The Same Sky, Differently

My favorite coffee shop keeps a sign behind the espresso machine: “Is Mercury in retrograde?” The barista uses a flip-style “yes/no” just below to indicate each day’s status. I’ve noticed that her greetings tend to be more along the lines of, “Be careful out there!” when the sign says yes versus, “Hey, enjoy the gorgeous day!” when the sign says no. I instinctually roll my eyes at that sign. Her cappuccino is the best in town, though, so I keep coming back.

She has no idea that I’m an astronomer by training. I’ve pondered speaking up. Would she be annoyed if I explained that “Mercury in retrograde” carries no cosmic implications—that it’s just an illusion? Maybe she’s all-in with astrology; maybe she’s a history buff who is fascinated by the ancient stargazers. Or maybe she follows Mercury just for fun. I decided not to press the issue, but our brief exchanges sparked a meaningful connection all the same.

Today when she gave me my cappuccino, the barista winked and told me to hang on tight and be brave, because Mercury is entering its period of retrograde motion. “Change is in the cosmos,” she said.

I smiled as I headed out into the bright sunshine. “It sure is. Always!” I replied. An astronomically true statement.

When I say that retrograde motion is an illusion, I don’t mean that it’s unimportant. On the contrary, it is a crucial visual clue that helped us figure out our place in the universe. The word “retrograde” comes from the Latin root grade, meaning “step.” Taken together with its prefix retro-, it’s most commonly defined as “backward motion.” More precisely, it is the apparent backward motion of planets in the sky. Scientific astronomy’s roots sprang from astrology; what ancient astrologers called appearances, modern researchers deem observational data.

The astrologers of long ago were impeccable record-keepers who paid close attention to those appearances. They realized that the positions of the constellations correlated with agricultural seasons. The steadily shifting orientation of the night sky throughout the year helped indicate when to plant and when to harvest. Even if they couldn’t discern the mechanisms behind those motions, ancient astrologers could intuit a relationship between the sky above and the seasons on Earth. And they wondered: If the stars matched seasons that affected feast or famine, what else might they influence?

If I had lived during those times, trying to make sense of illnesses, drought, or earthquakes without any knowledge of bacteria, climate patterns, or plate tectonics, I probably would have sought explanations in the sky as well. Nothing visible here on Earth could explain such phenomena, so perhaps the stars and planets—or the deities that set their courses—could.

Astrologers also observed that while the sky as a whole rotates during the night and slowly shifts over the course of the year, individual stars appear to be fixed in place within their constellations. Planets are a whole different story. The word planet comes from the Greek term for “wanderer,” and that etymology is accurate. Planets constantly wander about relative to the stars. If you keep track of, say, Mars for several months, you’ll be able to trace its path across a full constellation and into another.

Planetary paths aren’t willy-nilly, though. Planets all appear to travel along the same road in the sky, a path that ancient stargazers named the ecliptic. Most of the time they progress west-to-east. But every now and then, each planet deviates from its typical pattern. It slows down, stops, reverses, and then resumes its normal motion again. That behavior is counterintuitive, and to ancient astrologers it was baffling. Furthermore, each planet moves at its own, characteristic speed. That detail seemed significant, too.

As they wove these appearances together with storytelling, astrologers ascribed personality traits to the planets. Mercury displayed frequent bouts of retrograde motion across the twilight sky, which bolstered its reputation as a trickster—hence the word “mercurial.” Saturn moved slowest through their night sky, in a “saturnine” style. (Uranus and Neptune were unknown to humankind until the 18^th and 19^th centuries, when telescopes enabled us to see deeper.)

Astrologers sought to find order and meaning from all these motions. Those early sky-watchers, like the vast majority of pre-modern thinkers, believed that Earth was the center of the universe (Earth feels motionless, after all), with the Moon as our closest companion. From there, they rank-ordered the celestial bodies based on their speeds across the sky, so Mercury followed, then Venus. Geometry dictated that the Sun came next. The outer half of their Earth-centered model matched the true arrangement: Mars, Jupiter, and Saturn, in that order. Today, we know that gravitation explains why slower orbital speeds correspond to greater distances from the Sun. Astrologers inferred that arrangement from appearances.

Greek astronomers later constructed intricate models to describe how the planets moved in this geocentric system. They imagined a system of concentric, crystalline spheres, each carrying its own planet. Additional spheres carried the Sun and Moon, with the stars etched into the outermost sphere of the cosmos. Each layer rotated at its own pace within the hierarchy. These models provided such a powerful sense of cosmic order that they dominated Western thinking for nearly two millennia.

Retrograde motion proved to be an Achilles’ heel of the geocentric model, however. Astronomers contrived secondary circles of motion within the spheres as they attempted to replicate the backward motions of the planets. Greek stargazers named these sublayers epicycles. They even added circles upon circles upon circles to try to describe retrograde motion, but none of the Earth-centered models could get it quite right.

The real explanation for retrograde motion is breathtakingly simple. It just required a leap of imagination—a new way to think about appearances.

