Telescopes and observatories

When you look up at the night sky with your own eyes, you see only a tiny fraction of the universe. Even on a completely dark night you can see at most a few thousand stars, while our galaxy alone contains several hundred billion. And yet humans have studied the sky for thousands of years, trying to understand what lies out there.

Today telescopes and observatories help us see far further into the universe than our eyes ever could. With modern instruments we can observe galaxies billions of light years away, study planets around other stars, and even investigate what the universe looked like shortly after it was formed. Telescopes are therefore our windows to the cosmos, and the larger and more advanced they become, the more we can learn about the universe around us.

The first observatories

Humans began observing the night sky long before the telescope was invented. In ancient times, many civilisations used the sky as an important tool for understanding the world around them.

In Babylon, Egypt, and China, astronomers studied the movements of the stars to predict the changing of the seasons. These observations were crucial for agriculture, navigation, and calendar calculation. As the stars shifted position in the sky throughout the year, people could determine when it was time to sow or harvest.

Different civilizations also built structures that functioned as primitive observatories, some consisting of stones or buildings aligned with the positions of the Sun or certain stars. Stonehenge in England is one of the most famous examples, where large stones are positioned in a way that marks the Sun's position at events like the summer solstice.

In Mesoamerica, the Maya built advanced temples and observatories from which they tracked the movements of the planets. They developed calendars based on precise observations of the Sun, the Moon, and the planet Venus. Many of these early observations laid the groundwork for the astronomy we know today.

Myths in the night sky

Throughout history, when humans have looked up at the stars, they have not only tried to measure and understand them. They have also told stories about what they saw.

The patterns that stars form in the sky we call constellations. Many of the names we still use today come from Greek and Roman mythology. The constellation Orion, for example, represents a hunter from Greek mythology, while Cassiopeia represents a queen who, according to legend, was placed in the sky as punishment for her vanity.

Other cultures have their own stories about the stars. In Chinese astronomy there are entirely different constellations from those used in the West. Australian Aboriginal peoples tell stories about patterns in the Milky Way's dark dust clouds rather than the bright stars themselves.

The planets have also been named after mythology. Mars is named after the Roman god of war, Jupiter after the king of the gods, and Saturn after a titan from Roman mythology. Even in modern astronomy these ancient stories live on in the names we give to celestial bodies.

Danish pioneers

Denmark has played an important role in the history of astronomy. In the late 1500s, Tycho Brahe built one of the world's most advanced observatories on the island of Hven in the Øresund strait. The observatory was called Uraniborg and functioned as a research centre, laboratory, and home for the astronomers who worked there.

Tycho Brahe had no telescopes available, but he built enormous mechanical instruments capable of measuring the positions of stars with unprecedented precision. He recorded thousands of observations and mapped the night sky in far greater detail than anyone before him. His measurements were later used by the German astronomer Johannes Kepler to formulate his famous laws of planetary motion, which became a cornerstone of modern astronomy. Interestingly, Tycho Brahe also observed the planet Uranus with the naked eye, but because of its extraordinarily slow orbit around the Sun he did not notice its movement and simply recorded it as another star.

The Danish astronomer Ole Rømer also made a groundbreaking discovery in the 1600s. By studying the orbit of Io, Jupiter's innermost moon, he discovered that light does not travel instantaneously but has a finite speed. He noticed that the predicted moment when Io would emerge from Jupiter's shadow varied depending on the distance between Jupiter and Earth, something that could only be explained if light had a finite speed. Rømer thereby became the first person to measure the speed of light, a discovery that later proved crucial to the development of modern physics.

He also attempted to measure the distance to some of the brightest stars using the parallax method, the same principle we use in everyday life to estimate distances. Because we have two eyes, we see objects from slightly different angles. You can try holding a finger in front of your eyes and alternately closing one eye and then the other. The finger appears to shift position because our two eyes see it from different angles.

