Mohawk Valley Astronomical Society

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Chilled Blues

by Perry Pezzolanella

There are two distant worlds that are favorably placed for observing in the evening during the increasing chill of autumn evenings. Uranus and Neptune climb higher in the evening sky with each passing day as trees change color. Both planets were very close to each other in the sky during the 1990s and could be observed as a duo with hardly a little nudge of the telescope. Now they are far enough apart that several hours are needed for Uranus to clear the horizon after observing Neptune. September and October offer several clear nights to scrutinize them before the cold and stormy winter sets in. 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 +5.6) and Neptune (magnitude +7.8) are dim because they orbit the Sun at 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 2021 in Aries near the +5.6 magnitude star 37 Arietis making it easier to find as it is almost of equal brightness as Uranus. Neptune is in Aquarius well to the left of the +4.2 magnitude star Phi Aquarii which it was very close to in 2019. Neptune will be at opposition on September 14 while Uranus will be at opposition on November 4. 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 2022 with Neptune fading into the evening twilight by February and Uranus by April.

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. With its superior optics in the relatively dark skies south of Utica’s bright city lights, it 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-bluish, tiny 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. Both planets have displayed great storms appearing as large white spots that grow and 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 once reserved for the pros with giant telescopes. With the highest magnification that seeing will allow, there is a reasonably decent chance of photographing cloud detail. Observing these bluish worlds add to the chilly feel of the crisp autumn nights and are worth the warm satisfaction in locating them.