Mohawk Valley Astronomical Society

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Mystic Blues

by Perry Pezzolanella

The color blue, often considered a chilly color, is very fitting for two worlds at the outer reaches of the Solar System: the bluish-tinted Uranus and Neptune. September and October usually offer several clear nights and coincide with the oppositions of both planets. They are actually quite easy to find given clear, moonless skies in the dark countryside and 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 (3%) in Neptune’s atmosphere absorbs the red component of sunlight and scatters the blue creating a beautiful blue planet. Uranus does not appear as blue because it has a little less methane (2%) and unlike Neptune it has a ruddy haze which shifts its color towards the green giving it a turquoise hue. These colors are dramatic whenever they are near stars of contrasting colors.

Uranus spends 2018 straddling the border of Pisces and Aries well to the left of the Great Square of Pegasus near the bright star Omicron Piscum while Neptune is still in Aquarius left of bright star Lambda Aquarii. Neptune will be at opposition on September 7 while Uranus will be at opposition on October 23. Rising at sunset and setting at sunrise, both worlds will be up all night on these dates offering the best observing opportunities. They will remain in the evening sky for the rest of the year and into early 2019 with Neptune fading into the evening twilight by February and Uranus by March.

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 displays a chilly, icy-blue disc.

The Barton-Brown Observatory (BBO) at the Waterville Public Library houses a research grade, 16-inch Schmidt-Cassegrain telescope and its superior optics in the relatively dark skies south of Utica’s bright city lights consistently reveal Uranus as a true turquoise globe and most of the time two of the five largest moons, Oberon and Titania, are visible. The other three moons of Uranus: Umbriel, Ariel, and Miranda, are dimmer and have yet to be seen; all five moons shine at magnitude +14 to +15. Neptune is a tiny icy-blue globe with Triton shining nearby most of the time, but quite dim at magnitude +13.5.

Using a telescope of at least 16” such as the one at the BBO may increase the chances of seeing faint detail on Uranus and even Neptune. Uranus is no longer a dull, fuzzy planet as it has become very stormy with large white spots that have been observed appearing and disappearing with the change of seasons, and Neptune is even more active. The rapid advancement in technology and photography is making amateur photos rivaling the quality of what could be done with the giant professional telescopes decades ago. A magnification of at least 500x for Uranus and 900x for Neptune are highly recommended in order to have a chance at photographing any detail and using a yellow-green (Wratten #11) filter will improve the odds. Observing Uranus and Neptune at the BBO in Waterville, which is south and away from the bright lights of Utica, will make observing these chilly bluish worlds a mystic experience.