Only two planets didn’t jump out and bite us in the days of antiquity – Uranus and Neptune. Both escaped attention because they were too faint. Now all you need to see either is a pair of binoculars. Pop off those lens caps because we’re going to visit the solar system’s outermost planet, Neptune, now at its closest and brightest for the year.
Neptune reaches opposition today in the constellation Aquarius, a dim assemblage of stars to the west and south of the familiar Square of Pegasus. Opposition occurs when the Earth lines up on the same side of the sun as an outer planet. Not only are the planets at their closest, but Neptune remains visible all night long, rising at sunset in the southeastern sky.
‘Close’ in astronomy is always a relative term. At 2.7 billion miles from the good, green Earth, Neptune is one of the coldest places in the solar system. Where its methane-laced clouds meet against the vacuum of space the temperature drops to -360°F (-218° C). Powerful winds up to 1,250 mph (2,000 mph), stretch its chill blue clouds into subtle belts and bands that whiz through an atmosphere of mostly hydrogen and helium.
Deeper down, Neptune’s mantle resembles nothing on Earth – a superheated fluid of water, ammonia, methane referred to as ice but simmering under high pressure at temperatures between 3,000 – 8,500° F.
All these amazing facts are distilled into a minute blue-colored dot just 2.4 arc seconds in diameter (750 times smaller than a full moon) as viewed from Earth’s skies. Being so far away, Neptune takes 165 years to make a complete circuit around the sun. Since its discovery in 1846, the blue planet has completed just one single orbit. That was back in 2011. It’s a long time between birthdays on planet #8.
Despite its great distance, Neptune’s size and bright cloud cover make it a fairly easy find. At magnitude +7.6, you can spy it in 35mm or larger binoculars from the outer suburbs and countryside. What will you see? In binoculars, the planet looks like a dim ‘star’ that slowly creeps westward among the real stars. You can easily track its progress if you look one night, note the planet’s position, and look again a few nights later.
Discerning Neptune’s tiny disk will require at least a small telescope and magnification of around 100x. The planet looks like a pencil-point dot. I like to crank up the power to 250x on good nights to try and see its brightest, largest moon Triton, which looks for all the world like a 13th magnitude companion star. To know where to look for the moon at any time and date, visit Sky and Telescope’s Triton Tracker.
As we transition into fall, Neptune rises higher and earlier with each passing night. Take a look now and again to watch the slow gait of a world that’s been hidden from human eyes until only recently.