Mohawk Valley Astronomical Society

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Frosty Blues

by Perry Pezzolanella

Uranus and Neptune are chilly, blue-tinted worlds, which is fitting for the time of year they are best seen. Decades ago they were best seen during balmy summer nights, but are now best seen during frosty autumn and winter nights. September and October usually offer clearer nights, which happens to coincide with the oppositions of both planets. Under clear, moonless skies in the dark countryside, it is surprisingly quite easy 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 (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 that 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 2019 in Aries while Neptune is in Aquarius close to the bright star Phi. Neptune will be at opposition on September 9 while Uranus will be at opposition on October 27. 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 and into early 2020 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 during nights of perfect seeing, 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 globe with Triton shining nearby most of the time, but quite dim at magnitude +13.5.

Using a telescope of at least 16” may increase the chances of seeing faint detail on Uranus and even Neptune. Both planets have been extremely active with great storms appearing as large white spots that grow, then shrink and disappear. Uranus has developed a huge white north polar hood. Neptune is the more active of the two worlds, with another dark spot forming and more white spots, but it is more challenging due to its tiny size. Due to the rapid advancement in technology and photography, today’s amateur photos rival the quality of giant professional telescopes decades ago. A magnification of at least 500x for Uranus and 900x for Neptune are highly recommended to have a chance at photographing any detail (a yellow-green Wratten #11 filter will improve the odds). Observing these remote worlds is a rewarding experience for braving the frosty cold nights that lie ahead.