What is Astronomy? - Definition, History, Timeline & Facts

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  • 1:10 History & Timeline
  • 4:56 Facts
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Lesson Transcript
Instructor: Cassandra Marnocha
When you look up at a clear night sky, how many stars do you see? Probably more than you can even count! What's more incredible is just how large and far away those stars are, and how much we can learn about our universe by looking at starlight.


Astronomy is the study of the universe, the celestial objects that make up the universe, and the processes that govern the lifecycle of those objects. Astronomy is largely an observational science. Astronomers use the electromagnetic radiation emitted from stars and other celestial objects, which can include visible light, UV, infrared, and X-rays. Because the light from these objects is the primary means to study them, one of the most important tools for an astronomer is the telescope.

These images are looking at the same nebula, but using visible light and infrared. The stars visible in the infrared image (right) cannot be seen in the visible light image because of the optically opaque gas and dust.
Visible-infrared comparison

Given the size of the universe (which could be infinite), astronomy is an enormous field. The celestial objects that astronomers study include stars, galaxies, nebulae, and supernova. Because of the enormous distances between Earth and other objects, when astronomers look farther away, they are also looking back in time. This is because of the amount of time it takes the light we see to travel from the source to us here on Earth.

History and Timeline

Astronomical events have been observed and recorded since the dawn of civilization. Events such as the rising and setting of the sun, the movement of stars in the sky, and solar and lunar eclipses occurred with such regular patterns that ancient cultures couldn't help but notice and try to figure what was happening in the heavens. Many of these observations were deeply linked with cultural and religious beliefs.

The Ancient Greeks were especially interested in astronomy and were able to calculate some impressive figures, such as the size of and distance to the Sun. Persian, Arab, and Chinese astronomers supported astronomy during the Middle Ages, when little progress was made in Europe.

During the Renaissance, an interest in astronomy was revived in Europe and some of the most important names begin to show up in textbooks. Scientists like Copernicus, Galileo, Newton, and Kepler all lived and studied physics and astronomy around the 1600s. Hundreds of years later, the Space Age brought about a new age in astronomy, when massive telescopes could be constructed and even launched into orbit around the Earth to view objects further away than ever before.

Major discoveries are still made today in the field of astronomy. The Hubble Space Telescope was launched in 1990 and continues to send back data and breathtaking images of galaxies and nebulae. As you can see on screen, here's an imagine of the Pillars of Creation taken by the Hubble Space Telescope. This region is an incubator for new stars.

An image of the Pillars of Creation taken by the Hubble Space Telescope. This region is an incubator for new stars.
Pillars of Creation

The Hubble Telescope will eventually have a successor. The James Webb Space Telescope, is scheduled to launch in 2018.

Here is a brief timeline of important astronomical discoveries:

30,000 B.C.E: Bone carvings that show the phases of the Moon. The first records of early astronomical observers.

700 B.C.E: Babylonians record and predict lunar eclipses.

280 B.C.E: Aristarchus invents the first heliocentric, or Sun-centered, model of the solar system as an alternative to the Earth-centered model.

164 B.C.E: Earliest record of Halley's Comet.

140 C.E: Ptolemy models an Earth-centric solar system with circular orbits. This model accurately predicts the positions of the planets, but as we know, Earth isn't truly the center of the solar system!

1066 C.E: The appearance of Halley's Comet results in its inclusion in the Bayeux Tapestry.

1420 C.E: Ulugh Beg builds an observatory in what is now Uzbekistan. He compiles a star catalog based on his observations.

1543 C.E: Copernicus publishes his model of the heliocentric solar system.

1609 C.E: Galileo uses a telescope for his observations.

1682 C.E: Edmund Halley predicts the return of the comet, which is named after him - Halley's Comet.

1783 C.E: William Herschel describes the motion of the Sun through space.

1842 C.E: Christian Johann Doppler describes what is now called the Doppler Effect, or the change in wavelength of a wave if it moves toward or away from an observer.

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