Planet Neptune at Opposition! Close Visit by a Strange Planet


A size comparison of Neptune and Earth.

Photo Credit

Planet Neptune: 10 Facts About the Other Blue Planet

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On Tuesday, September 19, 2023, planet Neptune reaches its closest approach to us for the year. Why should we care about the only planet that cannot be seen with the naked eye? Well, if size matters, then it deserves attention on that basis alone; since it’s such an enormous blue ball, 58 planet Earths could fit inside it. Discover the “other” blue planet.

Discovering Neptune

We really should start this story in 1845 when French astronomer Urbain Le Verrier realized that Uranus—faintly visible to the unaided eye from dark rural skies and discovered 64 years earlier—was not moving in a normal way.  

Something was tugging at it. He calculated where this mysterious object should be located and tried to get any astronomer to aim a telescope there. Through his math calculations, he realized that Neptune has the gravitational pull of a planet.

No one in France was interested. Finally, Le Verrier convinced the Berlin observatory to point their small 9-inch telescope at his calculated position. In just one hour, Neptune was found within a single degree of the predicted spot, on September 23, 1846.

Image from Voyager 2 of Neptune. 
Credit: MSFC.

Neptune’s Moons

Seventeen days later, an enormous satellite was seen orbiting it and later named Triton, after the three-pointed spear carried by the god of the sea. Right from the start, however, screwy things unfolded.

  • Triton is the solar system’s only major moon that orbits its planet in the wrong direction.  Backward, meaning clockwise as seen from the north.  
  • Nor does it circle Neptune’s equator.  These oddities prove it was a traveling vagabond captured by Neptune’s gravity. Yet, all other captured satellites have highly elliptical orbits, not the kind of nice, near-circular one exhibited by Triton.
  • Triton also has geysers that continually spray frigid ice 5 miles into the atmosphere.

How many moons does Neptune have? 

It took another century to find a tiny second moon, and then more in the 80s, and another a few years ago, bringing the total to 14 moons. Two orbit at the astonishing distance of 30 million miles, the farthest moons from any planet by far. Those are farther from Neptune than the planet Venus is from Earth!

10 Facts About Neptune

But Neptune’s oddities don’t end there. We’re lucky the Voyager spacecraft whizzed past Neptune in 1989, giving us our only decent look at blue Neptune and Triton, which turned out to appear dappled, very much resembling a cantaloupe.

  1. Neptune has the windiest weather in the solar system—with fierce winds and clouds of frozen methane blowing five times faster than tornadoes.
  2. Neptune was named after the Roman god of the sea so it’s fitting that the planet’s color is ocean blue. But its deep blue color is a mystery. The planet has lots of methane gas that should make it greenish like Uranus, so something unknown must lurk in its atmosphere.  
  3. Although Neptune is the third largest planet, it’s so far away it only looks as big as a quarter dollar coin seen from a mile away.
  4. Neptune posseses the coldest surface in the whole solar system.  The average temperature is -353° F (-214° C)!
  5. The planet is not only very cold, but also very dark. Even high noon on Neptune is about as bright as Earth’s sunset. 
  6. Neptune’s magnetic field is 27 times mightier than Earth’s.
  7. A day on Neptune is 16 hours. A year on Neptune is 165 Earth-years long, so barely one Neptune year has passed since the planet’s discovery in 1846!
  8.  Neptune is the farthest known planet from the Sun (the 8th planet), about 2.7 billion miles away.
  9. It would take 12 years to journey in space from Earth to Neptune.
  10. If you weigh 100 pounds on Earth, you’d weigh 114 pounds on Neptune. 

Nothing about that place is normal.

Neptune was the Roman god of the ocean. (He was known to the Greeks as Poseidon.)

Viewing Neptune at Opposition in September 2023

Neptune will be in opposition on Tuesday, September 19.

When this outer planet is at “opposition,” that means that it’s at its closest point to Earth. Our planet flies between the Neptune and the Sun. This is the time when Neptune also appears the brightest, not that Neptune is that ever that close nor bright. With the naked eye, Neptune is about five times fainter that the dimmest star.

But this is your chance to see the Solar System’s most elusive planet if you have an optical aide.

You need either a low-powered telescope or binoculars.

  • The planet will rise in the east and set in the west at sunrise, climbing highest point in the sky about midnight Eastern time (or any time zone).
  • The Moon (near its First Quarter phase) sets after midnight so that’s another reason to look after midnight. See your Moonset time. Make sure you leave your eyes 20 minutes to adjust to the darkness.
  • The blue planet is now in front of the dim constellation Pisces. So get a sky app or sky chart and look for Aquarius. 
  • Another trick is to look for Saturn; which can be a good starting point to locate the area where Neptune can be found.

What You Can See

  • If you have a small telescope with 100x magnification, you should make out Neptune’s disc-shaped body as well as its blueish color.
  • If you have a slightly larger telescope (200x magnification), you could see possibly Triton’s moon. 
  • If you have a 300x telescope, you will definitely be able to see Triton and possible a second moon.

Here’s a good place to find a sky map and more detail

About The Author

Bob Berman

Bob Berman, astronomer editor for The Old Farmer’s Almanac, covers everything under the Sun (and Moon)! Bob is the world’s most widely read astronomer and has written ten popular books. Read More from Bob Berman

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