6 INTRODUCTORY. 



our calculations of the mutual effects of the planets upon 

 each other's motions depend upon an accurate knowledge of 

 their masses and distances. By these calculations, were 

 our data perfect, we could predict for all futurity, or repro- 

 duce for any given epoch of the past, the configurations of 

 the planets and the conditions of their orb itsand many in- 

 teresting problems in geology and natural history seem to 

 require for their solution just such determinations of the 

 form and position of the earth's orbit in by-gone ages. 



Now, the slightest error in the data, though hardly af- 

 fecting the result for epochs near the present, leads to 

 uncertainty, which accumulates with extreme rapidity in the 

 lapse of time ; so that even the present uncertainty of the 

 sun's distance, small as it is, renders precarious all conclu- 

 sions from such computations when the period is extended 

 more than a few hundred thousand years from the pres- 

 ent time. If, for instance, we should find as the result of 

 calculation with the received data, that two millions of 

 years ago the eccentricity of the earth's orbit was at a 

 maximum, and the perihelion so placed that the sun was 

 nearest during the northern winter (a condition of affairs 

 which is thought would produce a glacial epoch in the 

 southern hemisphere), it might easily happen that our re- 

 sults would be exactly contrary to the truth, and that the 

 state of affairs indicated did not occur within half a million 

 years of the specified date ; and all because in our calcu- 

 lation the sun's distance, or solar parallax by which it is 

 measured, was assumed half of one per cent, too great or 

 too small. In fact, this solar parallax enters into almost 

 every kind of astronomical computations, from those which 

 deal with stellar systems and the constitution of the uni- 

 verse., to those which have for their object nothing higher 

 than the prediction of the moon's place as a means of find- 

 ing the longitude at sea. Of course, it hardly need be said 



