biología bilingue


The science that studies the Universe is astronomy.

Universe (or Cosmos): is everything that exists (matter, energy, space and time).The origin of the Universe

The most accepted theory about the origin of the universe is the Big Bang Theory:

All matter was concentrated in one point; the temperature was extremely high.

13700 million years ago there was a huge explosion, and there was an expansion in all directions.

The first atoms (hydrogen atoms) were formed, and then the first molecules.

Matter and energy started to group together.

→ The theory of Inflation explains what happened one second after the explosion: the expansion caused cooling and created the particles (electrons, neutrons and protons) The position of the Earth in the Universe: Models

Geocentric model: the Earth is in the centre of the Universe. The Sun, the Moon and the planets revolve around the Earth. The stars are fixed in a dome, also revolving.

Heliocentric model: the Sun is the centre of the Universe. All the planets revolve around the Sun. The stars are fixed in a dome that does not move.

 Distances on the Universe: different units

Light year: is equivalent to the distance travelled by light in 1 year (approximately 9.5 million million km). We use light year to measure long distances, between stars, galaxies,…

Astronomical units (AU): is equivalent to the distance between the Earth and the Sun (150 million km). We use AU to measure distances in the solar system.

Galaxies: a huge group of stars, nebulae and interstellar dust, Galaxies can have different shapes (elliptical, spiral, irregular…) and group together in clusters…Nebulae: cloud of gas, formed by concentrations of hydrogen, helium and interstellar dust. Some nebulae are where stars are formed.


Our galaxy is the Milky Way; it is a spiral galaxy, located in a cluster called the Local Group. It is composed of millions of stars, and has 4 arms. The Sun is located in the Orion arm.

STARS: stars are large gas spheres, made mostly of hydrogen and helium. Inside, a reaction called nuclear fusion produces a lot of energy in the form of light and heat.

Stars are distinguished by their:

– colour: depends on its surface temperature. Blue- white- yellow- orange-red

– size

– brightness: varies according to how far away it is, the quantity of energy and its size.

⇒ Other bodies:

Planets: spherical bodies that move around the sun, in orbits that are not occupied by other bodies.

Dwarf planets: rocky bodies that move around the Sun in orbits that are occupied by other bodies.

Asteroid belt: irregular bodies that orbit the Sun between the orbit of Mars and Jupiter (there is a second asteroid belt beyond Neptune).

Comets: bodies composed of ice, rock and dust, which orbit the Sun in very elliptical and long orbits.


The sun, and the celestial bodies that orbit it (planets, dwarf planets, asteroids, satellites, comets). The orbits of the planets are elliptical, approximately in the same plane (ecliptic plane), and in an anticlockwise direction.

Revolution: movement of the planets around the Sun. The period of time that a planet takes to complete one revolution is a year.

Rotation: movement of a body around its own axis. The period of time that a planet takes to complete one rotation is a day.

The rotation of most planets is in anticlockwise direction, except for Venus (opposite direction), and Uranus (horizontal axis).


⇒ Inner(rocky) planets: they are small, they have very little or no atmosphere, and they have a rocky surface. 1, 2 or 0 satellites. Mercury, Venus, Earth, Mars

⇒ Outer (gaseous) planets: they are very large and have a gaseous atmosphere. They are surrounded by rings, they have many satellites and their surface is in a gaseous state. Jupiter, Saturn, Uranus, Neptune.


→ Revolution: it takes 365 and approximately 6 hours to complete one revolution.

  • We have to add  1 day every 4 years in our calendar (29 of february). This is called leap year.
  • The Earth’s axis is tilted, 23,5º. As a consequence, the amount and intensity of sunlight on the Earth varies during the year⇒ these variations cause the seasons:
    • During the summer (in the NH), the Sun’s rays hit the Northern Hemisphere directly (more heated), and the Southern  Hemisphere at an oblique angle (less heated).
    • In spring and autumn, the e Sun’s rays hit the Equator perpendicularly, and both hemispheres are equally heated.
  • The length of the days varies during the year:
    • Summer solstice: longest day of the year.
    • Winter solstice: shortest day of the year.
    • Spring and autumn equinoxes: day and night are the same length.

 Rotation: it takes 23 hours and 56 minutes to complete one rotation.

  • Rotation causes day and night. Living things can detect variations in the number of hours of daylight and temperatures, and they adapt to these variations; examples: trees lose their leaves, flowering, mating and breeding seasons or migrations


 The moon was probably formed when a Planet, Theia, hit the Earth 4500 million years ago. The Moon revolves around the Earth and rotates around its own axis, and these two movements have the same period, 28 days. That is why we always see the same side of the Moon from the Earth.

 The phases of the Moon are the different parts of the Moon we can see, as it moves around the Earth. New Moon, First Quarter, Full Moon, Last (or Third) Quarter.Eclipse: occurs when one body comes between two others, so that one is hidden from the other. Total or partial77

Tides: are the periodic rising and falling of sea levels due to the gravitational pull of the Moon (and the Sun).

  • During a day: 2 low tides, 2 high tides.
  • When the Earth, Sun and the Moon are aligned: Spring tides (twice a month, Full Moon and New Moon).