If you are traveling forwards in a train，the things around you outside seem to be moving backwards. / The sun seems to rise in the east, / move across the sky，/ and set in the west. / In other words? the sun seems to travel round the earth. / However, this is not really true. / It is the earth that is turning on its own axis. / The earth also travels round the sun on an unchanging path，/ known as an orbit. / There are seven other planets of different sizes，/ which all turn round on their own axes / and which all travel round the sun. / These are known as the solar system. / The planet nearest the sun is Mercury, and the planet farthest from the sun is Neptune. / The orbits of the earth and the other planets are not perfect circles. They are ovals. /