英文教材·1．1　 The Universe that Humans know
    The universe humans have observed at present

Human beings have recognized the universe for a long time. The astronomical universe discussed in ancient natural philosophy is no more than the earth and heaven. In l6th century, Copernicus proposed the heliocentric model of the universe. He believed the sun was the center of the universe. That means the universe is actually the solar system. In the 1800s, astronomers introduced the word “galaxy,” and, in some sense, it meant the same as the word universe. Since the 20th century, particularly the 1960s, with the use of large telescopes and the progress of space technology, the scale of astronomical knowledge has extends to more than 10 billion years and reaches back more than 10 billion light yearsjxfz0172_0003_0[1]. Thus, people\'s knowledge of the universe has increased.


① A nebula is a celestial body made of gas and dust. It is mainly made of hydrogen. The Crab Nebula is a shapeless expanded gas cloud in Taurus. Its size is 12 light years × 7 light years. Its luminosity is several thousand times greater than that of the sun.

② Saturn is a planet turning around the sun. It has bright and beautiful rings and several dozen satellites. Its volume is about 740 times that of the earth, and its mass is about 95 times that of the earth.

③ A meteoroid is an interplanetary dust particle or small fragment that exists between planets. The number of meteoroids is very large. A large group of meteoroids that go around the sun in the same orbit is called a meteor stream. When a meteor stream meets the earth, people can see an increasing number of meteors in a certain region of the sky at intervals of several hours, several days, or even longer amounts of time. Sometimes it looks like rain. This phenomenon is called a meteor shower. Most meteor showers are named after the constellation or the star near its radiation center. Astronomers in Spain took this photo of the constellation Leo in 1998 (there are five meteors in the photo).

④ A comet is a celestial body with a tiny mass, turning around the sun in a long and flat orbit. It has a nebulous appearance. The famous Halley\'s comet was the first which to have a successful forecast of its return. The revolution cycle of Halley\'s comet is 76 years.

The universe is a physical world, and it has different forms (Figure 1.1). On a clear night, with the naked eye or with the help of a telescope, we can see the gleaming stars, moving planets, and the moon changing between full and new cycles. Some times, we can also see nebulae with vague outlines, meteors flashing, trailing comets. We can also observe the gases and dust of interstellar space with the help of astronomical telescopes and other space surveying methods. All these are called celestial bodies. Celestial bodies are different in their size, mass, brightness, and temperature, etc.

The universe is in an unceasing movement and development. Stars and planets attract and turn around each other, forming the celestial star system. By now, the celestial star systems human have recognized rank from small to large (Figure 1.2).


The moon revolves around the earth, and they form the earth-moon system. The average distance from the earth to the moon is 384 thousand kilometers.

The planets such as Earth, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, and other celestial bodies, such as small planets, comets, meteors, etc., all revolve around the sun. They form the solar system. The sun is the central celestial body of the solar system. It makes up 99.86% of the total mass of the solar system. Pluto is the farthest planet from the sun. Its orbit diameter is about 12 billion meters.

The sun and thousands of other stars form a huge star group, called the Milky Way Galaxy. There are 200 billion stars like the sun in the Milky Way Galaxy. The diameter of the main body of the Milky Way Galaxy is 80 thousand light-years.

There are many other celestial bodies the same size as the Milky Way Galaxy. They are called the Extragalactic Galaxies, or galaxies for short. At present, with the largest telescope, we can observe billions of galaxies. The farthest of them is about 15 to 20 billion light-years away from earth. The Milky Way Galaxy and the other Galaxies that have been observed are together called the metagalaxy. This is the universe range we can now observe.

Celestial bodies in the universe were not formed at the same time. They all had their own origin, development, and decline. As a whole, the universe has developed from a high temperature to a low temperature, and from a high density to a low density.

The earth in the universe

The earth is a planet of the solar system. Its distance from the sun is 149.6 million kilometers.

The earth is a common planet in the solar system. From Table 1.1, we can see that among the 9 big planets of the solar system, the earth has its own characteristics in mass, volume, mean density, revolution and rotation, which are not uncommon among all the planets.


But the earth is special because it is a planet suitable for living things to live and increase. We believe there are other celestial bodies where living things can live too, but at the moment, they have not been discovered. Why did living things emerge on the earth? This answer is closely related to where the earth is, and the conditions on earth.

It has been a long time from the beginning of the solar system to the appearance of primitive living things. In this period of time, the sun changed little. The earth received steady sunshine. The living things evolved without interruption from low to high stages of development.

In interplanetary space, near the earth, big or small planets turn around the sun in the same direction, and in nearly the same orbital plane (Figure 1.3). Big or small planets all have their own paths and do not intersect with one another. This allows the earth stay in a secure universe environment.


The distance from the earth to the sun is moderate. This makes the average temperature of the earth\'s surface about remain 15℃. It is useful for the origin and development of living things. If the distance had been too small, the temperature would have been too high, so the heat would have stopped the atoms joining together. It would have been impossible for molecules to form, to say nothing of complicated living things. If the distance had been too large, the temperature would have been too low for the molecules to combine together in a solid or crystal shape. The living things would not have survived.

The volume and mass of the earth is moderate. The earth\'s gravity gathers a large amount of gas around the earth. The gas forms the atmosphere, which wraps up the earth. But the early atmosphere lacked oxygen. It was mainly composed of carbon dioxide, carbon monoxide, methane, and ammonia. After a long period of evolution, the atmosphere gradually became composed of nitrogen and oxygen, and became suitable for living things to breath.

Both the decay of radioactive elements inside the earth that generate heat, and the gravitational contraction of the original earth, caused the inner earth temperature to increase. The crystalline water turned to gas. The inner earth\'s activities, such as volcanic eruptions, accelerated the loss of water vapor from the earth\'s interior. As the temperature of the earth\'s surface gradually fell, water vapor gathered by condensation and rainfall. Eventually, it formed the proto-ocean. Primitive one-celled organisms on the earth came from the ocean.

So, the earth is in a steady and secure environment, and the earth itself has the essential conditions necessary for the survival of organisms. These are temperature, atmosphere, water and so on. So, there is no wonder that living things came forth and evolved.

　

Exploring extraterrestrial civilization

　

Modern astronomical observation and experiment gradually supports the view that if the conditions are suitable, life can be found on any celestial body in the universe, and it will gradually evolve to advanced forms of life. In order to find out whether there is any extraterrestrial life and to try to contact intelligent creatures outside the earth, human beings have taken a series of steps. For example, people have transmitted lots of radio waves by telegraph, radio, television, and radar for more than half a century. They have been sent out through dozens of light-years. In the meantime, people constantly increased the search of waves possibly transmitted by intelligent creatures from outer space. Additionally, people have sent space probes with a lot of data into outer space. They include images of the human body, maps of the solar system, some essential constants in the binary system, more than 100 slides showing the earth\'s civilization and landscapes, and various languages, music, and sounds, etc. recorded on a gold-plated copper board. People are waiting for a response from extraterrestrial intelligent creatures.

Activities

Give a short talk

Topics:

1．If we have an accurate understanding of the earth\'s place in space, we will reject blindness and superstition, which hurts our society.

2．Take good care of the earth, for we have only one earth.