Astronomy: How We Name the Planets – Then and Now

Astronomy: How We Name the Planets – Then and Now

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In this edition of Mobile Stargazing, we'll look at how the planets got their names (and why some didn't stick), what they are called in other cultures and where the names for newly discovered objects in the far-off Kuiper Belt come from. [Living on Other Planets: What Would It Be Like?]

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Spotting the planets

This November, most of the planets can be found in the early evening sky — except for the biggest one, Jupiter, which is easy to spot in the east before sunrise. The word "planet" is derived from the Ancient Greek term "astēr planētēs," which means wandering star. True to their name, the naked-eye planets are exhibiting noticeable location changes this month. Your favorite astronomy app will tell you where they are on any day of the month. In the SkySafari 5 search menu, you can open the Sun & Planets list and enable highlight symbols for all the solar system objects. Tap the List Settings icon to bring up the highlighting check box. 

Swift Mercury is climbing away from the sun, toward a rendezvous with ringed Saturn near the end of the month. With visual magnitudes around 0.50, both planets would be easy to see with the naked eye if they weren't sitting so low, embedded in the evening twilight. Glorious Venus and blood-red Mars are easy naked-eye objects in the early evening southwestern sky. King Jupiter is rising in the east before the sun. Its bright beacon is eye-catching, even as the sun rises. 

Uranus takes all night to cross the night sky in Pisces, while watery Neptune, fittingly in Aquarius for the next seven years, sets after midnight. Under very dark skies, Uranus is a naked-eye object, and Neptune is viewable with binoculars and small telescopes. Finally, distant dwarf planet Pluto requires a very large telescope to spot, although it, too, is in the early evening sky, between Capricornus and Sagittarius.

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In November 2016, there will be another opportunity to observe all of the planets. On Nov. 23, shown here at 5 p.m. local time, Mercury and Saturn will pass each other as Saturn is carried down into the sunset while Mercury climbs into a brief evening apparition. Regardless of whether they are visible to the naked eye or with a telescope, your astronomy app will indicate where each planet is located on any given date and time.

Credit: SkySafari App

Naming the classical planets

With only one, or perhaps two, exceptions — more on this later — the names of the planets in our solar system come from Roman mythology. The bright, naked-eye planets were known in ancient times — sometimes by other names — but we have adopted the names that were used in the Roman period.

Because of its swift motion through the sky, Mercury was named for the winged messenger of the Roman gods. The beautifully bright planet Venus, sometimes referred to as the morning or evening star, was named after the Roman goddess of love, beauty and sexuality. Earth's name predates the Greek and Roman cultures, but the influences of those periods remain in the terms "Geo" and "Terra," respectively. 

No surprise, Mars, tinted reddish by the pervasive iron oxide that forms on its surface due to weathering, has been named for the Roman god of war. Jupiter, the largest of the planets, was named after the most prominent of the Roman gods, despite the fact that Venus frequently shines more brightly! Saturn was named for Jupiter's father, the Roman god of agriculture, long before anyone knew about the planet's spectacular ring system.

In China and the Far East, the naked-eye planets are named after the traditional five elements — wood (木), fire (火), earth (土), metal (金) and water (水). These are central to the ancient Eastern philosophies that brought us the concepts of Yin and Yang, and Feng Shui. I've listed them in order of importance, and that is how they have been assigned to the planets' visual appearance. Jupiter is木星 (Mùxīng), the Wood Star. Mars is 火星 (Huǒxīng), the Fire Star. Saturn is 土星 (Tǔxīng), the Earth Star. Venus is 金星 (Jīnxīng), the Metal or Gold Star. And Mercury is水星 (Shuǐxīng), the Water Star.

Good old planet Earth is called 地球 (Dìqiú), which literally translates as "ball of earth." The distant ice giant planets, unknown to ancient Chinese astronomers, have been named as follows: Uranus is 天王星 (Tiānwángxīng), or Sky King Star, and Neptune is 海王星 (Hǎiwángxīng), or Sea King Star. And Pluto is called冥王星(Míngwángxīng), or Underworld King Star.

Ancient Chinese astronomers assigned the five elements wood, fire, earth, metal and water to the bright naked-eye planets. For example, Jupiter is the "Wood Star," and Mars is the "Fire Star." Planets discovered in the modern era use literal translations of the names from English into Chinese, such as "Underworld King" for Pluto.

Credit: Patricka via Wikimedia Commons

Discovering the outer planets

Individuals using telescopes or photographic plates visually discovered and named the smaller and dimmer outer planets between 1781 and 1930. Uranus is bright enough to have been observed with the naked eye in ancient times, but it was always assumed to be a star because of its almost imperceptible motion through the stars. The Greek astronomer Hipparchus included it as a star in his early star atlas of 128 BC. Between 1690 and 1769, telescope users frequently saw and documented Uranus. But only in the spring of 1781 did British astronomer Sir William Herschel observe Uranus and recognize that it was not a star, because it had a noticeable disk or comet-like form and it changed position compared to the surrounding stars in his telescope's field of view. 

As the discoverer, Herschel initially proposed to name the new planet "Georgian Sidus," or "George's Planet," after King George III. But after many decades of debate by the international science community (during which even "Neptune" was proposed!), Johann Elert Bode proposed the classic and decidedly non-Anglic name Ouranos, a Latinized form of Uranus. In Greek mythology, Uranus was the god of the sky, father of Saturn, and grandfather of Jupiter. I know many astronomy teachers of giggling school kids who wish the name "George" had stuck!

If you ever have a chance to see Neptune in a telescope, you'll understand why the beautiful blue planet was named after the Roman god of the sea. In a remarkable coincidence, Galileo Galilei observed Neptune on Dec. 28, 1612, but sketched it as a small star sitting near Jupiter in his telescope's field of view. Set your astronomy app to that date, center it on Jupiter, and zoom in. There's blue Neptune, sitting about 14 arc-seconds (half the Moon's diameter) from Jupiter. Even more fascinating, advance the date a few days to Jan. 3 and Jupiter passes directly in front (or occults) the giant planet! It's not known whether Galileo observed that event.

Unbeknownst to him, Galileo Galilei observed Neptune in 1612, long before it was officially discovered more than 200 years later. In his sketch of Jupiter and its moons from Dec. 27, 1612, Galileo noted a bright "star" to the far left of Jupiter — the planet Neptune. The lower panel shows the event replicated using the SkySafari 5 app. Notice that Galileo marked the positions of some of Jupiter's moons — Ganymede, at left, and Europa and Callisto, at right. His telescope was far too small to capture tiny Io crossing in front of the planet.

Credit: SkySafari App

In the middle decades of the 19th century, astronomers noted that Uranus was not orbiting precisely as expected, and it was theorized that the gravitational tug from an unknown planet was perturbing Uranus' path. By 1846, the French mathematician Urbain Le Verrier had calculated an estimate of where the missing planet might be found. Le Verrier sent a letter requesting that Johann Gottfried Galle of the Berlin Observatory search that patch of sky and, on the evening of Sept. 23, 1846, the same day he received the letter, Galle discovered Neptune within 1 degree of Le Verrier's predicted location! For a few months, "Le Verrier's Planet" was proposed for the name, but that risked Uranus reverting to "Herschel"! Before the year was out, Le Verrier's preference, Neptune, was accepted internationally.