FORENOON AND AFTERNOON. 493 



since the revolution of tlie earth around the sun once a year would 

 in the course of the year bring all sides facing the sun. Conse- 

 quently the earth makes one more revolution upon its axis each 

 year than the nimiber of solar days in that year, and a little con- 

 sideration of this fact will show that in each solar day the earth 

 makes one full revolution on its axis and about ^J-^- of another, 

 which fractional addition is occasioned by one day's progress of the 

 earth along its orbit. 



Another fact needs to be considered. Since the earth's orbit 

 is in the form of an ellipse, with the sun at one of the foci, the 

 earth must pass nearer the sun in some parts of its orbit than in 

 others. By the laws of gravity, when nearer, the attraction be- 

 tween the earth and sun is greater, and if this were not balanced 

 by increased velocity along its orbit the earth would fall into the 

 sun; and, on the other hand, when farther off this attraction is 

 less, and if this were not balanced by a diminution of velocity along 

 its orbit the earth would fly off into space. This varying velocity, 

 together with other complications too technical for a magazine 

 article, gives varying lengths of orbit to the several solar days of 

 the year. If the earth's orbit were laid out upon paper and, by 

 astronomical calculations, an exact proportionate section were 

 marked off for each solar day of the year, the variable lengths of 

 orbit for the different days of the year would plainly appear to 

 the eye. 



But, as before explained, the time of a solar day is the time of 

 one revolution of the earth upon its axis, together with the frac- 

 tional part of another revolution occasioned by one day's progress 

 of the earth along its orbit. Then it must follow that as the daily 

 sections of the orbit vary in length, the time of the solar day must 

 vary in length. ISTo clock could be made to keep the variable time 

 of true solar days, and if this were possible, the hour, minute, etc., 

 would be variable of length, and hence no standard for time meas- 

 urements. But by working a simple arithmetical problem of addi- 

 tion and division an average length of day for the year may easily 

 be found. This average day is the mean solar day adopted. Its 

 time is arbitrary and exact, forming a perfect standard for all time 

 measurements. From this the term mean time gains its significance. 



By referring to the foregoing earth's orbit laid out on paper, 

 with the true solar days marked off in sections of mathematical 

 exactness" it will be seen that by dividing each section into two 

 equal parts and marking the division point with red ink, the true 

 noon point of each solar day in the year will be conspicuous upon 

 the drawing, and in its proportionate relations in every way. If 

 now we set a pair of dividers or compasses so that the opening shall 



