Astronomy Terms

astronomytermsAstronomy is the study of the goings-on outside of the earth’s atmosphere. Astronomy is also sometimes described as the study of celestial bodies, which refer to basically any physical entity or structure that appears in our universe. This includes stars, planets, black holes, comets, asteroids, and anything else that astronomers can observe and study, anywhere in space beyond the boundary of Earth’s atmosphere. If you are unfamiliar with the science of astronomy, one way to familiarize yourself with the discipline is to study specific astronomy terms. Knowing what these terms mean will make the complex aspects of astronomy easier to understand.

History of Astronomy

Astronomy is possibly the oldest science that humans have studied. The word astronomy is derived from Greek words that translate to the law (nomos) of the stars (astron), and most ancient human cultures have left behind evidence that they studied the movements in the sky. Ancient mythologies, such as those of Greek and Roman cultures, often gave god and deity names to constellations of stars. The ancient Egyptian pyramids were built to line up with specific star patterns, and so were many other ancient structures built by varied groups of ancient peoples.

The beginnings of astronomy involved mapping the positions of stars, planets, and other celestial bodies visible from Earth, such as the moon. As civilizations developed, the study of astronomy expanded to include the movement and behavior of celestial bodies, and the reasons behind the movement. Early astronomer Ptolemy made famous the geocentric model of the universe, believing that the celestial bodies we see are rotating around our own planet, which sits at the center of the universe. The astronomer Aristarchus first explored the concept of the heliocentric model, in which the sun sits at the middle of the solar system, in the time of ancient Greece. Copernicus and Galileo furthered the heliocentric idea during the scientific revolution in the seventeenth century. Developments in technology, specifically in terms of telescopes and photography, led to developments in the field of astronomy. The twentieth century saw the introduction of the idea of our observable universe, including the arrangement of our solar system, our galaxy, and the presence of countless other galaxies. Today, satellites and space stations orbit the earth, massive telescopes and observation centers collect data from space, expeditions explore faraway planets and moons, and scientists’ ability to see farther into the past is ever-increasing. Astronomy has come a long way since our early ancestors began to wonder what the lights in the night sky could mean.

Basic Astronomy Terms

A study of astronomy should begin with an introduction to some basic astronomy concepts. Learning these terms will build a foundation on which you can further your study of the universe:

Gravity: Gravity is the invisible force that attracts physical bodies of matter to each other. It is one of the four fundamental forces of nature. They others are the forces of electromagnetism, the nuclear strong force, and the nuclear weak force. The pull of gravity is what is responsible for the concept of weight. How much an object or a person weighs is a result of its mass, which is attracted to the much more massive earth through the force of gravity. The greater your mass, the more gravity acts on you, and the more you weigh. The pull of Earth’s gravity is what causes a dropped object to fall to the ground; the story goes that Isaac Newton began to wonder about gravity when an apple fell from a tree, towards the earth, and struck him on the head.

Atmosphere: An atmosphere is a collection of gases that form a layer around a celestial body, such as a planet, a star, or a moon. Different celestial bodies have atmospheres composed of different gases. The atmosphere of Earth consists of oxygen, nitrogen, carbon dioxide, and a few other gases.

Planet: A planet is a celestial object of a certain mass that orbits a star or the remnants of a star. A planet must be large and massive enough that the pull of it’s own gravity gives it a round shape. The planets in our solar system orbit our Sun, which is a relatively small star in the scale of the universe. The ninth planet in our solar system, Pluto, recently lost its status as a planet, because it did not fit the definition accurately enough.

Star: A star is a sphere held together by its own gravity, much like a planet, but consists of mostly plasma and gas, unlike planets. The size of a star and its distance from Earth determine how brightly it shines to an observer. Stars are luminous because hydrogen is usually converted into helium in the core of a star, through a process called thermonuclear fusion.

Moon: A moon is a celestial body that orbits another celestial body, most commonly a planet. In order for a moon to orbit a planet, rather than a planet orbiting a moon, it has to have a smaller mass than the planet it orbits. The planet’s gravitational pull must be stronger than that of the moon. The earth has one moon, but larger planets in our solar system have several orbiting moons.

Comet: A comet is a small celestial body, composed mostly of rock and ice. Comets frequently orbit stars, and when they pass close enough to a star, they heat up enough to expel gas, which creates a small atmosphere around them. This small atmosphere can appear fiery, and is sometimes referred to as a tail.

Asteroid: An asteroid is like a very small planet, composed of rock, gases, and other matter. An asteroid will orbit a star, like a comet, but asteroids are not chemically active as comets are, and a star’s heat will not create an atmosphere around an asteroid.

The Solar System: The Solar System is the term used to describe the earth, our sun, and all other planets and celestial bodies that orbit our sun, including moons, comets, and asteroids.

Galaxy: A galaxy is a large group of solar systems bound by gravity. Our galaxy is named The Milky Way. A galaxy consists of stars, gas, dust, planets, matter, and other celestial objects.

Advanced Astronomy Terms

Beyond the basics of astronomy, there are more complex concepts associated with the study of the universe’s celestial bodies. These can be more difficult to understand, but are both intriguing and scientifically important:

The Observable Universe: The observable universe refers to anything in our universe that we can see from Earth. Since light travels at a certain speed, it takes time for light from celestial bodies to reach us on Earth. A star that is a million light-years away from us is emitting light that traveled for one million years before it reached Earth. In theory, there are parts of the universe that are not visible to astronomers on Earth because the light from those celestial bodies has not reached us yet.

General Relativity: Albert Einstein’s general theory of relativity is very complicated, but at its basis, it describes the force of gravity as a property of space and time. Einstein uses the term space-time to describe these two components, and suggests that they are intertwined with each other and share a dimension. According to this theory, there are curvatures in space-time throughout the universe, and traveling through space at a certain speed means traveling through time as well.

Light speed: Light speed is another way of describing the speed of light, which is the speed at which light waves travel from where they are emitted to where they are observed. The speed of light is around 186,000 miles per second. A light-year refers to the distance that light travel from one point to another in the span of one Earth year. People often think that a light-year is a measure of time due to the word year, but it is actually a measure of distance.

Black Hole: A black hole is a segment of space-time that behaves in a similar way to a vacuum. The mass of a black hole is so large, and therefore the gravity in a black hole is so strong, that nothing can escape from the area, including light. A black hole is black because its gravity literally swallows light. Outside of a black hole is a spot called an event horizon. The event horizon is the point past which nothing can travel and still escape the gravity of a black hole.

Broadening Your Knowledge

Now that you’ve become familiar with the foundations of astronomy, you can pursue further knowledge of this fascinating science. Learning about the universe beyond our own planet can be both exciting and overwhelming, It can make you wonder at how massive the universe is, and can make you wonder how small and insignificant human life seems on such a grand scale. No matter how overwhelming, however, developing an understanding of astronomy will expand your scientific knowledge and completely change the way you see our world.