Unit 1:  Our place in the Universe                                          1   2   3   4  

2. The origin of the Universe

2.1. First ideas about the Universe

Human being has felt always the necessity of explain the observed phenomena.

The early astronomers scrutinised the sky month after month as part of a religious ritual and observed that some stars seems to appear just before the sun and they set immediately after it.

They repeated their observations year after year and made measurements and annotations of the moments and positions in which these phenomena occur. Scientifics concluded that they repeated cyclically with each season. They realized that there was a big calendar written in the sky which allowed them to predict the favorable seasons to seed, hunt or navigate.

Babylonians sat the basis of the modern Astronomy. They described with precision the movement of the Sun, the Moon and the planets, they invented the sexagesimal system (the 360⁰ of the circumference) and established the Zodiac and the first calendars.

a) Geocentric model

    It is the simplest hypothesis to explain the movement of the celestial bodies

    and it considers our planet as the centre of the Universe.

    The observations which led to that conclusion were:

                     - The “fixed stars” and the Milky Way seems to move during the night

          like if they were joined to an invisible vault that was turning around

          a fixed point in the sky. First astronomers deduced that this celestial

          vault was similar to a huge sphere that surrounded the Earth.

                     - Other ones, however, did not behave like the stars. During part of the year

          they seemed to move forward and during other part backward.

          These celestial bodies were called planets.

    So that, according to the Geocentric model, the Earth, fixed and immobile,

    occupies the centre of the celestial sphere where are located the firmament,

    the Sun, the Moon, the planets and the fixed stars.

    Aristotle (384-322 B.C.) divided the Universe into two parts: the celestial

    and the terrestrial world. The terrestrial world was imperfect and it was

    formed by four elements: air, water, fire and earth. The celestial world,

    by contrast, was perfect and immutable. It was formed by the fifth element

    or quintessence, the ether (that means eternal) and its only movement

    has to be circular, because the circle is the perfect figure: it does not have end

    or beginning and it is equal in all its points.

    The Sun, the Moon, Venus, Mars and Jupiter were located in different

    transparent glass spheres, included ones in others and the Earth was in the centre.

    The stars were fixed and occupied the outermost sphere.

In the II century A.C., Claudius Ptolemy (Egypt, 100-170 A.D.) published

the Almagesto. In it he stated that the Sun, The Moon and the five visible planets from the Earth (Mercury, Venus, Mars, Jupiter and Saturn) moved in their own transparent spheres describing circular movements.

Ptolemy solved the discrepancies between observations and the aristotelian model establishing the hypothesis that the planets describe small circular movements (epicycles) which centres move in a circular orbit around the Earth.

This model persisted until the XVI century and it became in the essence

of the Church’s dogma about the nature of the Universe. According to this, by divine design, the Earth was the immobile center of everything and the sky was the absolute perfection.

b) Heliocentric model

The first heliocentric model was proposed in the 2^nd century B.C

by Aristarcus of Samos (Greece, 310-230 B.C.).

In this model the Earth and the rest of the planets revolved around the Sun

and the Moon revolved around the Earth. In addition, he defended that the Earth rotated around itself each 24 hours and it revolved around the Sun once a year. However, his theories found an unbridgeable opposition among the Greek philosophers and they never were accepted.

Most part of the early Grecian scientists’ works was lost during the fire of the Alexandria’s library and its knowledge was vanished. Only a small part of them

arrived to Occident through the Arabian culture.

During all the Middle Age the geocentric theory was supported, because it was

very well accepted by the ecclesiastical power. It was coherent that the creatures created by God inhabited the planet Earth that was located in the Centre of the Universe. What was no more than a mathematic instrument became in faith dogma. Others hypothesis were considered heresies and their authors condemned to the fire.

A cover of darkness and ignorance was extended by the world, and the ideas of many wisdom men and women were refused and set aside, being substituted by absurd and inconsistent models that however were according to the Sacred Scriptures.

Giordano Bruno (Italy, 1548-1600) was one of the first to accept and spread the heliocentric model of Copernicus. Following the logical thought that there must be an infinite number of worlds, he also proposed that there must be life in other parts of the Universe. These ideas, counter to Church’s ones, cost him his life. He was burned in the stake after pass 8 year imprisoned.

Animation: Astronomy timeline (NASA)

2.1. Relate each concept or discovery with the scientist who proposed it or made it:

a. Epicycles

b. Phases of Venus

c. First calendars

d. The planets move faster when they are near the Sun.

e. The existence of life beyond the Earth

f. The Earth rotates on its axis each 24 h.

g. The Sun is the center of the Universe.

h. The skies are neither immutable nor static.

i. The way celestial bodies move.

j. The Universe of transparent glass spheres.