What is astronomy? Why are we interested? Are we the center of the universe? What do we know about the universe, and how did we get there?
What is astronomy?
It is difficult to avoid cliches when talking about the sheer enormity of the universe. The scale of time and space is so mind-bending that we reach for the familiar to attempt to make sense of things. In terms of volume, the Earth could fit into the sun 1.3 million times. And our sun is a star of middling size, unexceptional among the 200 billion trillion stars estimated to exist in the universe.
Humans have always been captivated by the beauty of the night sky. Evidence suggests humans were making observations of the sky before recorded history, and astronomy is our oldest science.
Perched on our small planet in the vastness of space, we’re determined to understand the universe as much as possible. And what we know is constantly expanding and evolving. New concepts, technologies, and data challenge what we think we know about our universe all the time. As our technological capabilities expand, so too does the universe expand before us.
The branches of astronomy
Astronomy encompasses the study of everything in the universe beyond Earth’s atmosphere. It can be subdivided roughly into 4 main branches:
Astrophysics examines how the laws of physics apply in space and includes cosmology, which investigates the origins and expansion of our universe.
Astrometry is concerned with mapping celestial bodies.
Astrogeology involves examining terrains, rocks, and other materials in space.
Astrobiology describes our search for life beyond our own planet.
The wonders of the night sky – eclipses and auroras
Imagine walking into battle in the 6th century as a highly skilled soldier, when the sky suddenly turns utterly dark. It would be enough to strike fear in the stoutest of hearts, and probably enough to make you flee the battlefield and reconsider your career. According to Herodotus, this is exactly what happened during the Battle of the Eclipse. A sudden solar eclipse halted action, and the soldiers understandably lost their appetite for destruction under ominous skies.
Imagine seeing a blood-red moon as you hoped in vain for some auspicious event to come to pass. Or seeing – without explanation – curtains of flickering green light dancing in the sky, what we now call an aurora. Eerie and stunning phenomena in the sky have always had a powerful pull on human emotions.
Auroras result from particles from solar storms interacting with Earth’s atmosphere. Solar eclipses result from the moon passing in front of the sun, and lunar eclipses are because of the Earth passing between the moon and the sun. We know all this now. But still, the flickering green lights of auroras, the darkened day of a solar eclipse, and the startling red moon of a lunar eclipse are awesome to watch.
Ancient astronomy
An iconic example of ancient astronomy can be found in Stonehenge in England. The main stone circle of this prehistoric monument is thought to be around 4500 years old. Observations suggest that the axis maps out the path of the sun on its longest day, known as the summer solstice. It looks very much like it was built in a way that aligns with the summer and winter solstices.
In the 1890s, the astronomer Norman Lockyer used this and other observations to argue that Stonehenge was an astronomical temple, designed for ceremonies that celebrated our seasonal journey around the sun.
He further suggested that ancient people of all cultures tracked astrological cycles and used their observations to devise calendars. This idea seemed suspect at the time, but today, there is broad agreement that ancient architects from across cultures used knowledge of astronomy when creating monuments and other important buildings. The study of how prehistoric cultures looked at the sky is known as Archaeoastronomy.
Aristotle and the geocentric model
Aristotle was among the Greek philosophers credited with creating the scientific method as we know it. He lived around 384–322 BCE and devised a model of astronomy that dominated for 2000 years. In this model, the Earth is an unmoving sphere. He reasoned that we don’t feel the Earth move, so it must be still. Surrounding this central sphere are nested crystalline spheres carrying celestial objects. In the outermost sphere are the stars. This model – in which everything in the sky orbits around an unmoving Earth – is known as a geocentric model.
Ptolemy, a scholar from the library of Alexandria, made refinements to this model several hundred years later. He wrote an encyclopedia around 140 CE, which ran to 130 volumes and included a description of the geocentric model and a catalog of 1022 stars. Ptolemy’s descriptions served as the basis of astronomical thinking for the next thousand years.
Aristarchus and the heliocentric model
Even shortly after Aristotle’s time, the geocentric model was questioned. Aristarchus, living around 310–250 BCE, proposed a heliocentric model in which the Earth and planets orbited the sun.
The Greeks understood that we see the moon in reflected sunlight. Aristarchus used geometry to estimate the size of the moon and the sun relative to the Earth, using the shadow the Earth cast on the moon as a reference. He deduced that the Earth was larger than the moon and the sun was larger than the Earth.
