THE MOON, PLANETS, AND COMETS 251 



dates of ancient history. In certain cases writers have men- 

 tioned facts occurring many centuries ago, before our present 

 system of counting time was in use, and have stated that 

 certain events occurred at the time of a total eclipse of the 

 sun in a definite place on the earth's surface. The dates 

 of these eclipses have been determined by astronomers by 

 counting back across the centuries for more than two thou- 

 sand years ; and thus in our system of counting time the 

 dates of events which were given in some other system have 

 been determined for historians. 



256. Planets which revolve about the sun. Xext to the 

 moon the planets are most readily observed. The eight planets, 

 of which the earth is one, revolve about the sun. They are 

 observed by means of light reflected from the sun instead 

 of by means of their own light as is true of the sun and 

 other stars. It was supposed by the ancients that the appar- 

 ent motion of the sun was due to the actual revolution of 

 the sun around the earth. To most of them it was incon- 

 ceivable that any other body could be enormously greater 

 than the earth and that the earth should revolve around it. 

 But with the improvement of astronomical instruments and 

 methods it has been found that the earth revolves around 

 the sun in a path which is almost circular. 



The seven other planets which revolve around the sun 

 move in the same direction as the earth and almost in the 

 same plane. It is difficult to represent the orbits of all 

 the planets on the same scale on the page of the book, be- 

 cause those which are farthest from the sun are at such a 

 great distance compared to those which are nearest. Their 

 names, their proportionate size as shown by diameters, their 

 distances from the sun, their periods of revolution around 

 the sun (their years), and their periods of rotation (their 

 days) are shown in fig. 126 and in the table on page 253. 



It is seen from the table that those planets which are 

 near the sun complete their journey around it in a very 



