v The Earth a Planet 73 



accomplishes one revolution round the Sun is called a year, 

 and is the unit for long intervals of time. The unit for 

 shorter intervals of time is the solar day or apparent period 

 of the Earth's rotation. Unfortunately these two natural 

 units are incommensurable ; the revolution period of the 

 Earth with regard to the stars is not made up of an 

 even number of rotation periods or of solar days, but 

 consists of 365 days, 6 hours, 9 minutes, 9^ seconds. 

 The tropical year or time of apparent revolution is 365 

 days, 5 hours, 48 minutes, 46 seconds ; and it is in order 

 to fit in the extra 5 hours and odd minutes that the plan 

 of having ah extra day every fourth year (leap year), and 

 omitting it o^ a century, is adopted. If this were not 

 done, the same period of the year would not occur in the 

 same part of the Earth's orbit at each successive revolution. 



in. Solar and Sidereal Time. The revolution of the 

 Earth round the Sun once in a year accounts for the interval 

 between two successive transits of the Sun across the 

 meridian, the solar day being nearly 4 minutes greater than 

 the Earth's rotation period or sidereal day. While the 

 Earth is turning once round on its axis it advances so 

 far upon its orbit that nearly 4 minutes of turning more 

 than a complete rotation are necessary to bring the Sun 

 once more on the meridian. Since the Earth moves with 

 unequal velocity in different parts of its course, and its axis 

 is not perpendicular to the plane of its orbit, the day, as 

 measured from noon to noon, varies slightly in its length 

 throughout the year. The average solar day is taken in 

 order to calculate the solar mean time which is always used 

 in ordinary affairs. 



112. The Ecliptic. The Earth's orbit lies always nearly 

 in the same plane, because there is no force competent to 

 change its direction. That is to say, the Earth goes round 

 the Sun in limitless space as a boat sails round a ship on the 

 surface of a calm sea. We may imagine the plane to extend 

 beyond the Earth's orbit through all space so that it inter- 

 sects the dome of stars. The line of intersection is the 

 apparent yearly path of the Sun amongst the stars, and is 

 called the ecliptic ; the constellations it traverses are the 



