Astronomy (from Greek ástron "star" and nómos "law"): the science of the celestial bodies (not to be confused with ►astrology). Astronomy scientifically studies celestial bodies such as planets, moons, fixed stars, star clusters, galaxies, galaxy clusters, ►quasars, interstellar matter, cosmic radiation, and ►black holes. One subdiscipline of astronomy is cosmology, whose tasks include furthering our understanding of the temporal and/or spatial infinity of the ►universe.

Astronomy began with observations of the night sky and the discovery of regularities in the orbits of sun and planets. From these, certain laws of motion could be derived. Early astronomers were able to make precise predictions of solar and lunar eclipses on the basis of these laws and to achieve honor and fame with their predictions.

The first culture to afford professional astronomers was that of the ► Babylonians. The Babylonians used a numbering system based on the number 60 to record the positions of the moon, sun, and planets. In the Babylonian worldview, the Earth, surrounded by water, is the center of the universe. The moon, the sun, and the planets Mercury, Venus, Mars, Jupiter, and Saturn orbit around the Earth. A round firmament carrying the fixed stars stretches out beyond the planet Saturn. The whole is then surrounded, again, by water.

The ►Greeks at first simply adopted the Babylonians' worldview. Around 500 B.C., Thales of Miletus described the Earth as a flat disk surrounded by water. This world view was popular throughout the antique and is also found in the ►Bible. Subsequent Greek thinkers, however, modified this model considerably.

World view of the Hebrew Bible

Anaxagoras (around 450 B.C.) regarded the Earth as a vertical cylinder whose round front is inhabited by us. The moon is but a reflection of sunlight; lunar eclipses occur when the Earth's shadow covers the sunlight.

Eudoxus (around 350 B.C.) described the planetary motions around the Earth as small circles traveling on huge circular routes. This combination of circular movements was necessary to explain the strange route of the planets relative to the fixed Earth -- in particular, the fact that while the planets mostly seem to move in one direction, they sometimes seem to move in the opposite direction.

Aristotle (384 - 322 B.C.) revised Eudoxus's model. By adding additional circles he managed to establish a model of the planetary positions that was fairly consistent with empirical observations.

Aristarchus (around 250 B.C.) was the first to venture some rough estimate of the distance between the sun and the moon. He calculated the sun to be nineteen times further away from the Earth than the moon is. Since, however, the two appear to us to be of equal size, the sun must be nineteen times larger as well. Based on the calculations just mentioned, plus an additional estimate of the Earth's size as also small relative to that of the sun, Aristarchus then asked whether it makes sense at all to assume that the sun moves around the Earth. If the sun is in reality so much larger, he queried, why shouldn't it be the other way around?

Ptolemy (around 250 B.C.) revised the model established by Eudoxus and Aristotle and summarized the knowledge of astronomy available in his time in his book Amalgest. He designed a complicated model of planetary orbits whose centers in turn move on larger orbits, the so-called deferents. He continued to adjust the cycles until they were consistent with the observed planetary positions.

Ptolemy's Worldview

Thomas Aquinas (1225 - 1274 A.D.) turned Ptolemy's geocentric worldview into a Christian dogma that would dominate the official position of the Church for the next 700 years.

Nicolaus Copernicus (1473 - 1543 A.D.) created an entirely new cosmology with the sun in the center, surrounded by Earth and the other planets on their circular orbits. This model finally managed to explain the strange backwards planetary movement that always occurred when the Earth, in its orbit, passed one of the outermost planets. However, his simple model was contradicted by empirical observations. For this reason, he had to take recourse to the complicated scheme of cycles upon cycles proposed by Eudoxus and Aristotle.

Tycho Brahe (1546 - 1601 A.D.) was responsible for the most exact tables for the planetary motions that had yet been compiled, based on precise quadrant measurements. He developed a new model of the solar system according to which the other planets orbit around the sun but the sun, in turn, orbits around the Earth. Since empirical observation of two isolated bodies cannot tell which one of them orbits around the other, this model was empirically equivalent to the one offered by Copernicus.

Tycho Brahe's Observatory

Johannes Kepler (1571 - 1630 A.D.) first came up with the idea that the movement of Earth and the other planets around the sun may be elliptical rather than circular, and that in each case the velocity may depend on the turning radius. Finally, a simple model of the solar system had been found that perfectly corresponded to the observations.

Galileo Galilei (1564 - 1642 A.D.) was the first to explore the sky with a telescope. He discovered the moons of the planet Jupiter and noticed that the Milky Way consists of numerous individual stars. But the Church eventually forced him by threat of torture and death to publicly retract his endorsement of the heliocentric worldview.

Isaac Newton (1642 - 1727 A.D.) succeeded in providing a complete explanation of the planetary motions by means of a simple law of gravity. With this, the geocentric worldview was finally overthrown. Though it continued to be upheld by the Church into the 19th century, it was no longer taken seriously by scientists.

Wooden 40-inch reflecting telescope by Wilhelm Herschel (1785)

At the end of the 18th century,►Wilhelm Herschel built numerous highly efficient ►reflecting telescopes. He discovered the planet Uranus and, with assistance from his sister Caroline Herschel, compiled a complete catalog of all visible stars and galaxies.

