Mohawk Valley Astronomical Society

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Haunted Blues

by Perry Pezzolanella

Uranus and Neptune are chilly, blue-tinted worlds that will be favorably placed for evening observing during the approaching holidays, especially Halloween. Decades ago in the early years of the Mohawk Valley Astronomical Society both planets were well-placed during the balmy summer evenings, but are now better associated with crisp autumn evenings. September and October offer many clear nights and coincide with the oppositions of both planets when they are best seen. Under clear, moonless skies in the dark countryside, it is surprisingly 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 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 2020 in Aries while Neptune is in Aquarius to the left of the bright star Phi. Neptune will be at opposition on September 11 while Uranus will be at opposition on October 31. 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 2021 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 Barton-Brown Observatory (BBO) at the Waterville Public Library with its research grade, 16-inch Schmidt-Cassegrain telescope and superior optics in the relatively dark skies south of Utica’s bright city lights consistently reveals 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 chilly, ice-blue, 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 chances of seeing faint detail on Uranus and even Neptune. Both planets have displayed great storms appearing as large white spots that grow, then shrink and disappear with Neptune being the more active of the two. Modern technology is making it possible for amateurs to capture images of these storms on both planets. 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; using a yellow-green (Wratten #11) filter will improve the odds. The bluish tint gives these worlds a chilly, hauntingly remote feel, so why not take some time Halloween night to observe them!