Astronomy: Science that studies the stars

Astronomy Science that studies celestial bodies, their properties, nature and evolution. In the past it was customary to clearly distinguish a branch of the a., Astrophysics , which deals specifically with physical processes in celestial bodies; but today the rational and theoretical aspects of research are so advanced and inextricably linked to the observational aspects that the distinction has lost much of its meaning.


Astronomy is the “law of the stars”. But what does astronomy study? How has it developed over the centuries? And what is astrology? All the answers in detail

You have to know

  • The planets of the Solar System
  • Universe: definition, birth and galaxies
  • Comets: history, names and models of the stars

What you will learn

  • What is astronomy and what differentiates it from astrology
  • Astronomical theories between geocentrism and heliocentrism
  • What are the planets and minor bodies of the solar system
  • What are the motion of rotation and revolution of the earth


  1. Astronomy: law of the stars
  2. Difference between astronomy and astrology
  3. From geocentrism to heliocentrism
  4. The minor bodies of the solar system
  5. The rotational motion of the Earth
  6. The revolution motion of the Earth
  7. How long do the planets take to circle the Sun? Watch the video
  8. Key concepts


1Astronomy: law of the stars

The Universe, consisting of over 500 planets – Source: Shutterstock

Astronomy: meaning and history The term astronomy derives from the Greek and means law of the stars and indicates a science of ancient origins that deals with the physical and chemical laws that govern the universe and studies its origins and evolution.

For the ancients the sky represented a large vault (the celestial sphere ) in which they thought the stars were inserted, all placed at the same distance from the Earth , thought in turn at the center of the Universe .

What are the constellationsThe constellations are fictitious groupings of stars to which the ancients gave fictional names deriving from mythological legends.
The stars belonging to the same constellation do not constitute a “linked” system and appear close to us only for perspective reasons.

How do you indicate the brightness of the starsBy convention, each star of the same constellation is assigned a letter of the Greek alphabet starting from α for the brightest and so on until the Greek alphabet is exhausted and then passing to the progressive Arabic numbering.

Universe: definition, birth, galaxies

Stars in motion: why do they “move” in the sky?During the night the stars “move” in the celestial vault (this is an apparent motion due to the Earth’s rotation) and that is why, observing the sky for several consecutive nights and at the same time, the stars will not have the same position .
Even with the changing seasons, the sky takes on a different aspect: some constellations change position and remain visible all year round , while others are visible only at certain times .

What is a galaxyThe stars are not uniformly distributed but grouped along a band that the ancient Greeks called galaksìas , a term that today we translate as galaxy or Milky Way (i.e. the part visible to us of a structure that includes about 100 billion stars and which has a spiral).


Already in the eighteenth century the philosopher Kant had suggested that the “bright spots” visible in the sky were groupings of stars that he defined as “island universes” but the proof of the existence of galaxies came in 1923 when it was possible to observe with a telescope those that appeared like large whitish clouds in the shape of a spiral that seemed external to the Milky Way.

2Difference between astronomy and astrology

A special constellation: the zodiacAmong all the constellations, the best known are the zodiacal ones, which owe their name to the fact that they are generally figures of animals.
In a year, the Sun seems to pass in front of each constellation determining its visibility in the different months: when about two thousand years ago the astronomers identified the zodiac in the sky , the visible constellation was that of Aries (21 March, the day of the equinox of spring ) and that is why it is considered as the opening of the astronomical and astrological year :

  • Aries March / April
  • Taurus April / May
  • Gemini June
  • Cancer July
  • Leo August
  • Virgo September
  • Libra October
  • Scorpio November
  • Sagittarius December
  • Capricorn January
  • Aquarius February
  • Pisces March


The progress of scientific knowledge in the astronomical field has been accompanied, over the centuries, by representations of extraordinary precision and refinement, a bit like in the anatomical field. The illustrations by Albrecht Dürer (Nuremberg, 21 May 1471 – 6 April 1528) give us an idea of ​​the quality of the reproductions of the celestial hemispheres.

Astronomy and astrology: the differencesL ‘ astrology and astronomy , while having a common origin, are two very different disciplines and should not be confused.

  • Astrology is based on popular beliefs that arenot scientifically proven and aimed at predicting the future through the study of the sky. It is based on the assumption that the apparent motions of the Sun and the constellations of the zodiac are able to influence human events in some way.
  • On the contrary, astronomy has been based on the scientific methodsince the time of Galileo (as we will see in the next paragraph).

3From geocentrism to heliocentrism

Astronomy in Ancient GreeceAs we said before, astronomy was born in ancient times with the naked eye observation of celestial objects and cyclical variations in the appearance of the sky. The ancient Greeks thought that the Earth was a solid celestial body fixed in space , while the sky was made up of small bodies in continuous motion.