In 1838, ► Friedrich Bessel was the first to measure the distance of a star from the Earth using the parallax method (see ►distance). He calculated the distance to Star 61 Cygni to be 100 trillion kilometers -- the greatest distance that had ever been measured by human beings up to that point.

The Beginning of All Things

Until the beginning of the 20th century, astronomers assumed that the universe was ►eternal, ►unbounded (if not necessarily infinite), and unchangeable. In time, however, this picture of a static universe with neither beginning nor end was fundamentally overturned, just as Ptolemy's model had been completely overturned by Copernicus and Kepler 400 years earlier.

In 1929, ►Edwin Hubble measured the spectrum lines of distant galaxies and discovered a ►redshift in them that increased with decreasing illumination, that is, with increasing distance of the respective galaxy. Hubble assumed that this redshift was caused by a Doppler effect and that all galaxies were moving away from one another at a speed that increases with the distance between them. It follows that there must have been a time in the past at which all galaxies and stars were in the same place. Hubble assumed some kind of initial explosion as the cause of the galaxies' gradual drifting away from each other, which gave rise to the popular term "big bang" as a name for this initial point in time.

In the 1950s, the hydrogen bomb was developed. Since in principle a star is nothing but a permanent hydrogen bomb explosion under gravitational pressure, astronomers finally began to understand the nuclear mechanism inside a star. During its lifetime a star characteristically changes its degree of brightness and its temperature in proportion to its mass. Astronomers now had relatively reliable methods at hand to determine the ►distance between stars by measuring their temperatures and degrees of brightness.

It was thereby discovered that the most distant galaxies with known redshifts are several billion light-years (see below) away from us. Calculations based on the speed of these galaxies determined that they must have begun moving 10 billion years ago. This was the first rough estimate of the age of our universe.

Subsequent more precise measurements of distance showed that the redshift is not caused by the galaxies drifting away from one another, as Hubble had assumed, but rather by an expansion of space itself. This slightly altered the formula for calculating distance on the basis of redshift. (Some textbooks still erroneously cite the velocity-dependent Doppler effect as the cause of redshift).

A better understanding of quantum mechanics eventually gave rise to a new mathematical model of the ►big bang. According to this model, 400,000 years after the big bang, the initial plasma became transparent, enabling radiation that had previously been "captured" inside the plasma to escape. If this model is correct, this radiation must still be visible today. And indeed, in 1964,
two American physicists discovered the postulated cosmological radiation. This was the final proof of the big bang.

In the 1980s and 1990s, the ►Hubble Space Telescope and other state-of-the-art telescopes made it possible to conduct high-precision measurements of the distances between galaxies, particularly by means of analysis of the radiation of supernovas. It was thereby discovered that some galaxies are apparently older than 10 billion years -- which had until that point been believed to be the age of the universe. This puzzle was subsequently resolved by the further discovery that the universe expands more and more rapidly; until then, it had been assumed that the expansion was slowed down by gravity. The new data disclosed that the universe is in fact 13.7 billion years old.

We cannot in principle observe infinite distances in space. Astronomical observations are restricted to the Hubble volume -- a spherical area with a diameter of 46 billion light-years. However, our observations and the known physical laws do support justified hypotheses about the condition of the universe even beyond this observational limit (see parallel worlds).

One of the most important fields of astronomy -- perhaps the most important -- is the determination of distances to observed celestial objects. Astronomical distances are often not indicated in meters but rather in light-years or parsecs. One light-year* is the distance that a light ray in a vacuum travels in 365.25 days (9,460,528,000,000 kilometers). One parsec is the distance to a star that, as seen from opposite positions of the Earth's orbit, appears to be shifted against the background by exactly one arcsecond, which is 1/3600 angular degree (1 parsec = 3.26 light-years). Here are some known astronomical distances:

9 light-minutes

to the sun.

6 light-hours

to the end of the solar system (Pluto's orbit).

4.3 light-years

to the next visible** fixed star, Alpha Centauri.

365 light-years

to the nearest extraterrestrial civilization (no guarantee).

100,000 light-years

to the end of the Milky Way.

2.7 million light-years

to the next galaxy, the Andromeda galaxy.

250 million light-years 

to the Great Attractor.

32 billion light-years

to the currently most distant galaxy, Abell 2218/391.

46 billion light-years

to the end of the observable world.


Further: universe

* Journalists (and others) often confuse light-years, which are a unit of space, with years, which are a unit of time. Thus, when you read in magazines such as the German ►Der Spiegel that the universe came into existence 13.7 billion light-years ago, don't be surprised; but don't believe it, either. It's like saying that the US declared its independence from Great Britain over 200 kilometers ago.

** Alpha Centauri is actually a double star, that is, a system of two stars circling around each other. At a distance of 4.24 light-years from Earth, the red dwarf star Proxima Centauri is a little closer, but is too faint to be visible to the naked eye. For an observer in Europe, the Centauri stars are always below the horizon.

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