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Deep Blue Neptune

Credit: NASA Space Place

Neptune requires a modest telescope for direct observation. We’ll give him the honors this month, in part to make up for the slight he suffered as described below.

Neptune Needed to Wait

Neptune, god of the seas

Jupiter, Neptune, and Pluto were the chief Roman gods ruling over heaven, earth, waters, and the underworld. I was curious about the choice of the name Uranus for the next planet out from Saturn. Noted astronomical historian Owen Gingerich clears up the matter in this article. First, for all anyone knew, Uranus would be the last planet discovered. It was found by William Herschel who wanted to name it after England’s King George. Classicists objected.

Johann Bode, editor of the Astronomical Yearbook of Berlin, received many counter-suggestions including the name Neptune, but his own idea prevailed over all others. In mythology, Uranus was the god of the sky and husband of the Earth. He was the father of Saturn and the grandfather of Jupiter. Among Jupiter’s children were the inner planets Mars, Venus, Mercury, and also Apollo (the Sun). Putting Uranus at the outside over all would complete a perfect family album.

Physical Properties [courtesy of NASA Space Place]

Neptune is dark, cold, and very windy – the last of the planets in our solar system. It is more than 30 times as far from the sun as Earth. Neptune, like Uranus, is an ice giant. It is made of a thick soup of water, ammonia, and methane flowing over a solid core about the size of Earth. The methane gives Neptune the same blue color as Uranus. Neptune has six rings, but they're very hard to see. One day on Neptune goes by in 16 hours.

Neptune with rings
Credit: NASA Space Place

Neptune has 13 moons (and one more awaiting confirmation of discovery). It is the eighth and most distant planet from the Sun (since the demotion of Pluto). Only Voyager 2 has visited Neptune.

Neptune’s Discovery

Source: Wikipedia

The planet Uranus, discovered in 1781 by William Herschel, was not behaving exactly as Newton’s gravitational laws had predicted. Was Newton fallible, or was there a significant influence out beyond the Uranus orbit? Young Urbain Le Verrier envisioned yet another planet in our solar system and went to work on the extensive calculations needed to predict the mass, position, and orbit of Neptune. Those calculations bore fruit in 1846 when Johann Galle spotted the new planet from Berlin Observatory. It was the second major triumph of mathematics in the field of astronomy since the great Gauss’s paperwork parked asteroid Ceres in a one-car garage – so to speak.

Neptune had been observed many times prior to “discovery,” but since it moved so slowly around the Sun it was always mistaken for a blue star. Le Verrier surmised the necessary mass and distance from Uranus required to pull and push Herschel’s planet. He then would make adjustments to both figures using complex orbital calculations until they finally agreed with observations. Galle gets the spotter’s credit although he was out to a party or something and left his assistant Heinrich d’Arrest in charge. Just kidding. The two German’s took less than an hour to find Neptune within one degree of Le Verrier’s prediction.


Uranus and Neptune color comparison
Source: Worlds in Comparison

Color is a very subjective topic. NASA reports the same blue color for both planets. I suppose there are impartial instruments that can confirm the colors spectroscopically, but I’ve seen both planets fairly close to one another on the same night a few times. I always see Uranus as green or blue-green and Neptune as a pure blue. Even trained observers often disagree widely on perceived colors, so I guess we must leave it to the “instrumental” opinion.

February 2021

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