Farthest planet Neptune closest to Earth tonight – how to see it

Even though it’s 2.8 billion miles from the sun, Neptune shows seasonal changes in cloud patterns over its 165-year orbit. Photos made with the Hubble Space Telescope. Credit: NASA

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.

To find Neptune, face southeast around 10-10:30 p.m. Start with the bright star Altair in the bottom of the Summer Triangle in the south. Shoot a line from Altair about two outstretched fists long to the lower left to Beta Aquarii. Continue “sliding” in that direction to Theta. From there it’s just a short hop to dimmer Sigma Aquarii. Point your binoculars or scope at Sigma and use the map below to spot Neptune. You can also use the Y-shaped asterism nicknamed the Water Jar below the Square of Pegasus to navigate to Theta. Stellarium

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.

Earth, a rocky planet, has a diameter of 7,918 miles. Neptune is 30,600 miles in diameter and has a deep atmosphere with a mix of water and other ‘ices’ in its interior. At its center is a ball of rock made of iron and silicates with a mass of 1.2 Earths. Credit: NASA

‘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.

Hubble Space Telescope pictures showing Neptune and its system of dim ring arcs along with several of its moons. The 14th and newest is S/2004 N1, discovered last year. Credit: NASA

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.

A detailed map showing Neptune tracking near the star Sigma Aquarii in Aquarius over the next month. The planet should be easy to pick out as there are no stars of similar brightness close by to cause confusion. The field of view is about 1.5 degrees. Source: Chris Marriott’s SkyMap software

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.

4 Responses

  1. Troy

    If it clears up over the holiday weekend I’ll be sure to check out Neptune. Except for color it isn’t telescopically interesting, but I do get a feeling of awe at seeing something so far away.
    Interestingly enough the New Horizons probe just passed the orbit of Neptune after 8 or so years of flight. Considering it is the fastest thing ever launched it is mind boggling how far away it is.

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