Forget about planets for a moment. Imagine you’re in a car driving along the highway. There’s a long, gently arcing curve ahead, and another car is beside you. You’re on the inside lane, driving ever-so-slightly faster than the car in the outside lane. You know intuitively what will happen: Your car will move through the arc of the curve faster than the other car. But to your passenger looking out the window at the car next to you, there is no easy way to gauge your absolute motion. To the passenger, it seems as if the other car glides backward while you are overtaking it. Relative to the road, that’s untrue. Relative to you, though, the slower car appears to drift in reverse.

Hello, retrograde motion.

This phenomenon happens in lots of situations. If you are on an airplane as it taxis gently backward from the gate, it may appear instead that the gate is moving forward. Or, if you’re running around the outer lane of a track, you may feel as if you’re falling back when another runner passes you by.

So it is with Mercury’s loop-the-loop across the sky. Mercury laps the Sun every 88 days, four times as quickly as Earth does. If you could hover high above the solar system, you would see each planet moving in its own continuous oval path. Seen from our terrestrial point of view, though, Mercury sprints past us, goes backward, then overtakes us again as Earth plods along in its orbit.

Science is the common thread connecting these experiences. The laws of motion are the same everywhere. Equations allow scientists to describe these motions in terms of the relationships between force and acceleration, or between energy and mass. The planets move in their curved orbits around the Sun and a tennis ball flies through the air along a curved path when I play catch with my dog, both following identical principles of physics.

Popular history-of-science accounts often portray the old debate between the Sun-centered and Earth-centered models as a battle of science versus religion. It’s more accurate to describe it as a battle of mental constructs, as Copernicus and Galileo and many others attempted to untangle appearances in Earth’s sky from genuine patterns of cosmic motion. In the 17^th century, there was no way to fly a rocket that could watch our motions from afar. Even the first hot-air balloons were more than a century away.

Astronomers had to get creative, shifting their perspective while remaining anchored to the ground. They ended up envisioning a fundamental re-ordering of the cosmos. Copernicus’s heliocentric model provoked a collective jaw-drop not so much because it was more accurate than the earlier geocentric models, but because it solved the 2,000-year-old mystery of retrograde motion. In the Greek system of spheres, there is no reason for retrograde motion; it was added into the models as needed to match the observed wanderings of the planets. In the Sun-centered model, retrograde motion is logical and utterly simple—the result of two objects moving at different speeds along neighboring paths.

But that doesn’t mean that retrograde lost its astrological heft.

Throughout late summer and early autumn of last year, I hesitated to ask my barista how things were going. Not only was Mercury in retrograde, but for a few weeks every planet except Venus was in retrograde. Mercury, Mars, Jupiter, Saturn, Uranus, Neptune, and even not-quite-a-planet-anymore Pluto: all retrograde at the same time.

Astronomically speaking, this pattern means nothing. It’s a phase in the revolution of these particular planets as they orbit an ordinary star in an ordinary neighborhood of our ordinary galaxy. From Earth, we see each planet in retrograde at different times of year and for different durations, purely due to where they are in the speed- and distance-ranked hierarchy of orbits around the Sun. Correlation (the appearance of moving backward) is not the same as causation (varying orbital speeds).

And there is nothing special about Earth’s vantage point. If you were an astronaut on Mars, you would see Earth undergo retrograde motion. That’s right: Earth appears to move backward now and then in the skies of Mars. It really doesn’t feel like we’re at the center of the universe anymore, does it?

In one sense, the mystery of retrograde motion—which for centuries was deemed inexplicable—has become mundane. From a scientific perspective, it has no effect on the lives of Earthly bipedal, semi-intelligent hominids. Retrograde motion does not predetermine whether I’ll stumble and spill my cappuccino.

Newton’s laws of motion and universal gravitation make it possible to calculate forces throughout the cosmos, whether between distant galaxies or here on Earth. Taking an even deeper step into those physical laws, Einstein developed his theory of special relativity by pondering the extremes of relative motion: He imagined an observer traveling alongside a beam of light, just like two runners on a racetrack, and considered what that person would see. There are layers upon layers of connections here, not at all mundane, and plenty of still-unexplained phenomena.

Maybe I’ll invite my barista to chat one day, late in the afternoon, when the coffee shop isn’t too busy and the Sun—our star—is dipping toward the horizon. We could ponder the strangeness of that observation: We say that the Sun “sets” even though we know it’s just Earth’s rotation causing it to appear so. The motions, patterns, and positions of celestial objects in the sky do correlate with appearances. But only quite recently in human history have we figured out how to explain the “why” behind the “what” of those motions.

I like to think that this sense of wonder could open a conversation instead of closing it. As with most disparate points of view, mine and the barista’s are not necessarily mutually exclusive. Science bursts with marvels that cannot be fully described in terms of equations and data points. We are all coming to terms with the past, searching for deeper understanding, and longing for connections to the vastness of nature around us. My barista’s curiosity and her quest for meaning surely spring from the same roots that inspire me to study astronomy.

After all, I still make a wish on the first star I see each night. Why? Because it’s fun.