Revolution, of a planet; is either diurnal, or annual; the former, is its turning upon its own axis; the latter, is its passage in its orbit.
Satellites. Moons, secondary planets.
Segment of a circle. A portion, or part of a circle; called also, an arc of a circle.
Semi-diameter. Half the diameter. The semi-diameter of the earth, is the distance from its surface, to its centre.
Siderial. Belonging to the stars. A siderial day, is the time required for a star to reappear on a given meridian. A siderial year, the period in which the sun appears to have travelled round the ecliptic, so as to have arrived opposite to any particular star, from which his course was calculated.
Signs, or Constellations. Collections, or groups, of stars. Those of the zodiac are twelve, corresponding with the twelve months in the year. In the centre of these the ecliptic is situated. The sun appears to pass in succession through these signs; entering the first degree of Aries, which is accounted the first sign, about the 21st of March.
Sky. That vast expanse, or space, in which the heavenly bodies are situated. Its blue appearance is supposed to arise from the particles of which the atmosphere is composed, possessing the property of reflecting the blue rays, in greatest abundance.
Solar. Appertaining to, or governed by, the sun: as the solar system, the solar year, solar eclipses.
Solid. Not fluid. Having its parts connected so as to form a mass. Solid bodies, are not absolutely so, all undoubtedly containing pores, or spaces void of matter.
Solstices. The middle of summer and the middle of winter; those two points in the orbit of the earth, in which its poles point most directly towards the sun.