Encyclopedia of Questions and Answers [antikvár]

Earth's orbit round the Sun As the Earth orbits the Sun, the hemisphere of the planet that faces the Sun has its summer. The hemisphere facing away is in winter. How long is a day? A day is the time Earth takes to turn once. The stars come back to the same place in the sky every 23 hours 56 minutes 4.09 seconds (the sidereal day), Our day (the solar day) is 24 hours because Earth is moving round the Sun, and must turn an extra I ° for the Sun to return to the same place in the sky. How did the Eaitt i^ecitii' Arounp four and a half billion year5 ago, neither the earto nor any of the other planets existed.There was just this vast dark very hot cloud of gas and dust sv\/irling around the newl)/ formed Sun, Gradually, the cloud cooled and the gas began to condense into billions of droplets. Slowly these droplets were pulled together into clumps by their own gravity - and they carried on clumping until all the planets, including the Earth, were formed. But it took another half a billion years before the Earth had cooled enough to form a solid crust with an atmosphere around it. Why does the Earth spin? The Earth spins because it is falling around the Sun. As the Earth hurtles round the Sun, the Sun's gravity keeps it spinning, just as the Earth's gravity keeps a ball rolling downhill. The early Earth was a fiery ball, then the surface cooled to form a hard crust.