Astronomy Definition – What is Astronomy? Meaning & History.

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With more than 6,000 years of history, astronomy is probably the oldest of the natural sciences, its origins go back to ancient times, in prehistoric religious practices. Astronomy is the science of stargazing, seeking to explain their origin, their evolution, their physical and chemical properties. It should not be confused with the celestial mechanics which is only a particular domain. 

Astronomy comes from the Greek αστρονομ (? Στρον and ν? Μος) which means the law of the stars. 

Astronomy is one of the few sciences where amateurs can still play an active role. It is practiced as a hobby among a wide audience of amateur astronomers: the most passionate and experienced of them participate in the discovery of asteroids and comets. This is a hobby particularly popular in America 


Astronomy is considered the oldest of the sciences. Archaeology reveals that some civilizations disappeared from the Bronze Age, and perhaps Neolithic, already had knowledge of astronomy. They had understood the periodic character of the equinoxes and, no doubt, their relation to the cycle of the seasons, they also knew how to recognize certain constellations. Modern astronomy owes its development to that of mathematics since ancient Greece and the invention of instruments of observation at the end of the Middle Ages. If astronomy has been practiced for many centuries alongside astrology, the age of enlightenment and the rediscovery of Greek thought has given rise to the distinction between reason and faith, so that astrology is no longer practiced by astronomers 


In its beginnings, astronomy was simply the observation and prediction of the movement of celestial objects visible to the naked eye. Nevertheless, we owe to these different civilizations many contributions and discoveries: 

In the ancient antiquity 

  • Prerequisites: 
  • Needless to say: if all observations were made with the naked eye, the elders were helped in this task by the absence of industrial pollution and especially bright. For this reason, most ancient observations would be impossible today. 
  • It should not be mistaken, these observations sometimes relatively simple in appearance (simple drawing of four or five stars), already suppose a high advance in the Civilization, namely the existence of a group gathering at least: 

a writing or at least of its draft, a (proto-writing gathered together a set of signs representing the main objects and events) and a “system” including a cosmogony, a cosmology, a known chart of the sky without forgetting a calendar (sometimes very developed) and an observatory, which is often rudimentary. Without these prerequisites, there can be no recordable astronomical observation. 

  • For millennia, astronomy was not separated from astrology, which was also the primum-movens. Divorce will only occur in the Enlightenment to continue today. 
  • The best known if not the most developed systems are: 
  • in the Neolithic: all the great megalithic circles are in fact astronomical observatories, let us quote the best known: Nabta Playa 6,000 to 6,500 years old and Stonehenge (Wiltshire, England) 1,000 years later. Flammarion, who understood him as one of the first, will speak about the megalithic circles of “monuments with astronomical vocation” and “observatories of stone”. 
  • at the beginning of history: 
  • in the ancient world: 
  • Indian and Chinese astronomy: thus, the Rig-Veda mentions 27 constellations associated with the movement of the Sun as well as the 13 zodiacal divisions of the sky. 
  • Sumerian astronomy, and its derivatives the Chaldean, Mesopotamian, Egyptian, and Hebrew astronomies. So, the Bible contains a number of statements about the position of the Earth in the Universe and the nature of stars and planets. 
  • in the new world, the Amerindian astronomies are also already very developed especially the Toltec, the Zapotec (quite close) and the Maya quite original. Thus, without any optical instrument, Mayan astronomy managed to accurately describe the phases and eclipses of Venus! 

In classical antiquity 

The ancient Greeks made important contributions to astronomy, including the definition of the magnitude system. Thus, the Almagest of Ptolemy (90 – 168) already contains a list of forty-eight constellations. 

To sail on the sea but also in the desert, the Arab Civilization needed very precise data. Derived from Indian astronomy, Arab astronomy will culminate in 500, with the Aryabhata which presents a quasi-Copernican mathematical system, in which the Earth turns on its axis and considers the movement of the planets with respect to the Sun. This nearly 1,000 years before the West! 

Middle Ages 

At that time, astronomy cannot be studied without the contribution of other sciences that are complementary and necessary to it: mathematics (geometry, trigonometry), as well as philosophy. It is used for the calculation of time. 

On science and education in general in the Middle Ages: 

High Middle Age 

The role of Boethius as a founder in the sixth century of the quadrivium, which includes arithmetic, geometry, music, and astronomy, is noteworthy. After the barbarian invasions, astronomy develops relatively slowly in the West. It is however flourishing in the Muslim world from the ninth century: 

the Persian astronomer al-Farghani (805-880) writes a lot on the movement of celestial bodies; He makes a series of observations that allow him to calculate the obliquity of the ecliptic; 

  • Al-Kindi (801-873), philosopher and encyclopedic scientist, writes 16 works of astronomy; 
  • Al-Battani (855-923), astronomer and mathematician; 
  • Al-Hasib Al Misri (850-930), Egyptian mathematician; 
  • Al-Razi (864-930), Iranian scientist; 
  • Al-Farabi (872-950) great philosopher and scientist. 

