22 A SHORT HISTORY OF SCIENCE 



respondence with climatic conditions. A revision of the calendar 

 will become necessary. 



Still another natural period is introduced by the motion of the 

 moon, which seems like the sun to have a daily motion about 

 the earth, and also to describe a closed path among the stars in a 

 period of about 29 days. Unlike the sun, however, the moon has 

 during this period a remarkable change of apparent shape and 

 luminosity from " new " to " full " and back again. The study of the 

 day, the year, the month, thus naturally determined by the great 

 heavenly bodies has led to the development of the calendar with 

 greater and greater accuracy, the most recent rectification of 

 the length of the year dating only (in England) from 1752. The 

 difficulty of expressing the precise length of the month and the 

 year in days, causing the imperfection of early calendars, has, on 

 the other hand, reacted to the advantage of mathematical as- 

 tronomy by demanding the greatest possible precision both of 

 observation and of the computation based upon it. 



THE PLANETS. Another celestial phenomenon, though less ob- 

 vious than the foregoing, must have found wide recognition in pre- 

 historic times. The stars vary widely in grouping and individual 

 brilliancy, but in general their relative positions are sensibly 

 constant. To this constancy, however, five exceptions are easily 

 discovered in the wandering motion of the planets Mercury, Venus, 

 Mars, Jupiter, and Saturn, which like sun and moon have their 

 several paths among the stars but with seemingly irregular mo- 

 tions. Corresponding to these seven bodies there was set up by 

 prehistoric people an arbitrary division of time into weeks of seven 

 days, "the most ancient monument of astronomical knowledge." 

 The correspondence with the planets is still preserved in the 

 names of the days of the week in several modern languages. 1 The 



FRENCH ITALIAN 



1 Sunday dimanche domenica (Sun) 



Monday lundi lunedi (Moon) 



Tuesday mardi martedi (Mars) 



Wednesday mercredi mercoledi (Mercury) 



Thursday jeudi giovedi (Jupiter) 



Friday vendredi venerdi (Venus) 



Saturday samedi sabato (Saturn) 



