Top Image: This image was taken by the Hubble Space Telescope, and is called the Ultra Deep Field. The area covered is a very small patch of sky – about one-tenth the size of the full moon. The objects circled in red are stars in the Milky Way. Every single other object in this image is a galaxy, billions of light-years away. This image contains some 10,000 galaxies. Astronomers estimate the average galaxy consists of 100 billion stars. (Public domain image from NASA and the European Space Agency via Wikimedia Commons, red circles added by Richard Bloch)
A drawing of William Herschel on the night he discovered Uranus. Beside him sits his sister Caroline, herself a noted astronomer and comet hunter. Herschel was the best telescope maker of his time, and he discovered Uranus using a 6” reflector that he had built. (By Paul Fouché [Public domain], via Wikimedia Commons)
Ever look up at the night sky and wonder what’s out there? People have been doing exactly that for thousands of years. For most of that time, all astronomy was done using the naked eye, and steeped in mythology – but all that changed with the invention of the telescope in the 17th century. Through the work of Kepler, Newton, Galileo, and others, our knowledge of the universe was greatly expanded, both in terms of how it works as well as just what’s out there! By 1750, the works of these early giants of astronomy and physics were well known and accepted. But the story does not end there – in fact, much of our current understanding of astronomy has been shaped by research in the last 250 years.
For most of human history, the Solar System was thought to consist of just eight objects: the Sun, the Moon, and the first six planets from Mercury to Saturn. The reason for this was simple - these are the only objects in the Solar System visible to the naked eye. But with the invention of the telescope, the eye was no longer unaided, and in 1781 William Herschel became the first human in history to discover a planet. His discovery, Uranus, is actually bright enough to be visible under dark skies, however it would be too small for the eye to detect. To make matters worse, it would appear to move so slowly with respect to the background stars that if seen on two different nights, the eye would not notice the planet move. For this reason, Uranus eluded humans until just recently. Interestingly enough, Herschel was not the first person to see Uranus, but others before him did not think it was anything out of the ordinary.
An example of planetary motion. When looking at a patch of sky on two distinct nights, objects within the Solar System move with respect to the background stars. Astronomers can measure this motion and calculate an orbit to determine where and what the object is. (Richard Bloch)
Aside from planetary discovery, the increasing use of telescopes allowed for mapping of the sky to a level that was previously impossible. Most objects are too dim to be seen by the naked eye, but with larger telescopes and increased usage, our map of the universe increased dramatically. In the 1700s, there was no notion of galaxies as we know them today, but they were spotted in observatories and telescopes and their locations were marked down. At the time, they were considered to be nebulae - clouds of gas spread throughout our galaxy. It was not uncommon for these discoveries to be put into catalogues of objects, and it was in 1781 that arguably the most famous of these catalogues was published: the Catalogue of Nebulae and Star Clusters, today known as the Messier catalogue after its publisher, Charles Messier.
Messier was a French astronomer who spent his time at the telescope hunting comets, which appear in the eyepiece as small fuzzballs. Frustrated at stumbling upon the same fuzzy objects again and again only to confirm that they were simply small star clusters or nebulae, he compiled a list of these common objects, and between 1774 and 1781 added to it. While Messier was a successful comet hunter, discovering 13 different comets, his catalogue is his most well-known work today. It consists of 110 objects, which today we recognize as star clusters, nebulae, and galaxies - all of which are relatively easy to view through a modest telescope due to their brightness. While most are still too dim for the naked eye, these objects are among the closest and most well-studied and best-imaged.
While astronomers were busy working their way through the night sky, mathematicians and physicists were hard at work trying to understand it. One such mathematician was the astronomy-loving Pierre-Simon de Laplace, who in 1796 proposed an idea for the formation of the Solar System which is known as the 'Nebular Hypothesis.' This was just another way of saying 'the Solar System used to be a ball of gas that collapsed,’ which is essentially what the hypothesis suggests - a ball of gas in space was spinning and began to collapse, flattening out into a disk with the Sun at the centre, and the planets were formed out of the disk. This theory helped to explain planetary motion and composition, and while the whole story is a little more complex, the main concepts Laplace put forward were correct.
