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  • Writer's pictureLuke


Updated: Sep 17, 2021

I don’t get too excited about Neptune; even with the best conditions possible and a really good telescope it's not much to look at. So it’s no wonder that this is the first time I’m making a post about Neptune. Which really is a shame because although it doesn't look like much in a telescope it is a really interesting planet and definitely worth exploring here, where I’m not limited to showing what’s in the telescope, and I can share pictures from Voyager 2 and Hubble.

Neptune seen from the Voyager 2 flyby

Neptune’s discovery is an interesting story in itself. Because all the planets from the sun to Saturn have been known since before recorded history nobody really “discovered” them. Neptune, however is so dim that to even see it you have to have a telescope, making Neptune's discovery much more modern. The first recorded observation of Neptune was by Galileo Galilei in 1612. However Galileo is usually not credited with “discovering” Neptune since it’s unclear if he actually saw that it was a star and not a planet. Recent analysis of Galileo's writings indicate he might have known it was a planet.

Galileo's depiction of Jupiter in his notebook with Neptune depicted as a star near by to Jupiter (Right), A modern computer rendering of what Galileo would have seen in his telescope with Neptune labeled (Left)

Neptune was the first planet to be predicted before it was discovered. By the 1840’s it was clear to astronomers that the motion of Uranus (discovered in 1781) was abnormal and likely being influenced by another planet further out. Two researchers John Couch Adams, and Jean Joseph Leverrier were working separately trying to find this planet beyond Uranus. They used the motion of Uranus to predict the location of Neptune and in 1846 two astronomers Johann Gottfried Galle, and Heinrich Ludwig d’Arrest looked where it was predicted to be and found it. All four Galle, d’Arrest, Adams, and Leverrier share credit for Neptune's discovery.

Neptune is the smallest of the four gas giants (Jupiter, Saturn, Uranus, Neptune), however, it is still very big having a radius of 15,300 miles or, about 4 times Earth’s radius! It orbits about 2.8 billion miles away from the sun (about 30 times the distance from earth to the sun). It takes Neptune 165 Earth years to go around the sun once, meaning it had just been a little over a year on Neptune since its discovery (currently; 175 Earth years ago) in 1846. From Neptune the Sun is just a very bright star at 1/9000 the brightness we experience from the sun here on Earth (or about 450 times the brightness of the full Moon), Because of this Neptune is very cold.

Neptune size compared to Earth

Being so cold Neptune is usually called an Ice Giant however there are parts of Neptune that are quite hot. The atmosphere is thought to be mostly hydrogen and extends down about 15% of the way towards the center of the planet. Due to the thickness of the atmosphere the pressure on the surface of Neptune is about 100,000 times that of earth. This high pressure forms a hot dense ocean made up of water and ammonia. The high pressure at the surface of the ocean could mean that methane from the atmosphere would be under enough pressure to form diamonds and rain diamonds on the water-ammonia sea. The core of Neptune it thought to be made up of iron, nickle, and silicates making a dense rocky solid core.

Diagram of Neptune

There are currently 14 known moons of Neptune. The biggest Triton was discovered soon after the discovery of Neptune in 1846. Triton is about 78% the size of our Moon and orbits Neptune at about the same distance that the Moon orbits Earth. Triton is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. Because of its retrograde orbit and composition similar to Pluto, Triton is thought to have been a dwarf planet, captured from the Kuiper belt. The latest moon of Neptune to be discovered was discovered in 2013 by an astronomer analyzing photos from the Hubble telescope that were taken in 2009, it wasn’t even until 2019 that the new moon was given a name, Hippocamp (after Poseidon’s sea horse)

composite of Hubble photos of Neptune showing the newly discovered moon Hippocamp

Neptune has rings, the rings can actually be seen from earth but not easily. To see the rings you will have to wait until Neptune crosses in front of a background star. As Neptune approaches crossing in front of a star you can see a dip in the brightness of the star as the rings block out the light, then the star will brighten back up as it shines through the gap between the planet and the ring only to be blocked out by the planet soon after. Then the reverse can be seen as the star comes out from behind Neptune and seen in the gap between the rings to, then be hidden by the rings, to be observed again in full brightness after the rings have passed. The rings have only been observed up close during a fly-by of the Voyager 2 spacecraft which found Neptune to have 4 thin rings.

Neptune's rings seen by Voyager 2 (the planet is blotted out because it would have been too bright and out shined the rings).

Voyager 2 is the only spacecraft that we’ve sent to study Neptune. Since Neptune was the last planet Voyage 2 visited, the trajectory was changed so we could also get a peak at Triton. Voyager 2 made the closest approach to Neptune on Aug 25 1989. When doing so it was revealed that (like Jupiter’s red spot) Neptune had a ‘great dark spot’ that was a massive storm raging in the atmosphere of the planet. Since the fly-by we have relied on telescopes to study Neptune. In 1994 the Hubble Space Telescope noticed that the great dark spot was no longer there but in the other hemisphere another great dark spot was found. Neptune has also been observed to have gotten ~10% brighter between the 1980 to the 2000 this is thought to be due to more reflective white clouds in the atmosphere.

Voyager 1 and Voyager 2 flight path (Left), Neptune from Voyager 2 (mid), Neptune from Hubble (right). Notice how the dark spot flips from the southern to the northern hemisphere.

Observing Neptune is difficult for an amateur astronomer, and when found it offers very little to be seen. A telescope of about 6 inches or larger can resolve a small pale blue dot under a magnification of 95x or greater. But that’s about all there is to see, just that it’s not a point of light like a star. If you are really patient and track it down in a telescope and then proceed to wait a few months/years and do so again you will see how slowly it moves in the night sky. Today 9/15/2021 is technically the best time to see it all year since we are on the same side of the sun as Neptune. However, while being on the same side of the sun as Mars helps a lot, Neptune it makes little difference. Neptune is 30 times as far away from the sun so, if we were on a foot ball field the sun was on the end zone, earth at the 10 yard line Neptune would be at the 300 yard line (3 football fields away). Being on the same side of the sun as Neptune is like taking one step closer towards Neptune which helps but not dramatically.

The best way to see Neptune is in this Blog post but, if you want to try to see Neptune in a telescope with your own eyes the best way would be to join me on one of my tours. I’d love to try to show you Neptune in my large telescopes!

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