Mohawk Valley Astronomical Society

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Blue Odyssey

by Perry Pezzolanella

Jupiter and Saturn received much attention during the past several years during the summer and fall evenings while two other worlds further away have been steadily growing higher in the sky with each passing year. Uranus and Neptune are often thought to be too dim and small to seriously observe, but both are actually quite easy with a little determination. Under clear, moonless skies away from city lights it will be much less stressful to find Uranus and Neptune with the help of the finder charts that accompany this article.

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 slightly more methane (3%). These colors are dramatic whenever they are near stars of contrasting colors.

Uranus spends 2010 in Pisces just below the Circlet while Neptune is on the border of Capricornus and Aquarius. Neptune will be at opposition on August 20 while Uranus will be at opposition on September 21. 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 2011. Uranus will be very close to Jupiter on September 22 and next January 2 making a stunning color contrast through a low-power eyepiece, especially on September 22 with both near opposition. Neptune can be found not far to the upper left of the +5.1 magnitude star Mu Capricorni just to the east of the trio of vertical stars it was near last year.

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 challenging to find, but patience will pay off with a view of colors not commonly seen in the night sky. Finding and observing these distant bluish worlds is a rewarding odyssey.