436 



PHYSICAL SCIENCES. 



NOTE 43, p. 8. If the planet described a circle, $c. The motion of 

 a planet about the suii, in a circle A B P, fig. 10, whose radius C A is equal 



to the planet's mean distance from 

 him, would be equable, that is, its 

 velocity, or speed, would always be 

 the same. Whereas, if it moved in 

 the ellipse A Q P, its speed would be 

 continually varying, by note 39 ; but 

 its motion is such, that the time 

 elapsing between its departure from 

 P and its return to that point again 

 would be the same whether it moved 

 in the circle or in the ellipse ; for 

 these curves coincide in the points P 

 and A. 



NOTE 44, p. 8. True motion. 

 The motion of a body in its real orbit 



P D A Q, fig. 10. 



NOTE 45, p. 9. 

 fig. 10, at th 

 would accomplish a revolution in its elliptical orbit P D A Q. 



Mean motion. Equable motion in a circle P E A B, 

 e mean distance C P or C m, in the time that the body- 



Fig. 11. 



NOTE 46, p. 9. Tlie equinox. 

 Fig. 11 represents the celestial 

 sphere, and C its centre, where the 

 earth is supposed to be. q op Q .A. 

 is the equinoctial or great circle, 

 traced in the starry heavens by an 

 imaginary extension of the plane 

 of the terrestrial equator, and 

 E cyo e .A. is the ecliptic, or appa- 

 rent path of the sun round the 

 earth. cyD .A., the intersection of 

 these two planes, is the line of the 

 equinoxes; cyo is the vernal equi- 

 nox, and _A- the autumnal. When 

 the sun is in these points, the days 

 and nights are equal. They are 

 distant from one another by a semi- 

 circle, or two right angles. The 

 points E and e are the solstices, where the sun is at his greatest distance 

 from the equinoctial. The equinoctial is everywhere ninety degrees distant 

 from its poles N and S, which are two points diametrically opposite to one 

 another, where the axis of the earth's rotation, if prolonged, would meet the 

 heavens. The northern celestial pole N is within 1 24' of the pole star. 

 As the latitude of any place on the surface of the earth is equal to the 

 height of the pole above the horizon, it is easily determined by observa- 

 tion. The ecliptic E cyo e _A_ is also everywhere ninety degrees distant 

 from its poles P and p. The angle P C N, between the poles P and N of 

 the equinoctial and ecliptic, is equal to the angle e C Q, called the obliquity 

 of the ecliptic. 



