Mohawk Valley Astronomical Society

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Blue October

by Perry Pezzolanella

This year has been busy with the brighter planets competing for attention along with the Venus transit. Jupiter will be returning later in the year to once again be the main attraction of the holiday season, but two distant worlds are frequently neglected and deserve to be added to one’s list of observed celestial objects. 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 is possible 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 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 2012 in Pisces near the border of Cetus and to the left of the Circlet while Neptune is in Aquarius below the Water Jar asterism. Neptune will be at opposition on August 24 while Uranus will be at opposition on September 29. 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 2013.

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.

The new MVAS observatory in Waterville will house a research grade, 16-inch Schmidt-Cassegrain telescope with superior optics in the relatively dark skies far south of city lights. At its former location, the Seif telescope has already 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, but quite easy to identify.

Observing detail on each planet was once thought to be only possible with huge telescopes or sophisticated spacecraft. White spots and dusky banding have been noted on both Uranus and Neptune and amateur astrophotographers have been able to photograph these details. 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 can be a challenge to find, but patience will be rewarded with a color different from anything else in the night sky. October is one of the clearer months of the year with the haze of summer gone and the lake effect clouds of winter yet to take relentless hold. So take advantage of the crisp, clear October nights to see these blue-tinted worlds. Their color is especially dramatic when they are near stars of other colors and are definitely a worth pursuit.