Mohawk Valley Astronomical Society

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Misty Blue Orbs

by Perry Pezzolanella, MVAS

Jupiter and Saturn command attention for another summer of pleasant evening viewing, but two other worlds further away are sadly overlooked. Uranus and Neptune are dim and considered too remote to seriously observe; however, both will actually be easier to find than last year. Under clear, moonless nights away from city lights, the finder charts that accompany this article will make it surprisingly easy to find and track these remote worlds.

Uranus (magnitude +5.7) and Neptune (magnitude +7.8) are dim because they orbit the Sun at a distance of 1.8 and 2.7 billion miles, respectively. In the dim depths of the outer Solar System, daytime sunlight is no brighter than a clear evening sky 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 most of 2009 in Pisces near the Aquarius border while Neptune is in Capricornus. Neptune will be at opposition on August 17 while Uranus will be at opposition on September 17. 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 by February 2010. Uranus can be found below the Circlet of Pisces. Neptune has moved to the east of the 3rd magnitude star Delta Capricorni and a nearly vertical string of three 5th magnitude stars (42, 44, and 45 Capricorni) this year with the added bonus of being near Jupiter.

Given a night of steady seeing, a small telescope should be capable of resolving the discs and revealing the colors of these remote worlds; however, both planets are too far away to observe cloud detail or moons unless the telescope has an aperture of at least 16 inches. Uranus is 3.7 arcseconds across and Neptune is 2.4 arcseconds across. The planets appear distinctly different with Uranus having a rich turquoise hue while Neptune is a chilly, icy-blue disc.

The MVAS Apollo Observatory houses a research grade, 16-inch Schmidt-Cassegrain telescope and its superior optics in the relatively dark country sky has revealed Uranus as a true turquoise globe with two moons, Oberon and Titania, occasionally visible. The other three rather large moons of Uranus, Umbriel, Ariel, and Miranda, are more difficult to see, and all five moons shine at magnitude +14 to +15. Neptune appears like a tiny bluish globe with magnitude +13.5 Triton shining dimly nearby.

Observing detail on each planet is the biggest challenge, but white spots and dusky banding have been noted on both Uranus and Neptune. Amateur astrophotographers have been able to photograph a bright polar hood on Uranus and are beginning to coax out faint detail on Neptune. A magnification of at least 500x for Uranus and 900x for Neptune along with a yellow-green (Wratten #11) filter are required in order to have a chance at photographing any detail.

Uranus and Neptune may seem too remote and small to bother observing, but the misty bluish tint of these tiny orbs is an unusual color to observe in the heavens and viewing them in any telescope makes them a rewarding find.