Since both solar and sidereal time is estimated from the passage of the sun and the equinoctial point across the meridian of the place of observation, the time will vary in different places by as much as the passage precedes each. It being obvious that when the sun is in the meridian at any one place, it is midnight at a point on the earth’s surface diametrically opposite; so an observation taken at different places at the same moment of absolute time, will be recorded as having happened at different times. Therefore when a comparison of these different observations is to be made, it becomes necessary to reduce them by computation to what the result would have been had they been taken under the same meridian at the same moment of absolute time. Sir John Herschel proposed to employ mean equinoctial time, which is the same for all the world. It is the time elapsed from the moment the mean sun enters the mean vernal equinox, and is reckoned in mean solar days and parts of days. This difference in time is really the angular motion of the earth, and by measuring it the longitude of any place on the surface of the earth can be determined, provided we have a standard point of departure, and an instrument capable of accurately dividing the time into small quantities during its transit from the meridian on which it was rated. As will be hereafter shown, the axis of the earth’s rotation is invariable. Were the position of the major axis of the earth’s orbit as immutable, an observation of any star on the meridian taken at any place would always be the same. Again, the form of the earth has an important effect; the equatorial diameter exceeds the polar, thus giving a large excess of matter at the equator. Now the attraction of an external body not only draws another to it in its whole mass, but, as the force of attraction is inversely as the square of the distance, it follows that the attracted body would be revolved on its own centre of gravity until its major diameter was in a straight line with the attracting body. The sun and moon are both attracting bodies for the earth; the plane of the equator is at an angle to the plane of the ecliptic of 23° 27´ 34´´.69, and the plane of the moon’s orbit is inclined to it 5° 8´ 47´´.9 Now from the oblate form of the earth, the sun and moon, acting obliquely and unequally, urge the plane of the equator from its own position from east to west, thus changing the equinoctial points to the extent of 50´´.41 annually.