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What is astronomy? Definition & History

Astronomy of the Helix Nebula (NGC 7293) in the constellation of Aquarius, imaged from the European Southern Observatory’s La Silla observatory in Chile
Astronomy of the Helix Nebula (NGC 7293) in the constellation of Aquarius, imaged from the European Southern Observatory’s La Silla observatory in Chile (Image credit: ESO)

Every night, using the science of astronomy, the entire universe can be revealed above us. Although at some point we've all had that "blanket of stars" moment, it is an illusion. The visible planets and the bright stars you can see with your eyes are mostly very close to us — in cosmic terms — but the night sky has incredible, almost unfathomable depth. Not only can our own galaxy, the Milky Way (opens in new tab), be navigated and known but other galaxies can be probed using telescopes, on Earth (opens in new tab) and in space, and in various wavelengths of light from all kinds of cosmic objects. 

Here’s everything you need to know about what astronomy is, what it’s not, and how recent developments within the field of astronomy are making it more exciting than ever before.

Definition of astronomy

What does astronomy mean? A dictionary will tell you that it’s the branch of science that deals with celestial objects, space, and the physical universe as a whole. 

Astronomy is the study of everything in the universe that’s beyond our own planet’s atmosphere. The planets in our own solar system, our own star the sun, and the bright stars can all be seen with the naked eye. However, astronomy can go much deeper, taking advantage of telescopes and other scientific instruments to study other stars and their planets in our galaxy, as well as distant galaxies beyond our own. It can gather clues about the nature of the physical, chemical and biological universe itself. 

On July 2, 2019 a total solar eclipse passed over the European Southern Observatory's (ESO’s) La Silla Observatory in Chile  (Image credit: ESO)

What astronomers do (and don’t do)

Astronomers aren’t stargazers. Or, at least, there’s no longer any need for them to be. If you think an astronomer treks up mountains to spend night after night behind the eyepiece of a giant telescope, think again. These days telescopes can be controlled remotely, so it’s common for modern astronomers to simply make requests for observations and then download computer-generated data and images the next morning for their analysis. That includes space telescopes like the Hubble Space Telescope (opens in new tab).

People very often confuse astronomy with astrology. Every professional astronomer has had to hear someone say to them, "so you’re an astrologer, right?" Astrology and astronomy are not the same thing, but they used to be. Observational astronomy can be traced back to Ancient Egypt (opens in new tab) and Mesopotamia (opens in new tab) as far back as 3,000 B.C., but the calculating of solar eclipses, the movements of the planets and theories about how the night sky works was the job of ancient astrologers who presumed that celestial events and alignments had a direct impact on human affairs. 

Modern astrologers attempt to do something similar, making predictions about human lives based on pseudoscience. Astrology (opens in new tab) is not a science. 

A full-scale model of NASA's James Webb Space Telescope

A full-scale model of NASA's James Webb Space Telescope (Image credit: NASA)

How many types of astronomy are there?

In the past century or so, astronomy has been broadly split into two camps — observational astronomy (using telescopes and cameras to collect data about the night sky) and theoretical astronomy (using that data to analyze, model and theorize about how objects and phenomena work). 

They complement each other, yet within these two broad categories modern astronomy includes many subsets, from astrometry to exoplanetology, that intrinsically overlap yet help explain the many things astronomers do. Here’s what they all mean: 

  • Astrometry: This ancient branch of astronomy concerns precise calculations of the motions of the sun, the moon (opens in new tab) and the planets. It includes predictions of solar and lunar eclipses, and meteor showers. It also includes exoplanetology, a relatively new and very exciting field that concerns itself with the discovery and characterization of planets outside of the solar system. 
  • Planetary astronomy: How did the solar system (opens in new tab) come to be? This is the central question penetrating planetary astronomy, which focuses on the formation, evolution and death of planets, moons and other objects. In the solar system it also includes planetary geology.  
  • Astrophysics: Astrophysicists apply the laws and theories of physics to astronomical observations. It’s an attempt to understand the mechanism behind how the universe was created and how it has and will evolve.
  • Astrochemistry: Astrochemists study the composition and reactions of atoms, molecules and ions in space. 
  • Astrobiology: This emerging and, for now, largely theoretical field of astronomy is the study of life beyond Earth (opens in new tab)
  • Stellar astronomy: The study of the life cycle and structure of the sun (opens in new tab) and the stars, stellar astronomy concerns the classification of stars, and populations of stars. 
  • Solar astronomy: Galactic astronomers study our galaxy, the Milky Way (opens in new tab), while extragalactic astronomers peer outside of it to determine how these groups of stars form, change and die. 
  • Cosmology: Although it’s sometimes used to mean astronomy, strictly speaking, cosmology (opens in new tab) refers to the science of the origin and nature of the universe. The key concept in cosmology is the Big Bang Theory (opens in new tab), the most widely accepted explanation of how the universe began. Cosmology also includes purely theoretical subjects including string theory, dark matter (opens in new tab) and dark energy (opens in new tab), and the notion of multiple universes.

This illustration shows a group of galaxies merging and interacting in the early Universe about three billion years after the Big Bang

This illustration shows a group of galaxies merging and interacting in the early universe about three billion years after the Big Bang  (Image credit: ESO / M. Kornmesser)

What are optical, infrared and radio astronomy?

All astronomy is the study of different wavelengths of the electromagnetic spectrum, which comprises radio, microwave, infrared, visible, ultraviolet, X-ray and gamma rays. To get the full picture of what’s out there astronomers need to study various wavelengths of light. 

Optical astronomy is the study of celestial objects using telescopes and in visible light; all of the biggest telescopes on Earth (opens in new tab) are optical. Infrared light can be detected outside of the Earth’s atmosphere, so by space-based observatories like the Hubble Space Telescope and the James Webb Space Telescope (opens in new tab). Radio astronomy is the study of the sky in radio frequencies; radio telescopes detect and amplify radio waves from space.

Artist’s rendering of the Extremely Large Telescope

Artist’s rendering of the European Southern Observatory's Extremely Large Telescope  (Image credit: ESA)

The problem with astronomy

However they observe the universe, astronomers only ever get a snapshot of the planets, stars and galaxies they study. So although there are dozens of different branches of astronomy, in practice many of them must overlap for an astronomer to get as full a picture as possible of objects that exist for millions to billions of years. 

We’re on the cusp of some tremendously exciting new technology that looks set to revolutionize astronomy. The most obvious is the James Webb Space Telescope, which from 2022 will probe the cosmos to uncover the history of the universe. 

Though just as exciting is the Vera Rubin Observatory (opens in new tab) all-sky survey and the a new generation of massive ground-based telescopes like the Extremely Large Telescope (opens in new tab), which should all see ‘first light’ in the mid-2020s. The Square Kilometre Array, the world’s largest radio telescope, should also be operating by the late 2020s. 

Astronomers are about to see deeper into space to observe regions and objects never seen before. 

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Jamie is an experienced science, technology and travel journalist and stargazer who writes about exploring the night sky, solar and lunar eclipses, moon-gazing, astro-travel, astronomy and space exploration. He is the editor of (opens in new tab) and author of A Stargazing Program For Beginners (opens in new tab)