Density (earth's = 1) - 0.60419, or 3.444 the density of water (water being unity). Surface area, about 14,600,000 square miles (earth's surface area, 196,870,000 miles) Earth's surface area = 1, moon's - About 2/27 or 0.07407. Action of gravity at surface - 0.16489 or 1/6.065 of the earth's. Surface of moon never seen - 0.4100. Surface of moon seen at one time or another - 0.5900. Synodical revolution, or interval from new moon to new moon (commonly called a lunation) - 29 d. 12 h. 44 m. 2.684 s. - 29.5305887 days. Sidereal revolution, or time taken in passing from one star to the same star again - 27 d. 7 h. 43 m. 11.545 s. - 27.3216614 days. Tropical revolution, or time taken in passing from "the first point of Aries" to the same point again - 27 d. 7 h. 43 m. 4.68 s. - 27.321582 days. Anomalistic revolution, or time taken in passing from perigee to perigee - 27 d. 13 h. 18 m. 37.44 s. - 27.55460 days. Nodical revolution, or time taken in passing from rising node to rising node - 27 d. 5h. 5m. 35.81 s. - 27.21222 days. Distance (mean) in terms of the equatorial radius of the earth - 60.27. Distance in miles (mean) - 238,840 miles. Distance, maximum - 252,972 miles. Distance, minimum - 221,614 miles. Mean excentricity of moon's orbit - 0.05490807. Inclination of moon's orbit to the ecliptic (mean) - 5 deg. 8 min. 39.96 sec. Inclination of moon's axis to the ecliptic - 87 deg. 27 min. 51 sec. Inclination of moon's equator to the ecliptic - 1 deg. 32 min. 9 sec. Maximum libration in latitude - 6 deg. 44 min. Maximum libration in longitude - 7 deg. 45 min. Maximum total libration from earth's centre - 10 deg. 16 min. Maximum diurnal libration - 1 deg. 1 min. 28.8 sec. Angle subtended by one degree of selenographical latitude and longitude at the centre of the moon's disc, when at its mean distance - 16.566 sec. Length of a degree under these conditions - 18.871 miles. Selenographical arc at the centre of the moon's surface, subtending an angle of one second of arc - 3 min. 37.31 sec. Miles at the centre of the moon's disc, subtending an angle of one second of arc - 1.139 [It must be remembered that this value is _increased_, in departing from the centre, in the proportion of the secants of the angular distance from the centre.] Period of similar phase - 59 d. 1h. 28m. = 2 lunations. Or, more accurately - 442 d. 23 h. = 15 lunations.