Neptune reached opposition on Sept. 5th, when it was closest to Earth for the year, making now through December an ideal time to observe the solar system’s most distant planet. Opposition occurs when both the Earth and an outer planet line up on the same side of the sun. Since Earth’s in the middle, the sun and Neptune appear in directly opposite parts of the sky: we see the planet rise in the east the same time the sun sets in the west. Now past opposition, Neptune rises even earlier and is already well placed for viewing in binoculars and telescopes as early at 9 p.m. local time.
Neptune shines at magnitude 7.8, so it’s visible in binoculars as small as 7×35 from a reasonably dark sky. Since light pollution is so ubiquitous these days, I’d recommend using something a bit larger, say 8×40, 7×50 or 10×50. Point your binoculars at Lambda and use the detailed map to find Neptune. Observers using large 70mm binoculars may notice the planet’s blue color, otherwise it appears no different than a star.
But there’s one way you can tell Neptune apart from a star, and that’s to watch it move night to night. Stars keep their places in the sky, but planets orbit the sun, so they’re on the go. Because of its great distance, currently 2.7 billion miles (4.3 billion km) from Earth, Neptune moves more slowly than the other planets but not so sluggishly that we can’t see it in binoculars. Plus, we get some help this week.
Neptune passes very close to a star nearly its equal in brightness, HD 216777. Tonight (Sept. 19th), the planet appears just 2 arc minutes (1/15th the diameter of the moon) southwest of the star. In binoculars, they’ll look like a temporary “double star.” The separation increases by a little more than 1 arc minute each night as Neptune scoots to the right (west), making it easy to gauge the planet’s travels night by night.
At this point, you might ask — don’t planets travel to the east (left) as they orbit the sun? Yes, they do! But around the time of opposition, all outer planets pause in their eastward journey and then back up and go in the “wrong” direction, to the west. It’s called retrograde motion and occurs because Earth moves so much faster than Neptune, it passes the planet like a car passing another on the freeway. For a time, the car, or in this case the planet, seems to move backwards, falling behind as we pass it.
If you own a telescope, you can find Neptune by pointing again at Lambda and using the detailed map. At low magnification, it still looks just like a star, but if you power up to 100x or higher and the air is steady enough to provide sharp images, you’ll be able to make out a tiny blue-tinted disk. It look similar to what the Earth must look like from Neptune’s perspective. Earth’s oceans absorb red and orange light from the sun, making them look blue. Neptune is all atmosphere with no surface. Most of the air is hydrogen and helium like all the large, outer planets, but it has a trace of methane gas, which absorbs the same warm colors as ocean water, giving the planet its blue hue.
With all the hurricanes in the news this season, Neptune’s winds provide some perspective. Winds there are the fiercest in the solar system with speeds of 1,500 mph (2,400 kph), some eight times faster than a typical Category 5 hurricane. Neptune’s a big planet, too with a diameter of 30,598 miles (49,244 km), nearly four times as large as the Earth and encircled by five, skinny, dark rings coated in some type of organic material. Taking 165 years to orbit the sun, no human has ever seen one complete perambulation of the planet around the sky in their lifetime.
Triton, Neptune’s largest moon, revolves backwards, the only large moon in the solar system to do so. At magnitude 13.7, you’ll need a 10-inch or larger telescope to see it, but if you’d like to try, I recommend Sky & Telescope‘s Triton Tracker. Select a date and time, and you’ll get a map showing you exactly where to look for this oddball. It’s surprisingly easy to see. Just be sure to use high magnification of 200x or more, so you can separate it from the planet’s glare.
Be a deep space explorer this fall — let Neptune take you to the edge.