Unit 1: The Earth in the Universe
1. The Universe and the Solar system.
The Universe is everything we can observe: space, matter and energy. Everything that exists is part of the Universe.
1. 1. Historical theories to explain the Solar System.
– The geocentric model. According to this model, the Earth is the centre of the Universe, and the Sun, Moon and planets revolve around the Earth.
– The heliocentric model. The Sun is the centre of the Universe, the Moon revolves around the Earth and the Earth rotates on its axis. The Earth and the other planets revolve around the Sun at different distances.
If you compare these models with the actual model, you will find both models were wrong.
1. 2. The actual model of the Universe.
The Solar System consists of the Sun and all the celestial bodies travelling around it. Celestial bodies are the planets and their satellites, dwarf planets, comets and asteroids. The Sun is just another star among millions that form part of the Universe.
1. 3. The Big Bang theory.
The Big Bang theory says that a giant explosion created the Universe. Matter, at a very high temperature, expanded and got cooler to form huge nebulae of gas, mainly helium and hydrogen. Those nebulae developed into galaxies formed by thousands of millions of stars.
1. 4. Galaxies and stars.
Galaxies are groups of stars and clouds of gas and dust. In the sky at night and looking from north to south you can see a wide band of light. This is the Milky Way, and it is our galaxy.
Stars are celestial bodies which emit their own light. The Sun is the star we know the best. Stars have a beginning and an end: they are born, they develop and they die.
Activities
1. Complete the sentence:
According to the Big Bang theory an ______generated the______. Matter reached a very high ______and expanded and got cooler to form very big ______of gas. Nebulae became ______formed by thousands of millions of stars.
2. Complete the table with the correct form of the verb move:
Geocentric / HeliocentricThe Sun
Earth / Doesn't move
The planets
The stars
2. The celestial bodies in the Solar System.
2. 1. The planets.
Planets are spherical bodies which move around the sun in an anticlockwise direction and have cleared their orbit of other celestial bodies.
The Solar System has eight planets. We classify them into two groups:
– Terrestrial planets. Mercury, Venus, Earth and Mars are terrestrial planets. They have a compact rocky surface like the Earth.
– Gas giant planets. Jupiter, Saturn, Uranus and Neptune are gas giant planets. They are liquid and gaseous. They are much bigger and colder than terrestrial planets. These four planets have similar atmospheres made of hydrogen, helium and methane.
2. 2. Distances in the Solar System.
The approximate distance between each planet and the Sun is expressed in astronomical units (A.U.): 1 A.U.=150.000.000 km, which is the distance between the Sun and the Earth. For instance, Neptune is 30 times farther away from the Sun than our planet.
2. 3. Other celestial bodies.
Other celestial bodies in the Solar System include dwarf planets and other small bodies such as comets and asteroids.
2. 3. 1. Dwarf planets.
Dwarf planets are round celestial bodies orbiting the Sun which have not cleared their neighbouring regions of other bodies.
Today we know of four dwarf planets: Pluto (which until recently was considered a planet), Ceres, Eris and Makemake.
2. 3. 2. Small bodies.
They are made of rocks and ice and some are made of metals. We know this because sometimes they fall to Earth and we can study them. There are two types:
– Asteroids: Most of them are located in a ring around the Sun between the orbits of Mars and Jupiter which is called the Asteroid Belt (Ceres, the dwarf planet, is there). If an asteroid collides with our atmosphere it gets very hot. If it does not vaporize and reaches the surface of the Earth, we call it a meteorite.
– Comets: They move around the Sun, but their orbits are very eccentric and they complete their cycle in periods that go from 5 years to 300 years.
Activities
1. What is the difference between a planet and a dwarf planet?
2. Where are the most asteroids found?
3. Why is Ceres a dwarf planet? Can you remember where is located?
4. Make a list of the different components of the Solar System. Order them according to their distance from the Sun.
3. Planet Earth.
3. 1. Movements of the Earth.
The Earth moves in two ways: rotation and revolution.
– Rotation: This is the movement of the Earth around its own axis and it causes night and day.
– Revolution: The Earth orbits the Sun. The shape of the orbit is an ellipse. It takes the Earth 365 days, one year, to orbit the Sun.
3. 2. The seasons.
The tilt of the Earth's axis means that the rays of the Sun illuminate the Earth in different ways. The Sun heats the northern hemisphere or the southern hemisphere more strongly depending on the time of year. This, together with the revolution of the Earth, produces the seasons.
The days when seasons change are called the equinox and the solstice.
Activities
1. Which of these sentences are true?
– The days when the season changes are called ellipse and ecliptic.
– Spring equinox is on 21st June.
– In the northern hemisphere days are longer than nights during the winter.
– The Moon.
4. Characteristics and movements of the Moon.
The Moon is the Earth's only satellite. Its surface is full of craters, caused by the impact of huge numbers of meteorites.
Although it is smaller than the Earth, the Moon is still very big and it is very close. These characteristics cause important phenomena, such as eclipses and tides.
The Moon has two movements:
– Rotation: It revolves on its own axis; this movement takes 28 days.
– Revolution: It orbits the Earth; this movement also takes 28 days.
As both movements take the same amount of time, we always see the same side of the Moon and we cannot see the other side, the far side.
4. 1. Phases of the Moon.
According to the changing positions of the Sun, the Moon and the Earth, the Moon shows four different phases:
– New Moon: In this phase we cannot see the Moon. Its hidden side is the one illuminated by the Sun.
– First quarter: We can see the right side of the Moon illuminated; it has the shape of a D.
– Full Moon: In this phase we see the whole Moon illuminated by the Sun.
– Last quarter: We see the left side of the Moon illuminated; now it has the shape of a C.
4. 2. Tides.
Tides are periodic movements of the level of the seas due to the mass of the Moon and to the attraction that the Moon and the Sun have on the Earth. The water level rises on the part of the Earth nearest the Moon and also on the opposite side, so that we have high tide. On those parts of the Earth not exposed to the Moon, the water level goes down and there we have low tide.
4. 3. Eclipses.
An eclipse occurs when a celestial body hides another temporarily, either partially or completely.
Activities
1. Why can't we see the Moon during the period of the New Moon?
2. Which has more influence on tides, the Moon or the Sun?
3. What do we call the side of the Moon we cannot see?
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