All light bends like visible light. Infrared light, which lies just beyond the red part of the visible spectrum, bends as shown here. By moving a thermometer toward this part of the spectrum, the temperature is seen to increase. (Richard Bloch)
Perhaps one of the most important moments for physics during this period came in 1800. William Herschel (the Uranus guy) was using a prism to break sunlight into its component colours in order to determine if there was a difference in heat among the colours. He placed a thermometer in each region of the spectrum, and noticed that as he approached red, the temperature would increase relative to a thermometer placed away from the experiment. Then he moved his thermometer beyond the red light, where there was no more colour from the prism, and saw the temperature rise even more! Herschel had inadvertently discovered infrared radiation, and it was the first experimental evidence that there was light beyond what the eye could see. In further experiments, Herschel verified that infrared light could be transmitted, refracted, reflected, and absorbed, just like visible light. While Herschel is most famous for his discovery of Uranus, his discovery of infrared radiation was just as profound!
A NASA diagram of asteroids in the inner solar system. Many asteroids do not remain in the asteroid belt, and NASA keeps an eye on ones that get too close for comfort. The label PHA refers to Potentially Hazardous Asteroids, and a sample orbit it shown. NEA refers to Near Earth Asteroids, and another sample orbit is shown. Each dot represents a known asteroid. (NASA)
In 1801, our idea of the Solar System began to expand once again. It was the opinion of many astronomers and mathematicians that there ought to be a planet between the orbits of Mars and Jupiter, and many astronomers spent their nights looking for just such a planet. On January 1 1801, Giuseppe Piazzi discovered an object in such an orbit - but this was no planet. Instead, Ceres was the first asteroid ever discovered, and further observations of this orbital region resulted in the discovery of more asteroids. By the middle of the 1800s, only about five had been discovered, but it became clear to astronomers that there would not be a single planet in this region, but rather a number of smaller rocky bodies. Today, it is believed that the asteroid belt is actually home to millions of asteroids of various sizes, and we know that asteroids can be found in many other locations around the Solar System too!
Feeling a little jealous of its neighbour, Neptune decided to show up in 1846. Prior to its observation, astronomers had noticed that Uranus did not orbit the Sun exactly as predicted - there appeared to be irregularities in its orbit. These irregularities were hypothesized to be the result of another major planet which had not yet been discovered. Using observations of Uranus' orbit and performing clever calculations, the French mathematician Urbain Le Verrier predicted where such a planet would be if it did indeed exist.
Neptune, the furthest planet from our Sun, as imaged by the Voyager 2 spacecraft in 1989. 200 years ago, nobody knew this planet existed.
In September of that year, Neptune was successfully observed where Le Verrier had predicted, and after thousands of years of human history we had finally tracked down all our planets! Nobody was happier than Neptune.
It may seem like the years between 1750 and 1900 were filled with boring legwork and record-keeping with only a few exciting moments, but this was an indispensible period of work for astronomy. Like all science, astronomy is about evidence - and astronomical evidence comes in the form of observations and records. Our image and knowledge of the universe doesn't come from taking pretty pictures and seeing exciting things in a telescope - it comes from years of work across hundreds or thousands of observations. Without this 150 year period, there would have been very little basis for much of the discovery and theory that came in the 20th century - discoveries that have changed our ideas of both the universe and reality itself.
No ideas were more dramatic than those put forth by Albert Einstein. In 1905, Einstein published his theory of special relativity, which stated that the speed of light is a universal constant, and in 1915 published his theory of general relativity, which stated that space and time are actually one entity called spacetime.
An artist’s conception showing a probe examining Earth’s gravity. The mass of the earth curves the otherwise flat grid into a bowl shape. Objects travelling along a straight path appear to curve inwards to the Earth along this bowl, which we see as gravitational acceleration.
He stated that spacetime is warped and bent by mass, and this explains the movement of objects in the vicinity of a planet or a star. It also predicted that the path of light should bend in the presence of mass, and sure enough, a few years later during a solar eclipse, this phenomenon was verified experimentally, giving huge credibility to Einstein's theory.
