If we walked straight ahead, without food or sleep, for 24 hours a day, we would be able to walk back to the same place for about a whole year.But there are so many mountains and seas along the way that it could take years to paint on foot.
But if we take a plane, and the plane goes straight ahead, it takes us about 40 hours to go around the earth.Many people can travel around the world in a few months by means of transportation, with stops in cities on the way.If we think of the world as the earth, then man can travel around the world today in not too long a time.
But if we leave the earth, we will find that the world is much, much bigger.It takes eight minutes for a beam of light to travel from the sun to be received by the earth.Light, by comparison, circles the earth seven and a half times a second.
There are eight big planets in the solar system. The farthest one is Neptune, which is about 30 times farther from the sun than the sun is from the earth.There are many other small objects orbiting the Sun beyond Neptune, which can be as far away as 1 or 2 light years.Which means that light takes a year or two to reach the boundary of the solar system.
At night we can see a lot of stars in the sky, these stars are also the same as the sun's luminous celestial bodies.Each star has its own "solar system".The sun and these stars form a large group called the Milky Way.From one end of the Milky Way to the other, light takes nearly 100, 000 years to travel.
But the Milky Way is still not the whole of the world we know.Outside the Milky Way, there are numerous clusters of stars like the Milky Way.The most distant galaxy we can observe today is 32 billion light-years away.Light
would have to travel 32 billion years to reach this galaxy.By comparison, Earth is only 4.6 billion years old.
And beyond our observation, space is still stretching, and the world may well be infinite.
