In the same year, Johannes Kepler published his Astronomia Nova, in which he introduced his laws of planetary motion to the Solar System.
The year 1609 marked a real revolution in astronomy. Since then, astronomical observations with telescopes have become increasingly the norm, even today, when institutes around the world host giant telescopes that work every available second to collect immense amounts of data.
Each technological advance has brought new, and often totally unexpected, discoveries about our Universe, enriching our cultural heritage.
In 1669, a few decades after the invention of the refracting telescope, a lens-based design, Isaac Newton introduced the first practical reflecting telescope using mirrors.
Over the next 300 years, these two telescope design concepts competed and evolved into increasingly powerful research facilities.
For nearly two centuries, refractory telescopes have been at the forefront, overcoming image quality issues with smart choices of optical designs and glass combinations. Refractor technology reached its peak in the late 19th century with the large Lick and Yerkes refractors, which used lenses measuring three feet and three feet in diameter, respectively. However, these lenses and their mounts proved to be the largest that could be built in practice, and so reflective telescopes finally made the day.
Reflective telescopes in the 19th century suffered from the poor reflectivity and thermal properties of their mirrors. Despite this limitation, William Herschel and William Parsons, the third Earl of Rosse, were able to build reflectors with diameters ranging from 1.25 to 1.80 meters around the turn of the 18th century, with which they discovered more planets and moons in the Solar System, further expanding the limits of the then-known Universe.
The mirror efficiency problem was not resolved until the mid-19th century, when silver coating of glass became feasible. This paved the way for early modern telescopes such as the 2.5-meter Hooker telescope (1917) and the 5-meter Hale telescope (1948). With the new giant telescopes came the next revolution in knowledge as well: the Sun, the most prominent object in the sky, was demoted to a mere dwarf star; it was shown that the Milky Way is just one galaxy among millions, and the Universe, considered static and eternal, was expanding and having a finite age! In the mid-20th century, our view of the world had little in common with that prior to the invention of the telescope.
Since then, progress has continued. Telescopes also expand the observable wavelength domain. Over the past sixty years, astronomers have developed telescopes capable of observing the entire electromagnetic spectrum. Antennas for low frequency observations - radio, millimeter and submillimeter - were built, allowing for many scientific advances, such as the discovery of quasars, pulsars, cosmic background radiation and much more.
Furthermore, space observatories allowed observations to be pushed to shorter wavelengths, in the ultraviolet, X-ray and gamma regimes. This opening of the high-energy frontier has spawned a new flood of discoveries such as X-ray stars, gamma-ray bursts, black hole accretion disks and other exotic phenomena. Previously unknown physical processes were taking place in the Universe around us.
These discoveries led to a series of Nobel Prizes in Physics (in 1974, 1978, 1993, 2002 and 2006) and giant leaps in our understanding of the cosmos. Although astronomy has expanded into these new wavelength ranges, many discoveries are still being made in the visible and near-infrared regimes, where stars predominantly emit their light.
Technological advances in the 1980s and 1990s allowed scientists to build ever-larger telescopes and increasingly sensitive cameras. These instruments opened up new areas of study. For example, the first exoplanets (planets orbiting other stars) have been detected, and the current generation of 8- to 10-meter telescopes has even allowed us to take the first pictures of some of these objects.
Our knowledge in astronomy continues to progress at an incredible pace, answering many questions, but also raising interesting new ones. The European Extremely Large Telescope (E-ELT) will address these new questions, and in the following sections we try to give you an idea of the kind of fundamental questions it will ultimately answer. However, just as Galileo was surprised to find mountains on the Moon and moons orbiting Jupiter, the most exciting discoveries are probably ones we haven't even imagined yet!
Credit ESO
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