The Outer Planets Move In, plus an "Old Moon" Observing Challenge

As the bright planets march westward, Uranus and Neptune become the fresh new faces of fall. And if you've never seen an ultra-thin lunar crescent, here's your chance.

It won't be long before the four bright evening planets dwindle to just one — Mars. Venus and Jupiter are already difficult to spot unless you've got an unobstructed view to the west-southwest. From the central U.S. Venus currently stands only about 3° high 15 minutes after sunset with Jupiter at 14°. Both are deeply buried in the twilight glow. Trees block sinking Saturn from my house, but Mars still dominates the southern sky even as it fades from magnitude –1.2 to –0.6 this month.

Meanwhile, Uranus and Neptune climb higher in the east. Befitting the cooler season, these outer planets harbor temperatures around –350°F (–212°C) at cloud-top level due to their enormous distances from the Sun (2.9 billion and 4.5 billion kilometers, respectively). Neptune, now 3 weeks past opposition in Aquarius, is comfortably placed for viewing as soon as the sky gets dark. Thanks to its northerly declination in Aries, we don't have to wait long to view Uranus either, which clears 25° before 10 o'clock local time despite opposition being nearly 3 weeks away (October 23rd).

Because of their great distance and subsequent slow apparent motion, the two planets have occupied the same seasonal sky now for decades. Faster Uranus passed Neptune in Sagittarius in November 1993 and these ice giants have been slowly separating ever since. They won't pair up in conjunction again until January 2165.

Tracking down the fainter outer planets takes only a little more effort than seeing the bright ones. They're a nice change of pace after observing the many galaxies October brings to the table. At magnitude 5.7, Uranus is relatively easy to see with the naked eye from a rural sky once you know exactly where to look. I usually can spot it with averted vision on moonless nights once it climbs to a healthy altitude. How about Neptune?

I've never seen it, nor do I know of any naked-eye observations, but according to the Bortle Dark-sky Scale, stars of magnitude 7.6–8.0 are visible to the suitably-trained naked eye. We'll assume we're looking relatively high up in the sky where atmospheric attenuation is a minimum. In that case Neptune, at magnitude 7.8, might be seen with averted vision. For most of us, getting to 6th magnitude is challenging enough, what with airglow and light pollution. That's why binoculars are so useful. With a typical 50-mm pair, you can pick out stars as faint as 10th magnitude, but factoring in light pollution and shakiness from hand-holding, the typical limit is closer magnitude 9.

If you switch to a telescope and use a magnification of 100× or higher, Uranus stands out clearly as a pale, blue-green disk 3.7″ (arcseconds) across. Seeing a planet four times bigger than Earth look so teeny helps us to appreciate how truly remote it is. Neptune looks bluer than Uranus to my eye and, of course, considerably smaller, just 2.3″ in diameter. I use a magnification of at least 200×–350× to see this pencil-point-wide planet.

Those with 10-inch and larger telescopes can seek Neptune's largest and brightest moon, Triton. At magnitude 13.5, it's totally within range of even a smaller telescope, but challenging because its distance from the home planet varies from just 9″, when closest, to 14″ at greatest elongation over its 5.9-day orbit.

If you're lucky enough to get a few clear nights in a row, you'll appreciate Triton's truly unique qualities: it revolves backwards — counterclockwise around Neptune — and swings north or south of the planet at greatest elongation instead of east or west due to its steep orbital inclination. To cleanly separate planet from moon, I use a magnification of 220× or higher. Knowing just where to find the moon is easy with the Sky & Telescope Triton Tracker.

Though closer to the Sun, the largest and brightest moons of Uranus, Titania and Oberon, are roughly half the size of Triton, with magnitudes as dim as 13.9 and 14.1, respectively. Their apparent distances at elongation are also considerably farther from the planet — 40″ for Oberon and 30″ for Titania. That might make you think they're easier catches than Triton, but just the opposite is true because Uranus is two magnitudes brighter than Neptune. That added glare gets in the way.

Still, under a moonless sky with Uranus well up in the east and a steady atmosphere, both these moons aren't too difficult to spot. Use the S&T Moons of Uranus tool to track them. I've seen both in scopes as small as 10 inches using 220× and higher. An occulting bar to block the light from the comparatively brilliant planet makes the task much easier. Ariel (magnitude 14.3) and Umbriel (15.0), which are closer in yet at just 18″ and 12″ at greatest elongation, are targets for 16-inch and larger telescopes. I've never positively identified Umbriel but about flipped out when I finally spotted Ariel a year ago in a friend's 24-inch.

While both planets will remain far apart for more than a century, Mars will share a close conjunction with each as it hurries east across the fall sky. The Red Planet passes only 2′ (arcminutes) north of Neptune on December 7th around 14^h UT as seen from the eastern hemisphere. From the Americas, their separation will vary from 15′ to 11′ from the east coast to the west the evening of December 6th. On February 12, 2019, Mars glides 58′ north of Uranus.

Fast Facts

While you're out tracking down the planets and their moons, here's fuel for the imagination:

• • Neptune has the strongest winds of any planet in the solar system. They’ve been clocked at over 2,100 km/hour. Scientists speculate the planet’s extreme cold, coupled with the fluidity of the gases in its atmosphere, reduces friction to very low levels, which allows the winds to reach incredible speeds.
  • Neptune is the coldest planet in the solar system with temperatures dipping to –366°F (–221°C) in its upper atmosphere.
  •  With a diameter less than 1/30 the width of the full moon, the sun would be too small to see as a disk with the naked eye from Neptune but would still shine with the brightness of several hundred full Moons.
  • At a depth of 7,000 kilometers, the temperature warms to between 3,140° and 8,540°F (1,725°–4725°C). Research suggests that the combination of high temperatures and pressures causes methane to transform into diamond dust that “rains” down onto Neptune's core.
  • Because it revolves around the planet in the "wrong" direction, Triton is likely a captured asteroid.
  • If you could fly to Uranus in a transcontinental jet at a cruising speed 550 mph, it would take 373 years to arrive.
  • Uranus's axis is tilted 98°, so the planet orbits the Sun on its side with its poles periodically facing Earth. Each season on the planet lasts about 21 years. Either pole is plunged into complete darkness for 21 years during every orbit.
  • Uranus and Neptune are known as “ice giants”. Eighty percent of their masses is made up of a hot, dense fluid of water, methane, and ammonia that envelopes a rocky core.

Observing Alert: Most thin crescent aficionados look for young Moons in the spring evening sky, but old October ones work just as well. A thin-as-air crescent Moon will rise shortly before the Sun on October 8th. From the East Coast, the Moon will be just 17 hours before new; 16 from the Midwest; 15 from the Mountain states, and 14 hours from the West Coast.

If you're game, click here to get your local moonrise time, find a spot with as near as perfect an eastern horizon as possible (a lake is ideal), and start watching about 35 minutes before sunrise. The Moon will stand about 3.5° high some 20–25 minutes before sunrise from many locations in the Americas. This rare opportunity comes courtesy of the nearly perpendicular angle the ecliptic makes to the eastern horizon at dawn this month. Bring binoculars! If you see the crumbly crescent, please let us know.

Amateur astronomer Steven James O’Meara spied a 15-hour-32-minute crescent for the naked-eye record, while Mohsen G. Mirsaeed broke the record for youngest Moon ever seen with optical aid at 11 hours, 40 minutes. For more on thin moons (and more records!), click here.