Mohawk Valley Astronomical Society

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The Twilight Fringe

by Perry Pezzolanella, MVAS

A spectacular summer of planetary gatherings and the autumn arrival of another great opposition of Mars may divert our attention from the two dimmer planets. Uranus and Neptune are at their best this year as the nights lengthen, but finding them is a challenge that can be a lot of fun. The challenge will be easier using the finder charts that accompany this article provided the night is clear, moonless, and away from city lights.

Uranus and Neptune are dim because they orbit the Sun at a distance of 1.8 and 2.8 billion miles, respectively. In the dim depths of the outer Solar System the daytime Sun is no brighter than a clear evening 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 (2%) absorbs the red component of sunlight and scatters the blue creating a turquoise hue for Uranus. Neptune appears even bluer since it is not as hazy and has more methane (3%). These colors are dramatic whenever they are near stars of contrasting colors.

Uranus spends all of 2005 in Aquarius while Neptune is in Capricornus. Neptune is at opposition on August 8 at magnitude +7.8. Uranus is at opposition on August 31 at magnitude +5.7. 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 until they are lost in the twilight during January 2006. Uranus can be found to the southwest of the 3rd magnitude star Lambda Aquarii. Neptune will spend most of the time between the 4th magnitude stars Iota and Theta Capricorni.

A small telescope is capable of resolving their discs and revealing the colors of these remote worlds given a night with steady seeing. The planets appear distinctly different with Uranus having a rich turquoise hue while Neptune is a chilly, icy-blue disc. Uranus is 3.7 arc-seconds across and 1.77 billion miles away at its closest point this year. Neptune is 2.4 arc-seconds across and 2.70 billion miles away. Both planets are too far away to observe cloud detail or moons unless the telescope is at least 16 inches.

The MVAS Apollo Observatory has just such a telescope, the Seif Telescope: a 16-inch Meade Schmidt Cassegrain. Given the new optics and dark country skies, it has already revealed Neptune's largest moon, Triton, shining steady and true at magnitude +13.5 on September 10, 2004. The larger moons of Uranus: Oberon, Titania, Umbriel, and Ariel, are fainter than Pluto at magnitude +14 to +15 and are elusive, but on the night of September 10, 2004 Oberon and Titania were faintly seen with averted vision within the glare of Uranus.

The Seif Telescope has given Uranus and Neptune distinct personalities and opens up a new project for these remote worlds. Uranus is a true turquoise globe instead of a disc with two faint moons nestled in its glare and even Neptune appears like a tiny bluish globe with dim Triton hovering close by. A fun project would be to plot the positions of the moons each night as they orbit each planet and to make note of how long it takes each to go around once. No cloud details can be seen on either of the planets and a magnification of at least 500x for Uranus and 900x for Neptune are required. A yellow-green (Wratten #11) filter will help. The Hubble Space Telescope continues to reveal an increasing number of white spots erupting on both worlds.

A Uranus flyby mission and Neptune orbiter mission are under study for the next decade, but their vast distances on the twilight fringe of the Solar System will challenge mission planners for years to come as to how to get to them there quickly and economically. So when looking through the Seif Telescope, the pale bluish light of these worlds is free for all to explore.