NOMENCLATURE. 473 



9. Declination is the distance of the heavenly bodies from the equinoc- 

 tial measured as a meridian. 



The Tropics indicate the limits of the sun's declination. 



i o. Disc is the term applied to the apparently flat surface of a planet, 

 such as the moon, for instance. 



1 1 . The Orbit is the path described by a planet revolving round the 

 sun. The plane of the orbit is an imaginary surface cutting through the 

 centre of the sun and the planet, and extending to the stars. The diagram 

 shows the plane of the earth's orbit. The circle, ABCD (fig. 501), is the 

 ecliptic. The inclination of an orbit is the plane of the orbit with reference 

 to the plane of the earth ; and, supposing the shaded part of the illustration 



Fig. 502. Conjunction of Venus and Saturn. 



to be water, a hoop held inclined towards the earth, with one half in and the 

 other half out of the water, will describe the planetary orbit. 



12. Nodes are the opposite points of a planet where its orbit cuts the 

 ecliptic or the earth's orbit. 



13. Apogee is the point of a planet's orbit farthest from the earth. 

 Perigee is the nearest point. 



14. The terms Culmination, Conjunction, and Opposition require no 

 special explanation. But planets are in conjunction with each other when 

 in the same sign and degree. A planet with the sun between it and the 

 earth is in conjunction with the sun. With the earth between it and the 

 sun it is in opposition. 



1 5. Latitude and longitude upon a celestial globe are known respectively 

 as " Declination " and " Right Ascension." 



1 6. The Radius Vector is a line drawn from a planet to the sun, 

 wherever the planet may be (see fig. 497). 