Rømer's idea was to perform the same experiment on a cosmic scale by observing the stars Sirius and Vega six months apart. At the first observation Earth would be on one side of the Sun, and six months later on the other side, effectively giving the same effect as our two eyes. Unfortunately he chose two stars that lie relatively far away, and the telescopes of his time simply could not detect the tiny difference. With today's telescopes it is perfectly achievable, and is in fact one of the methods we use to measure distances to the nearest stars.

The invention of the telescope

In the early 1600s, an instrument was invented that truly transformed astronomy: the telescope.

The first telescopes were probably developed in the Netherlands, but it was Galileo Galilei who began using them systematically to study the sky. With his telescope, Galileo discovered mountains and craters on the Moon, thousands of previously unseen stars, and four large moons around Jupiter. These observations made it clear that the universe was not as simple as previously believed.

Galileo's telescopes were so-called refracting telescopes, which worked by bending and focusing light using a set of glass lenses, a little like glasses or binoculars but considerably more powerful. Shortly afterwards, Isaac Newton developed a new type of telescope using mirrors instead of lenses to gather light. This type, called a reflecting telescope, is still used in many modern observatories. The advantage was that it is much easier to produce large mirrors than large lenses, meaning Newton's design could capture far more light.

Astronomers later built ever larger and better telescopes. In the 18th and 19th centuries, William Herschel built some of the largest telescopes of his day and discovered the planet Uranus among other things. In the 20th century, Mount Wilson Observatory in California became home to some of the world's largest telescopes, where astronomers discovered among other things that the universe is expanding.

Large telescopes on Earth

Today astronomers build enormous telescopes to study ever fainter and more distant objects in the universe.

One of the most advanced observatories is the Very Large Telescope in Chile. It consists of four large telescopes that can work together as one gigantic instrument, achieving a resolution equivalent to a telescope over a hundred metres in diameter.

On Hawaii sits the Keck Observatory, consisting of two telescopes with mirrors ten metres in diameter. These mirrors are composed of many smaller hexagonal segments that together function as a single large mirror.

Other large telescopes include the Subaru Telescope on Hawaii, the Gran Telescopio Canarias on the Canary Islands, and the Gemini Observatory, which has telescopes both on Hawaii and in Chile.

One of the most exciting recent projects is the Vera C. Rubin Observatory in Chile, where astronomers are conducting a ten-year survey of the entire night sky, recording millions of objects that change over time, such as supernovae, variable stars, and asteroids.

Telescopes that see invisible light

Although many telescopes study visible light, the universe also emits radiation across many other wavelengths. Astronomers therefore build telescopes capable of detecting radio waves, infrared radiation, X-rays, and gamma rays.

Radio telescopes capture radio waves from space. One of the most advanced is ALMA in Chile, consisting of 66 antennas working together. ALMA can study cold gas clouds where new stars and planets are being formed.

In China sits FAST, the world's largest single-dish radio telescope with a diameter of 500 metres, used among other things to study pulsars and search for faint radio signals from distant galaxies.

Other large radio telescopes include the Green Bank Telescope in the USA and the upcoming Square Kilometre Array, which will be one of the largest astronomical instruments ever built.

Many of these radio observatories work by combining measurements from multiple telescopes to effectively create a larger telescope than we could physically build. This same principle was used when astronomers wanted to photograph a black hole. The problem is that the two black holes they wanted to observe lie so far away that they appear incredibly small in the sky, comparable to trying to photograph an orange on the surface of the Moon. To do that you would need a telescope the size of Earth, which we obviously cannot build. Instead, data from radio telescopes around the world were combined, and by recording radio radiation over a long enough period of time, together with a great deal of data processing, the same result could be achieved. 

The same principle is used for the Event Horizon Telescope (EHT), a global network of radio observatories spanning four continents that functions as a virtual telescope the size of Earth itself. In 2019 it produced the first ever image of a black hole, the supermassive black hole at the centre of galaxy M87, 55 million light years away! In 2022 it followed this with the first image of Sagittarius A*, the black hole sitting at the heart of our own Milky Way. Both images were the result of years of data processing by hundreds of scientists around the world, and represent one of the greatest collaborative achievements in the history of astronomy. 

