THE LEGAL TIME IN VARIOUS COUNTRIES. 1 



[With colored map.] 



By Dr. M. Philippot, 

 Astronomer at the Royal Observatory of Belgium. 



TIME IN GENERAL. 



Time is measured by the rotation of the earth about its axis. A 

 day is defined as the time taken for one complete rotation. It is 

 assumed that the axis is fixed in the earth and that the rotation is uni- 

 form. In order to measure the time taken for this rotation, it is nec- 

 essary to have reference marks both in the sky and on the earth. 

 For the latter the meridian is chosen, which "is the plane passing 

 through the earth's axis and vertical to the place where the time is 

 measured. Two points are used in the sky: The first, the vernal 

 equinox, which is the ascending node (intersection) of the ecliptic 

 upon the equator; the second is the sun's center. 2 



The vernal equinox serves to determine the sidereal day, which is 

 the time between two successive passages of the equinoctial point 

 over the upper meridian of a place. The moment of this passage is 

 taken as the beginning of the sidereal day. The hour angle of the 

 vernal equinox gives the local sidereal time. For the affairs of civil 

 life sidereal time is inconvenient and not used. It is used only for 

 astronomical purposes. 



The center of the solar disk is used to define the true solar day. 

 On account of the variable movement of the sun along the ecliptic, 

 the length of the true solar day varies from day to day and it is not 

 feasible to make mechanisms or clocks keeping time with these irreg- 

 ularities. A fictitious sun has therefore been imagined, running its 

 course along the ecliptic at a regular rate and reaching the points of 

 its orbit nearest to and farthest from the earth at the same times as 

 the true sun. A second imaginary sun is likewise supposed to pass 

 along the celestial equator at a uniform rate and to be at the vernal 

 equinox at the same moment with the first fictitious sun. This sec- 

 ond imaginary sun is called the mean sun. The day measured by it 



i Translated by permission, with revisions by the author, from Annuaire Astronomique pour 1912 

 Belgium. 



« The ecliptic is the intersection with the celestial sphere of the plane passing through the earth's orbit. 

 The equator is the intersection with the celestial sphere of the plane passing through the earth's equator. 



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