August 11 at magnitude +7.8. Uranus is at opposition on September 5. Rising at sunset and setting at sunrise, both worlds will be up all night on these dates. They will remain in the evening sky for the rest of the year until they are lost in the evening twilight during January 2007. Uranus can be found to the east of the 3rd magnitude star Lambda Aquarii. Neptune will spend most of the time above the +4.3-magnitude star Iota Capricorni.

A small telescope is capable of resolving the discs and revealing the colors of these remote worlds given a night of steady seeing. The planets appear distinctly different with Uranus having a rich turquoise hue while Neptune is a chill icy-blue disc. Uranus is 3.7 arc-seconds across and 1.77 billion miles away at it closest point this year. Neptune is 2.4 arc-seconds across and 2.70 billion miles away. Both planets are too far away to observe cloud detail or the moons unless the telescope is at least 16 inches.

The MVAS Apollo Observatory houses a 16-inch Schmidt-Cassegrain telescope and its clean, new optics in the relatively dark country sky has already given Uranus and Neptune distinct personalities. Uranus appears as a true turquoise globe instead of a tiny disc and two moons, Oberon and Titania, can occasionally be seen within its glare. The other three rather large moons of Uranus, Umbriel, Ariel, and Miranda are more difficult to see and all five moons shine dimmer than Pluto at magnitude +14 to +15. Neptune appears like a tiny bluish globe with magnitude +13.5 Triton shining dimly nearby.

A good project in this digital age is to photograph each planet on as many nights as possible in order to plot the orbits of the moons and to estimate how long it would take each to orbit once around its respective planet. No cloud detail can be seen on either of the planets and a magnification of at least 500x for Uranus and 900x for Neptune are required. A yellow-green (Wratten #11) filter will help and if done digitally it may be possible to tweak out some detail by stacking the best pictures and processing accordingly. The Hubble Space Telescope continues to monitor active weather on both worlds with several new white spots and bands along with darker belts developing and evolving.

When viewing the azure worlds of Uranus and Neptune far away in the cold and dim outer Solar System, it is easy to think of them as a gateway to the infinity of space. What is really incredible is that nothing more than a small telescope is needed to experience this passage into the vast universe beyond.