Nicolaus Copernicus was the first to lead to the affirmation of the heliocentric theory, already advanced by Aristarchus of Samo – Source: Ansa

Plato and the geocentric hypothesisPlato (427-347 BC) imagined the stars as eternal and immutable celestial objects that described circular orbits around the Earth : all this was the foundation of the geocentric hypothesis , which placed the Earth at the center of the Universe and was accepted for two thousand years. .

A first heliocentric hypothesis: Aristarchus and the criticisms of his thoughtThe first heliocentric hypothesis was advanced by Aristarchus (310-230 BC) in the third century BC but was strongly criticized because it contradicted the philosophical doctrines of the time. With Ptolemy (100-170) the geocentric hypothesis found a new impulse and remained in vogue for about 15 centuries, until the Copernican revolution.

The Copernican Revolution: The Rise of the Heliocentric TheoryWith Nicolò Copernico (1473-1543) the reference system moves from the earth to the Sun, placing the latter and not the Earth at the center of the Universe. The Earth, at the center of the lunar sphere, turned out to perform a daily rotation around an axis and this is why the motions of the Sun, planets and stars begin to be defined as apparent motions (derived from the motion of the Earth).

Tycho Brahe and the “tychonic system”An intermediate hypothesis between the heliocentric hypothesis and the geocentric one was the one formulated by Tycho Brahe (1546-1601) according to which all the planets orbited the Sun which, in turn, revolved around the Earth ( tychonic system ).

Tycho Brahe’s system of planetary rotation. It’s 1560 – Source: Getty-Images

Kepler and his theoryIn 1600 the astronomer Giovanni Kepler (1571-1630) was hired as an assistant by Brahe . The Kepler’s theory was based on the assumption of a close correlation between the orbits of the planets known at the time 6 and 5 geometric solids (cube, tetrahedron, octahedron, dodecahedron, icosahedron).

How the Kepler model worksAccording to the model developed by Kepler, these solids were placed one inside the other and formed a structure in which:

  • four planets occupy the spacebetween one solid and another
  • a planet was inside
  • the other was located outside this structure.

The planets, moved by an undefined force, revolved around the Sun, motionless in the center of this system. Kepler accurately determined the orbit with which the Earth rotated around the Sun (it was almost circular with the Sun placed in an eccentric position) and detected a greater speed when it was closer to it.



From geocentrism to heliocentrism


The results obtained by Kepler can be summarized in three laws .

Kepler’s first lawAccording to Kepler’s first law (or law of orbits) the planets describe elliptical orbits and the Sun occupies one of their foci . According to Kepler’s first law, since the orbits described by the planets are not circular, there will be a point of maximum proximity to the Sun (perihelion) and one of maximum distance (aphelion) .



Planets and Kepler’s laws: explanations and demonstrations


Kepler’s second lawThe Kepler’s second law (or law of areas) defines the speed of the planets in the various points of their orbits : the carrier radius joining the center of the Sun with the center of a planet sweeps out equal areas in equal times.

Kepler’s third lawFrom the studies carried out by Kepler to identify mathematical correlations between the motions of the planets, the third law or law of times

 arose .

This law harmonizes the motions of the planets and is expressed by the formula: T2 = K d3 where

  • Tis the period of revolution, which is the time it takes for a planet to make a complete orbit around the Sun.
  • Kis a constant that depends on the celestial body around which the orbit takes place
  • The value d, on theother hand, can be approximated to the average planet-Sun distance

Galileo Galilei and the first telescopeThe accuracy of the Copernican hypothesis was confirmed by the observations made by Galileo Galilei (1564-1642) who was responsible for the construction of the first telescope . This instrument allowed to obtain a magnification equal to three times the original and allowed Galileo to identify, for example, cavities and protrusions on the lunar surface, thousands of stars of the Milky Way not visible to the naked eye and the 4 satellites of Jupiter (Io, Europa, Ganymede and Callisto).


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Galileo Galilei: biography and discoveries


The endorsement of the Copernican hypothesis, the trial of the Holy Inquisition and the abjurationIn the Dialogue on the two greatest systems of the world (1632), Galileo clearly refuted geocentrism on the basis of experimental observations and, before the publication of this work, the court of the Holy Inquisition had “invited” Galileo not to support the Copernican hypothesis , contrary to the Holy Scriptures.
At the publication of the Dialogue, Galileo was tried because he defended the clear separation between scientific topics and religion and assimilated the intuition of a scientist almost to a divine revelation. In exchange for abjuring the Copernican theory , he obtained home confinement and the indexing of his work.

An image of Galileo Galilei, the first to confirm the Copernican theory and to introduce the experimental method in scientific research – Source: Ansa

Galileo’s legacy to science: the scientific method and experimental demonstrationFrom the death of Galileo (1642), a phase of renewal began in the scientific field due to the application of the scientific (or Galilean) method , based on the observation and experimental demonstration of the hypotheses built to formulate theories of universal validity.

