8. Meridians are circles passing through the poles at right angles to the equinoctial. Every place is supposed to have a meridian, but only twenty-four are upon the globe, and they represent the sun’s, or the planets’, “movements” every hour—15° being one hour, 360° being twenty-four hours (see fig. 500). One quarter of a degree equals one minute of time. Parallels of latitude are familiar circles parallel to the equator. Latitude in astronomy is the distance from the ecliptic at a right angle north or south. This will be explained as we proceed.
Fig. 501.—Orbit of planet.
9. Declination is the distance of the heavenly bodies from the equinoctial measured as a meridian.
The Tropics indicate the limits of the sun’s declination.
10. Disc is the term applied to the apparently flat surface of a planet, such as the moon, for instance.
11. 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, A B C D (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 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.
Fig. 502.—Conjunction of Venus and Saturn.