Uranus: the Threshold Planet of 2006

By Gary T. Nowak

Let's face it- the latter half of the year is going to be disappointing for the planet viewers. The unaided-eye planets are now too low for a good steady view or poorly placed in the glow of twilight. One planet, however, has been slowly and steadily becoming more northerly over the years: Uranus.

Most planetary observers hardly give Uranus a glance. The main reason is this planet represents the threshold of observed planetary features. Uranus' size, distance, and brightness make this planet a challenging object for observers of all types.

The observing window for Uranus is from Aug. to early Dec. 2006. The planet is at opposition in Aquarius on 5 Sep. 2006. It shines around +5.8 magnitude and its apparent diameter is 3.7". Uranus is a gas giant which is severely tipped on its axis (98 degrees tilt). This tilt means that at various times in its 84-year orbit, different regions come to view. This year the Southern Hemisphere Equatorial region is angled towards Earth. This allows for unique observing challenges with various level of equipment.

Unaided Eye: Uranus can be seen with the unaided eye under a clear dark sky. During our observing window it is never more than 3 degrees away from the star Lambda Aquarius. Uranus is a challenge to see with the unaided eye. Because of Light Pollution this planet may well be our Solar System's threshold planet.

Binoculars: Handheld (e.g. 7 x 50) binoculars will reveal the planet. Large binoculars (e.g. 20 x 80) make the planet much easier to locate. A good finder chart for Uranus appears in the RASC Observers Handbook 2006, p196. If you plot the observed position of Uranus on a star chart, then observe the planet a few days later the planet's motion is quite noticeable.

Small Telescope (5" (125mm) and under): What size telescope and magnification will reveal the small disk? Some sources claim that a 3" (75mm) telescope at 100x will reveal the disk. With this size telescope 3" (75mm) the disk appears washed out or a gray color. What size telescope and magnification will easily reveal the green colored disc?

Medium Size Telescope (6" (150mm) - 9" (225mm)): Most observers with these medium size telescopes can resolve the light green disk and detect some limb darkening. Limb darkening is caused by the absorption of Sunlight by the planet's dense atmosphere so that the edge of the planet seems darker than the center of the planet. Some observers complain that limb darkening makes it hard to determine when the planet is in focus.

Over the years some observers with 6" (150mm) telescopes working at 250X - 300X have claimed to have seen some small scattered bright spots or ghostly cloud bands on Uranus. What will you see when you observe Uranus?

Colored Filters: Since Uranus is a small dim planet; colored filters don't work well in a small telescope and work marginally in a medium telescope. Very light colored filters such as W#8 Pale Yellow or W#82a Pale Blue are often used. Large telescopes usually can employ such light filters to their advantage.

Large Size Telescope (10" (250 mm) or larger): Users of large telescopes have 2 challenges awaiting them:

• The challenge to see faint cloud markings on Uranus. This challenge is very similar to the medium telescope challenge. Usually these markings are hard to see and elusive in a 10" (250mm) at 300x. Also the atmospheric seeing must cooperate. Large telescopes have enough light gathering power so that light colored filters may be used to see the faint markings.
• The challenge to see Uranus' moons. Two of Uranus moons, Titania (Mag +13.5) and Oberon (Mag +14.0) are very difficult targets for large telescopes. There have been some sightings of these moons with a 10" (250mm) or larger scope but this is not easy. Again, atmospherics must be favorable. Sky & Telescope Sep 2006, p51 has an article and position chart about Uranus' Moons.

Astrophotographers: The challenge of Uranus' faint markings and moons await your imaging skills. Can you capture these features with your equipment?

The late summer and fall evening skies present you with an opportunity to observe Uranus. What challenging features will you see on the planet?