Isaac Newton and the law of universal gravitationIn those years, the question on which most scholars focused industry concerned the force acting on the planets and determine the orbit around the sun . It must be Isaac Newton (1642-1727) the formulation, in 1687, the law of universal gravitation : the gravitational force with which attract two bodies is directly proportional to the product of their masses (m1 and m2) and inversely proportional to the square of their distance (r): F = G m1m2 / r2. The value of G, the universal gravitational constant , was calculated in 1798 by Henry Cavendish .


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The planets of the solar system: structure, characteristics, size


How the law of universal gravitation worksThe Sun therefore exerts its gravitational force on all the planets of the solar system , and the planets on their satellites .

From the law of universal gravitation onwards, the planets Uranus, Neptune and Pluto have been identified, which due to their great distance were confused with the stars. Recall that the planets are distinguished from the stars because they change position, with respect to other bodies, with regular periodicity .

The planets of the solar systemThe planets of the solar system ordered by increasing distance from the Sun are:

  • Mercury
  • Venus
  • Land
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune
  • Pluto

Taking the Earth as a reference point, Venus and Mercury are defined as internal planets as they are located between the Sun and the Earth (they therefore have a smaller orbit than the Earth’s). Similarly, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are called outer planets and have a larger orbit than that of the Earth.

4The minor bodies of the solar system

The minor bodies of the solar systemThe minor bodies of the solar system can be classified as asteroids, meteoroids and comets .

The asteroidsThe asteroids or planets are fragments of rock material in orbit around the sun (between Mars and Jupiter and the orbit of Jupiter).

The meteoroidsThe meteoroids are divided into micrometeorites (meteors) and meteorites , celestial objects traveling through space. The former, in contact with the terrestrial atmosphere , heat up and ionize the atmospheric gases causing the formation of light trails ( falling stars or meteors). Meteorites are larger meteoroids.

Comets are made up of rock and ice materials and are located on the edge of the solar system. – Source: Getty-Images

CometsComets are made up of rock and ice materials and are located on the edge of the solar system . They revolve around the Sun and when they are far from it they appear as rather small luminous objects; as they approach, their ice core sublimates, producing a luminous “crown” and a “tail” that stretches for millions of kilometers.

5The rotational motion of the Earth

The rotational motion of the EarthThe rotational motion of the Earth is the motion that it completes around its axis in approximately 24 hours. The direction of rotation is counterclockwise (from west to east), the linear speed varies with latitude while the angular one is constant at any point on the Earth.

An observer on the earth’s surface unwittingly participates in this motion and within 24 hours he has the impression of identifying in the celestial sphere a movement of rotation in the opposite direction (apparent motion of the celestial sphere).



Earth rotation: the experiments of Guglielmini and Foucault


The Earth’s rotation and the observation of the trajectory of a body in free fallGiovanni Battista Guglielmini (1763-1817) and Léon Foucault (1819-1868) experimentally demonstrated the earth’s rotation , the first by observing the trajectory of a body in free fall

 . The test was conducted from the Torre degli Asinelli in Bologna (height of about 100m) in 1791 and it was observed that the body did not follow a perfectly vertical trajectory but reached the ground about 17mm further east.

Foucault and the plane of oscillation of the pendulumFoucault experimentally proved in 1851 that rotation occurs counterclockwise . The physicist hung from the dome of the Pantheon in Paris a pendulum made up of a 68m wire from which hung a 30 kg sphere at the base of which he had placed a metal tip. The latter drew a line on the sand placed on the ground capable of indicating the plane of oscillation of the pendulum .

Over time, these traces indicated a gradual movement of the plane clockwise and the complete rotation was obtained in about 32 hours.

6The revolution motion of the Earth



The Earth and its movements: motion of rotation and motion of revolution


The revolution motion of the EarthThe motion of revolution of the Earth is the motion made by the Earth around the Sun, in analogy with the other planets.

An indirect proof of this motion is constituted by the apparent movement of the Sun along the Zodiac in the different months of the year. This shift is apparent as the observer ideally projects the Sun on the celestial vault, making it complete a full circle in 365 days.

An indirect proof of the motion of revolutionAnother indirect proof of the motion of revolution is constituted by the periodicity of the meteor showers that the Earth encounters at certain dates of the year, ie when it passes near them, causing the phenomenon of ” shooting stars “.

Astronomy has taught us that we are not the center of the universe, as has long been thought and as someone wants us to think even today. We are just a tiny planet around a very common star. (Daisy Hack)

Why do we study astronomy In ancient times, astronomy was practiced with the naked eye by the first priest-astronomers for the definition of agricultural calendars linked to the cyclical nature of celestial phenomena . By observing the night sky, man is

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