He suggested it was reasonable to assume smaller objects orbited larger objects. So if the sun was bigger than the Earth, it was reasonable to think the Earth orbited the sun.
In the absence of compelling evidence, though, and since we don’t feel the Earth moving under our feet, Aristotle’s ideas prevailed over the heliocentric model for thousands of years.
Beyond ancient Greece
Astronomy in Western Europe made little progress during Roman times or the subsequent dark ages. However, astronomical advances continued to be made by cultures around the world.
Arabic astronomers preserved Greek books and improved the models put forward by Ptolemy. Arab astronomers measured the circumference of the Earth in 840 CE, and their result was only 4% off the actual answer.
In 600 CE, the Indian astronomer Aryabhata proposed that Earth turned on its axis, and that this accounted for the movement of the stars we see in the sky. The prevailing wisdom at the time was that the stars revolved around the Earth. Some have suggested that a heliocentric model underpinned his thinking, but this has been disputed.
The court astronomers of China created a continuous record of observations spanning 2000 years, which is still consulted by active astronomers today.
Astronomy was integral to Mayan culture and supported by the state. Major decisions, such as the timing of wars, were made based on astronomical observations.
Unfortunately, many astronomical records from cultures around the world have been lost to turbulent and destructive periods in history.
The Copernican Revolution
Copernicus recognized that predicting the positions of the planets became a lot simpler if you assumed the sun was at the center of a system and the planets orbit around it. He published his theories at the end of his life in 1543.
Although his model was simpler, it wasn’t any better at predicting the positions of the planets than Ptolemy’s.
In the years that followed, the work of science continued to build on the ideas of Copernicus. Tycho Brahe established what is thought of as the first modern observatory in Europe – a facility used specifically to observe celestial events.
This observatory was known as Uraniborg or ‘The Castle of Urania’. Tycho took many accurate observations from Uraniborg, and Johannes Kepler used these observations to make breakthroughs in our thinking about how planets moved. Galileo used a telescope of his own making to make observations of Venus and other planets, as well as the moon, confirming the heliocentric view of the universe.
Our place in the center of the universe was swept away. Astronomy would never be the same.
A tale of decreasing importance
The Copernican Revolution completely changed our understanding of our importance in the universe. If the Earth orbits the sun, we should see changes in the relative positions and brightness of the stars in the sky. This is due to an effect known as parallax, which makes objects appear to move when the observer’s position changes.
To experience parallax now, line up your thumb with an object in the near distance and look at it with your left eye. Now switch to looking with your right eye. Your thumb appears to jump because each eye observes from a slightly different location. But the stars don’t seem to jump about in the sky as we look up from Earth. This suggests the universe must be much bigger than we first thought.
As our technology advances, our sense of our importance within the universe diminishes. We have now discovered many planets beyond our own solar system. As of 2022, we’ve discovered more than 5000 of them. We know that we inhabit just one of an estimated 100 billion galaxies in the universe. And we know that the universe appears to contain large amounts of dark matter and dark energy – neither of which is fully understood. It is clear that we are far from the center of the universe.
How do you measure the sky?
As our understanding of the enormity of space grows, the units of distance we use on Earth seem inadequate in measuring astronomical distances.
Modern astronomers use 3 fundamental units of distance:
The light-year is the distance traveled by light in 1 year. It’s easy to grasp and gives some sense of how large interstellar distances are. One light year is around 5.88 trillion miles (9.46 trillion kilometers).
An Astronomical Unit (AU) is roughly the distance from the Earth to the Sun. It is about 93 million miles (150 million kilometers). It is particularly useful for measuring distances between objects in the solar system.
The Parsec has its origins in the angular measurements astronomers used to work out widths in the sky. One parsec is about 3.26 light-years. Larger distances within the universe are measured in megaparsecs, which is 1 million parsecs.
What does the future of astronomy look like?
Since Galileo pointed his home-made telescope at the sky, the science of astronomy has evolved into something entirely new. We now have telescopes in space, such as the Hubble Space Telescope and the James Webb Space Telescope, sending us images of the distant universe. Because of the speed light travels, we can look back in time as well as outwards in space.
We have such large volumes of data about the universe that volunteer citizen scientists have been recruited to help sort through projects such as the Galaxy Zoo. We can examine the chemical composition of planets in space. And yet, so many questions are still unanswered.
What is dark matter? Is the universe infinite? Is there life beyond Earth?
We don’t have the answers to these questions, and we might not even have all the right questions yet. But we can keep trying to learn more about the universe through astronomy.