These theories revolutionized astronomy. By setting a speed limit on light, astronomers could calculate the properties of distant objects by how the observed light appeared to them. In later years, when the realization came that the universe had a beginning, this cosmic speed limit gave the universe a horizon - if light can only travel so fast, then any object shining far enough away would not be visible if the universe hadn't been around long enough for the light to travel from there to here. Einstein's work on gravity and relativity is still applied to modern astrophysics, and a large number of experiments and technologies (such as GPS) have consistently verified his theories throughout the last century.
In 1923, another major change occurred in our knowledge of the universe. Believe it or not, before this time, there was no understanding of 'other galaxies' like we have today. Galaxies were taken to be nebulae of gas within our own Milky Way, but by 1923 astronomers had started to wonder if perhaps these objects were something special.
The Andromeda galaxy. This galaxy is visible to the naked eye in dark skies, and was measured by Hubble to be beyond the reaches of our own galaxy, proving that other ‘islands’ of stars exist independently of the Milky Way.
The big bright circle on the rim of the galaxy is actually a separate galaxy, as is the fuzz patch at the bottom.
(Richard Bloch and Slavik Koustov)
Edwin Hubble (after whom the famous space telescope is named) used the most powerful telescope on Earth at the time to look at the Andromeda Galaxy. By then, astronomers knew that this cloud was made up of stars, but Hubble managed to measure its distance. Astronomers had figured out the rough size of the Milky Way by then, and Hubble found Andromeda to lie at least ten times as distant as the edge of our galaxy. This discovery led to further observation which confirmed the existence of other galaxies, dramatically expanding our idea of the universe.
And while we're on the subject of expansion, Hubble was also the first to show that the universe is expanding. By measuring the distances and speeds of different galaxies, Hubble showed that the farther away things are, the faster they appear to be moving away from us - his 1929 paper demonstrating this fact was another bombshell in the world of astronomy. In fact, it was this expansion that really got people talking about a 'beginning' to the universe - if everything was expanding, then if you rewound time there should be a point where everything was in the same place! While there was some debate prior to Hubble's work about a 'steady state' universe (one that's been around forever) versus a universe with a beginning, Hubble's work really gave evidence to the latter.
As a result, the Big Bang theory grew over the coming decades, and explained a lot about our universe - where matter came from, why things look as they do now, and much more. But there was a need for evidence of such an event. Physicists predicted that if the Big Bang really did occur, there should be a radiation field permeating the entire universe – a leftover of such an event. In 1965, researchers Arno Penzias and Robert Wilson accidentally discovered this radiation field while conducting unrelated research at Bell Labs. While they saw it as annoying noise in their experiment, physicists and astronomers were quick to realize that it was the radiation about which everyone had been theorizing, and Penzias and Wilson were later awarded the Nobel prize for their discovery - we know it today as the Cosmic Microwave Background.
The most up-to-date image we have of the Cosmic Microwave Background, taken by the Planck spacecraft in March 2013. This all-sky map shows an image of the temperature of the universe when it was only 370,000 years old. The different colours correspond to tiny temperature fluctuations in the universe.
The history of astronomy goes back thousands of years, but it is a story that is still being written today. We end this story in the present day not because we've discovered everything, but because we can't see the future. Two of the greatest mysteries in astronomy are recent discoveries - dark matter in the 1960s and 1970s, in which it seems as though 27% of the mass of the universe is actually invisible; and dark energy as recently as 1998, in which the expansion of the universe was found to actually be accelerating by some mysterious force. Even exoplanets - planets beyond our Solar System - are a recent discovery. The first was discovered in 1988 (although it wasn't confirmed for another 15 years), and we now count almost a thousand planets, with that number expected to go up with the 2009-2013 data from the Kepler mission.
Astronomy is a dynamic field with a dynamic history. From long nights of hard observation to accidental discovery, to some of the most famous minds in human history, the development of our knowledge in this field is an ongoing process. Twenty years ago, some of our common knowledge today was only pure speculation. Textbooks from a hundred years ago may almost read like a good joke today. Although humans have always lived in this universe, after thousands of years we may just now be beginning to understand its true nature.