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Shades of Blue

by Perry Pezzolanella

Planetary observing is often reserved for the Fab Five: Mercury, Venus, Mars, Jupiter, and Saturn. After all, these are the brightest planets, making them rather easy to observe. Little thought is given to Uranus and Neptune. Too dim, too tiny, why bother? The reason to find them is because both belong to a class of planets known as Ice Giants. Unlike Jupiter and Saturn, these worlds are frigid and contain ices such as water, ammonia, and methane. Another reason is to observe their chilly shades of blue, a rare color among celestial objects. 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 1.8 and 2.7 billion miles from the Sun, 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 toward green giving it a turquoise hue. These colors are dramatic when they are near stars of contrasting colors.

Uranus spends 2023 in Aries growing closer to the beautiful Hyades-Pleiades region of Taurus, which it will cross from 2024-26. It is getting easier to locate with several brighter stars to hop to it when very high up around midnight. Neptune has moved into Pisces, below the Circlet, also making it easier for star hopping. Neptune will be at opposition on September 19 while Uranus will be at opposition on November 13. 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 2024 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 3.8 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 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” may increase the chances of seeing faint detail on Uranus and even Neptune, but more likely imaging may be necessary. Both planets have been active for several years with a bright polar hood on Uranus as its north pole points nearly straight towards Earth and large dark spots on Neptune. 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 will put an end to the “why bother” attitude once and for all.