Last week, I tried to respond to a question that many fourth grade students asked me when I was director at Perkins Observatory: “Who discovered the first planet?”
On one level, the question is unanswerable. You can’t answer the “who” without answering the “which planet.”
The problem is made more difficult because that discovery happened long before people recorded such matters in writing. Also, the ancient discoverers needed to at least recognize that they were discovering something new, i.e., that they were seeing something other than a star.
As a result, I claimed that the first person who discovered a planet was William Herschel, who used his home-built telescope to discover Uranus on March 13, 1781.
I imagine that fourth grader responding, “But that’s not what I meant. I meant the planets they could see without a telescope.”
Okay. We must imagine a time before written history. We must imagine some unnamed, primitive human who first noticed that one of the sky’s bright things was different from the rest. Thinking like an early human is a difficult task, but let’s try anyway.
The ancients though they were at the center of the universe. They thought that things in the sky moved above the surface of the ground.
Because the ancients had absolutely no sense that the planets orbited the sun, we must return to what they did know.
What made a planet different? As we did last week, let’s return to the ancient Greek definition. The stars moved around the dome of the sky, but they were fixed with respect to each other.
Some objects called planetes, or literally “wanderers,” didn’t fit in because they moved against the starry background. Their ability to move at will meant that they must be gods. The wanderers included Mercury, Venus, Mars, Jupiter and Saturn. But they also included the sun and the moon.
The moon is very obviously different than the stars. The stars are numerous, and they are points of light.
There’s only one moon. It subtends about half a degree of arc, about the width of your little fingernail held at arm’s length.
The moon was important to precivilized people. When Luna is above the horizon, it provides sufficient illumination for night hunting and harvesting and relative safety against predators.
Early people must surely have noticed that the moon goes through a monthly set of phases. In fact, the “moonth” (and not the year) was our first way of measuring the passage of calendar time.
As it orbits our planet, it moves 13 degrees every day. Thirteen degrees is a bit more than the width of your fist held at arm’s length. If you observe the moon at about the same time every night, Luna moves from one side of the sky to the other over the course of two weeks. Some ancient human must have noticed that motion. So the answer must be the moon …
… Unless it’s the sun. Our day star is dazzlingly bright, so it’s dangerous to your eyes to look at it. But some early sun gazers must have noticed that they could see the sun’s disk, which is about the same size as the full moon, through thin clouds at sunrise or sunset. (You should never, never try it yourself!)
The sun moves across the sky over a day, disappears at night, and reappears again at sunrise. Some ancient observer must have noticed that the sun also moves against the background of the sky and that the motion was connected to the seasons.
Their lives depended on that motion. During the lifegiving summer months, old Sol rises far to the northeast and sets in the far northwest. As a result, it sits very high in the south at noon.
In deadly winter, the sun rises farther to the west and sets farther to the east. It never gets very high in the sky, even at local noon. The snow falls. The crops die.
We tend to notice things first when they have practical usefulness. Yes, it must be the sun.
My fourth grader replies, “Come on. I meant regular planets.’” I must resist the temptation to say, “That’s what comes of applying a modern definition of the word planet to a term the ancients used differently.”
But okay. So we are applying the ancient definition of planet, but subtracting the more recent notion that the sun and the moon aren’t planets.
Saturn is probably out. It’s the dimmest of the naked-eye planets and wanders very slowly. As the farthest planet, Saturn has the farthest to go, so it takes almost 30 years to make one circuit around the sky. Our ancient discoverer certainly would not have noticed its motion against the starry background from day to day or even month to month. Even if they had, they wouldn’t have noticed it first.
The same goes for Jupiter, which takes 12 years to circumnavigate the sky. It traverses only one zodiacal constellation every year. Noticing its motion would take careful observation over months or years. Jupiter is bright, but it still looks like an unmoving star at first or second glance.
Mercury is the quickest “wanderer” because it is so close to the sun. However, it never gets farther than a couple of hand widths above the horizon. To this day, many stargazers have never seen Mercury. I have only seen it in twilight when there are no nearby visible stars to measure its motion. Ancient stargazers may not have even known about Mercury’s existence.
They would have known about Mars and Venus. Mars is a likely candidate because it moves quite quickly. The Red Planet traverses the 360 degrees of the sky every 26 months or so. Mars varies in brightness somewhat over its 687-day orbit, but its motion against the background of stars is quite noticeable even over a period of days.
But if we look at the ancient written records of early civilizations, Mars didn’t get that much attention compared to another planet. It’s hard to believe that precivilized stargazers noticed the motion of Mars before they noticed that of brilliant Venus.
If you don’t believe me, go out in the evening and observe orange Mars in the southern sky. Then go out in the predawn and look at Venus in the east.
Mars is bright. Venus is a stunner — the brightest object in the nighttime sky after the sun and moon.
The trouble is that Venus is pretty close to the sun, although not nearly as close as Mercury. It’s tough to locate reference stars when sunlight blocks the view. However, over a period of days, it’s quite easy to see that Venus changes position in the sky if you use the horizon as your measurement and observe it at about the same time every day.
Just a few weeks ago, Venus was quite high in the sky during morning twilight. Now, Venus is quite low at about the same time. Certainly, ancient stargazers must have noticed that motion.
However, that assumption is problematic. Because of its proximity to the sun, we tend to see it in only the morning or evening sky. We don’t see it all night the way we sometimes do with Mars.
As a result, the early Greeks thought Venus was two planets. The morning appearance was called Eosphorus, which means “dawn-bringer.” Hesperus, Venus’s appearance at dusk, means “evening.” The early Greeks regarded them as two separate gods, and therefore as two separate planetes.
The ancient Egyptians made the same error. They referred to Venus’s two incarnations as Tioumoutiri, the morning star, and Ouaiti, the evening star.
It’s hard to imagine that ancient stargazers before the Greeks and Egyptians made all the connections to classify Venus as a single object and also a wanderer.
However, perhaps some cultures older than the Greeks or Egyptians did.
We might get a hint by looking at the very earliest of western cultures, which arose in the Middle Eastern region known as Mesopotamia.
The first surviving record of Venus’s movement across the sky is a Sumerian cylinder seal dated to about 3,000 BCE. It seems to indicate that the Sumerians knew that Venus was a single planet.
The Sumerians worshipped Venus as the goddess Inanna, who embodied powerful forces like love, war, and political power. The “Queen of Heaven,” as she was also known, was among the most important of Sumerian gods.
The planet was later worshipped as Ishtar, the most important god of the Babylonians. They recorded detailed observations of Venus as a single planet on the Venus tablet of Ammisaduqa during the first millennium BCE. However, the observations on the tablet date back as far as the 1600s BCE.
Therefore, perhaps the first ancient planet discovered was Venus. It depends a lot on how you define “planet.”
So what was the first planet discovered? Who discovered it? I dunno. That’s my definitive answer, and I’m sticking with it.
Tom Burns is the former director of the Perkins Observatory in Delaware.