422 EXPLANATION OF THE PRINCIPAL TERMS

Evection, an inequality in the motion of the moon,by which, at her quarters, her mean place differs fromher true one by about 2 s degrees more than at her con-junction and opposition.

Ether, a fine subtile fluid, which is supposed to sill thewhole celestial space between the heavenly bodies andour a»mosphere.

F.

Foci of an ellipsis, two points in the longest, or trans-verse axis, on each fide of the centre; from each ofwhich it any two right lines be drawn to meet eachother in the periphery, their sum will be always equalto the transverse axis.

G.

Galaxy, or the Milky-way, a large irregular zone orband of light which encompasses the heavens.

Gemini, the Twins, a zodiacal constellation consistingof ninety-four stars.

Geocentric place of a planet, is that position which ithas when seen from the earth.

Gecrgium Sidus, a new planet lately discovered byDr. Herschel, being the seventh in order from the sun,and the most distant of any in the system.

Gibbous, a term used in reference to the enlightenedparts of the moon, whilst she is moving from the firstquarter to the full, and from the full to the last quarter,on account of the dark parts appearing falcated, orhorned, and the light ones convex.

Golden Numbers, a series of numbers proceeding fromone to nineteen, which are used in the almanack fordetermining the times of new and full moons.

Grego-