At the end of the 10th century, a large observatory was built near Tehran by astronomer al-Khujandi. 

Philosophy (Plato and Aristotle) is an integral part of all other sciences (medicine, geography, mechanics, etc.). This great renaissance movement called the golden age of the Arab-Muslim civilization. 

Saint Bede the Venerable in the eighth century developed in the West the liberal arts (trivium and quadrivium). It establishes the rules of computation for the computation of the mobile feasts, and for the computation of the time, which required elements of astronomy. 

Other elements were introduced in the West through Gerbert d’Aurillac (Sylvester II) shortly before the year 1000, with the philosophy of Aristotle. It is difficult to know exactly which Muslim astronomers were known to Gerbert d’Aurillac. Gerbert is important for understanding the historical development of all Western knowledge, which included philosophy. 

Low Middle Ages 

The work of Al-Farghani is translated into Latin in the twelfth century, along with many other Arab treatises and the philosophy of Aristotle. 

In the Muslim world, we can mention: 

  • in Persia, Omar Khayyam (1048-1131), who compiles a series of tables and reforms the calendar; 
  • Ibn al-Haytham (965-1039), mathematician and Arab-Islamic physicist; 
  • Al-Biruni, (973-1048), mathematician, astronomer, encyclopedist, etc.; 
  • Al-Tusi (1201-1274), philosopher, mathematician, astronomer and theologian (considered one of the founders of trigonometry); 
  • Al-Kashi (1380-1429), in current Iran and Uzbekistan. 

We can also mention Al-Maghribi, Al-Sufi. 


During the Renaissance, Copernicus proposed a heliocentric model of the solar system. This idea is defended, extended and corrected by Galileo and Kepler. Galileo imagines the telescope to improve his observations. Based on very accurate observational records by the great astronomer Tycho Brahe, Kepler is the first to imagine a system of laws governing the details of the movement of planets around the Sun, but is not able to formulate a theory going beyond the mere description presented in its laws. 

It is Isaac Newton who, in describing gravitation by his laws of motion, makes it universal and finally makes it possible to give a rational explanation to the movement of planets. He also invented the reflector telescope, which improves observations. 

Contemporary period 

We discover that the stars are very distant objects: the nearest star of the solar system, Proxima Centauri, is more than four light-years away. 

With the introduction of spectroscopy, we show that they are similar to our sun, but in a wide range of temperature, mass and size. The existence of our galaxy, as a separate set of stars, is only proven at the beginning of the 20th century because of the existence of other galaxies. 

Shortly after, we discover the expansion of the universe, a consequence of Hubble’s law, establishing a relation between the speed of distance of other galaxies from the solar system and their distance. 

Cosmology made great progress during the twentieth century, especially with the Big Bang theory, largely supported by astronomy and physics, such as cosmological thermal radiation (or fossil radiation), and the different theories of nucleosynthesis explaining the abundance of chemical elements and their isotopes. 

In the last decades of the twentieth century, the advent of radio telescopes, radio astronomy, and computer processing means, authorize new types of experiments on distant celestial bodies, by spectroscopic analysis of the emission lines emitted by the atoms and their different isotopes during quantum jumps, and transmitted through space by electromagnetic waves. 

Disciplines of astronomy 

At its beginning, during ancient times, astronomy consisted mainly of astrometry, that is to say, the measurement of the position in the sky of stars and planets. Later, Kepler and Newton’s work gives birth to the celestial mechanics that allow the mathematical prediction of the movements of celestial bodies under the action of gravitation, especially the objects of the solar system. Most of the work in these two disciplines (astrometry and celestial mechanics), previously done by hand, is now highly automated with computers and CCD sensors, to the point that they are now rarely considered as discrete disciplines. From now on, the movement and position of objects can be quickly known, so modern astronomy is much more concerned with the observation and understanding of the physical nature of celestial objects. 

 Since the twentieth century, professional astronomy has tended to split into two disciplines: observational astronomy and theoretical astrophysics. Although most astronomers use both in their research, because of the different talents needed, professional astronomers tend to specialize in one or other of these areas. Observational astronomy is mainly concerned with data acquisition, which includes the construction and maintenance of instruments and the processing of results. Theoretical astrophysics is mainly concerned with the research of the observational implications of different models, that is to say, it seeks to understand and predict the phenomena observed. 

 Astrophysics is the branch of astronomy which determines the physical phenomena deduced by the observation of the stars. Currently, astronomers all have extensive training in astrophysics and their observations are almost always studied in an astrophysical context. On the other hand, there are a number of researchers who study exclusively astrophysics. The work of astrophysicists is to analyze data from astronomical observations and to deduce physical phenomena. 

The fields of study of astronomy are also classified into two other categories: 

  •  By subject, usually according to the region of space (for example, galactic astronomy) or the type of problem addressed (formation of stars, cosmology) 
  • By the mode of observation, according to the type of particles detected (light, neutrino) or the wavelength (radio, visible light, infrared). 

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