A casual familiarity with astronomy exposes a variety of confusing terms that inaccurately describe certain phenomena. However, if we investigate the history of their use, we find them to be artifacts from a previous era, words that reveal outdated interpretations “before we knew any better.” Tradition has allowed these archaic phrases to stick around despite their now erroneous interpretations. Some examples include lunar maria, shooting stars, planetary nebulae and asteroids.
While these expressions provide amusing conundrums for the etymologist in all of us, they also serve as reminders that science is a progress report, an assessment of the properties of our universe based on current observations and interpretations; with further study, scientists refine this understanding. In 1862, for example, Lord Kelvin estimated the age of Earth, based on a model of its rate of cooling, at 98 million years old. In the following century scientists developed radiometric dating techniques that allowed for more accurate evaluations and today the accepted number is about 4.5 billion years.
With that said, let’s look at the history of the terms listed above and what they reveal about their times. When Neil Armstrong first stepped on the moon at the Sea of Tranquility, his feet settled onto a surface of powdery rock rather than sinking into water. This came as no surprise to scientists of the day but it might have to ancient Greeks, who believed the dark areas of the moon to be bodies of water. The Italian astronomer Giovannia Ricciolo paid homage to this interpretation in 1651, when he designated many of the names for lunar features that we still recognize. He assigned Latin terms, hence Mare Tranquillitatus (Sea of Tranquility), Oceanus Procellarum (Ocean of Storms) and Sinus Iridium (Bay of Rainbows).
For years, people thought the bright flashes of space debris burning up in our atmosphere that we know today as meteors were instead stars in the throes of death, streaking across the skies toward their demise. They were called shooting stars, a term first used in print by none other than William Shakespeare in his 1593 play "Richard II." In Act 2, Scene 4, the Earl of Salisbury says, “Ah, Richard, with the eyes of heavy mind/I see thy glory like a shooting star/Fall to the base earth from the firmament.”
Many astronomical bodies weren’t known until the telescope came along at the dawn of the 17th century. Even with these early telescopes — crude by today’s standards — sky watchers discovered an array of previously unimaginable features of all sizes and shapes. In 1764 the French astronomer Charles Messier recorded one of these, a small, fuzzy sphere, as the 27th object in his catalog of astronomical phenomena. Several years later, in 1781, William Herschel discovered the planet Uranus. Perhaps this discovery led Herschel to have “planet” on the brain, because in 1784 or 1785 he coined the term "planetary nebula" for those fuzzy features first detected by Messier. Herschel thought these resembled planets but later observations showed they were not related to planets, but instead consisted of concentrations of nebulous material—gas and dust—surrounding an ancient red giant star.
Another astronomical feature not discovered until the advent of the telescope was the asteroid. The Italian priest and astronomer Giuseppe Piazzi discovered the first asteroid in 1801 and others soon were found. Our old friend William Herschel, after considering other options such as planeret, planetking, and planetling, finally settled on calling these bodies asteroids (“star-like” or “star-shaped” in Greek) because they appeared, like stars, as points of light.
We know today that these are more related to planets than stars, and in fact are called minor planets by many scientists. However, as in the case of lunar seas, shooting stars, and planetary nebula, the familiar yet inaccurate name remains the most commonly used.
Kevin Schindler is the content specialist for Lowell Observatory.