Telescopes in space

Although telescopes on Earth are very powerful, Earth's atmosphere still affects observations. The air causes stars to twinkle and blocks certain types of radiation. Astronomers therefore also send telescopes into space.

The Hubble Space Telescope has since 1990 produced some of the most iconic images of the universe. Still operational today after more than three decades, it has helped astronomers measure the age of the universe, study distant galaxies, and track changes in our own Solar System. Hubble sees primarily in visible and ultraviolet light, giving us crisp, detailed views of the universe in the wavelengths our own eyes are familiar with.

The James Webb Space Telescope (JWST), launched on Christmas Day 2021, takes things to an entirely new level. Webb orbits the Sun 1.5 million kilometres from Earth, far beyond Hubble's orbit of 560 kilometres above Earth, and observes the universe in infrared light, allowing it to see through clouds of cosmic dust that would  entirely block Hubble's view. This makes it possible to take a look into stellar nurseries where new stars are being born, study the atmospheres of planets around other stars, and look back to the very dawn of the universe. JWST has already observed a galaxy that existed just 290 million years after the Big Bang, breaking records for the most distant object ever observed. It has also examined the atmospheres of potentially habitable exoplanets and found unexpected early galaxies that are challenging our understanding of how the universe formed. Hubble and Webb are not rivals but rather partners, and astronomers regularly use both together to build a more complete picture of the cosmos than either could provide alone.

Other space telescopes have had more specialised tasks. The Kepler telescope found thousands of planets around other stars, while the Chandra X-ray Observatory studies the universe in X-ray radiation.

Listening to the universe

Astronomers do not only study light. Some observatories investigate entirely different signals from the universe.

A global network of interferometers measures gravitational waves, tiny ripples in spacetime itself that arise when massive objects such as black holes or neutron stars collide. These include LIGO in the USA, Virgo in Italy, and KAGRA in Japan, working together to detect and localise signals from across the cosmos. In 2015 LIGO detected gravitational waves for the first time, opening an entirely new way of studying the universe, sometimes described as giving astronomy a sense of hearing to go alongside its sense of sight! 

The detectors work by measuring tiny changes in the length of laser beams caused by passing gravitational waves, changes smaller than the width of an atomic nucleus. Since the first detection in 2015, dozens of events have been recorded, from colliding black holes to merging neutron stars. You can read much more in our dedicated section on Gravitational Waves.

Researchers have also built enormous neutrino observatories. Neutrinos are ghostly subatomic particles produced in enormous quantities by stars, supernovae, and other powerful cosmic events. They have almost no mass and barely interact with anything, making them difficult to detect but also extraordinarily useful because, unlike light, they can travel through entire galaxies without being absorbed or deflected, carrying information directly from the heart of the most violent events in the universe.

One of the best known neutrino observatories is IceCube in Antarctica, where sensors are placed deep within the ice, detecting the rare occasions when a neutrino interacts with the ice. The Sun in particular produces enormous numbers of neutrinos, and in fact around one hundred trillion (100,000,000,000,000) pass through your body every second without you noticing. IceCube therefore only detects a tiny fraction of the neutrinos that pass through it, but even that tiny fraction tells us a great deal about the universe.

An ever sharper view of the universe

Astronomers continue to build larger telescopes and develop new ways of studying the universe. One of the most anticipated is the Extremely Large Telescope (ELT) in Chile, which is currently under construction and it is expected to have preliminary test observations in March 2029, and full scientific operations beginning in December 2030. It is going to be the largest optical telescope ever built, with a primary mirror nearly 40 metres across, and capable of imaging planets around other stars in direct detail for the first time while also exploring deeper into the early universe than ever before!

Looking even further ahead, ESA's LISA mission (scheduled for launch in 2035) will take gravitational wave astronomy into space for the first time. Consisting of three spacecrafts flying in a triangular formation millions of kilometres apart, LISA will be sensitive to gravitational waves that ground-based detectors like LIGO cannot detect, opening a window onto the mergers of supermassive black holes (learn more about black hole classification) across the universe. 

Every time we build a new telescope, we open a new window to the universe. And almost every time, we discover something we did not know existed!