DISTANCE, SIZE, AND MASS.

15

when she is actually overhead; and she is more de-pressed the nearer she is to the horizon.

It follows that wheresoever the observer may bestationed on the earth, the moon cannot appear tomove as she would if she could be watched from thecentre of the earth. If M E M s M lv (fig. 5, Plate I.)represent her path as supposed to be seen from thecentre of the earth, then the actual path she follows isas shown in the dotted line m 1 m s m s , her observedplace being always vertically below her true place(for we may consider her place as supposed to beviewed from the earth’s centre, her true place, since itis only as so viewed that her motions could showtheir true uniformity). This apparent displacementof the moon is called her parallax.

Hence, for any observer not placed at a stationwhere the moon rises actually to the zenith, it is nother total displacement when on the horizon, called herhorizontal parallax , which is to be compared with hertrue placing as she is seen on the zenith; but theformer displacement is to be measured against thedisplacement which she shows when highest in theheavens. It is seen from fig. 6, Plate I., that when themoon rises high above the horizon this difference willbe appreciable if the moon’s horizontal parallax is ap-preciable. For let M represent the moon’s place whenshe is 50 degrees above the horizon; then, as seen fromP, she would lie in the direction P M; but from Eshe is seen in the direction E M, which is the sameas Pin. (drawing P m parallel to E M). Thus the