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The planet Saturn

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Nebulae M42 and M43 in Orion

   Nebula M42 in Orion



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Astronomy means "law of the stars" (from ancient Greek: astros + nomos) and it is the most ancient science. It involves the observation and explanation of events occurring outside Earth and its atmosphere.

Astronomy is not to be confused with astrology, which assumes that people's destiny and human affairs are correlated to the apparent positions of astronomical objects in the sky. The two fields share a common origin: most astronomers up the 17th century were also astrologers, including Ptolemy, Brahe and Kepler (and possibly Galileo, the father of modern science). Since then, astronomers have embraced the scientific method, while astrologers have not.

Astronomy has 3 traditional branches: astrometry and celestial mechanics are the classical fields of astronomy; astrophysics is the modern field. Astrometry and celestial mechanics are so related that they may be considered a single branch.

Short history of astronomy

In its earliest days, going back to ancient Greece and other ancient civilizations, astronomy consisted largely of astrometry, concerned with measuring positions of stars and planets in the sky. Ancient men used to group stars in constellations, that are still used today: there are 88 of them at this time. Astronomy was mostly stagnant in medieval Europe, but flourished meanwhile in the Arab world.

During the Renaissance, Copernicus revived and developed the heliocentric model of the Solar System that was proposed 18 centuries earlier by Aristarchus, a Greek astronomer and mathematician. Copernicus' work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo added the innovation of using telescopes to enhance his observations. Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to Newton's invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope to further enhance observations.
Read also: Celestial mechanics.

Stars were found to be faraway objects. With the advent of spectroscopy it was proved that they were similar to our own sun, but with a wide range of temperatures, masses, and sizes. The existence of our galaxy, the Milky Way, as a separate group of stars was only proven in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Cosmology made huge advances during the 20th century, with the model of the big bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's Law, and cosmological abundances of elements.

In the twentieth century the field of professional astronomy has tended to split into observational astronomy and theoretical astrophysics. Of course astronomers incorporate elements of both into their research, but most of them tend to specialize in one or the other. Observational astronomy is concerned mostly with getting data and this branch is still referred to as astrometry or simply as astronomy. Theoretical astrophysics is concerned mainly with figuring out the observational implications of different models, and involves working with computer or analytic models.

Astronomy is one of the few sciences where amateurs can still play an active role, due to their observations. Several comets, asteroids, variable stars, novae and supernovae have been discovered by amateur astronomers. even though their telescopes and instruments are far less powerful than professional ones. However, most of modern astronomical research involves a substantial amount of physics and can be considered astrophysics, so it's studied by professional astronomers and astrophysicists.

The current high status of astrophysics is reflected in the naming of University institutions involved in astronomical research: the oldest ones are "astronomy" departments or institutes, the newest ones include "astrophysics" in their names, often excluding the word "astronomy".

Read more about astrophysics, astrometry and celestial mechanics.

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