The Earth has only one satellite which is known as moon. As the Earth rotates on its axis and revolves around the sun, similarly the moon rotates on its axis and revolves around the Earth. Like the Earth’s orbit around the sun, the moon’s orbit around the earth is elliptical. The axis of the moon is inclined at an angle of about 58°43´ with respect to the plane of the ecliptic. As the moon revolves around the earth along an elliptical path, the distance of the moon from the earth keeps on changing. It is said to be in perigee when it is nearest to the earth at a distance of about 3.56 lakh km. As against this, it is said to be in apogee when it is farthest from the earth at a distance of about 4.07 lakh km.
Thus the mean distance of the moon from the earth is 3.85 lakh km. The diameter of the moon is 3478 km which is aboutof the earth’s diameter: mass and volume of the moon are just 1/80th is 1/50th part of the earth respectively. The moon’s surface is marked by a large number of craters which are much bigger than the craters found on the earth’s surface. It revolves around the earth on an elliptical path and completes one revolution in 27 days, 7 hours 43 minutes and 11.6 seconds or roughly 271/3 days. This period is called sidereal month. The moon’s period of revolution with reference to sun is 29 days, 12 hours 44 minutes 2.8 seconds or roughly 29.53 days. This period is known as synoptic month.
Since the moon’s period of rotation around its axis and that of revolution around the earth is the same, we can see only one side of the moon from the earth. Thus only 59 percent of the moon’s surface is visible at the earth and the remaining 41 percent is never visible to us. As a matter of fact, a maximum of 50 percent of the moon’s surface can be seen from the earth at a given time. The remaining 9 per cent can be seen at dierent times depending upon the shiof the apparent edges of the illuminated part of the moon due to the relative motions and changes in the angle of view from the earth. It is worth noting that the moon does not have its own light. It appears bright because it reflcts light received from the sun.
Phases of the Moon
Each night, the moon appears in dierent shapes. When the moon is exactly between the earth and the sun, it is called conjunction and the moon at this time is called new moon. In this position, the part of the moon facing the earth is in complete darkness. About 3½ days aer the conjunction, the moon is visible as thin crescent and the moon in this position is known as the crescent moon. Aer about 7 days of the conjunction, the phase of the moon is called first quarter. Half of the moon is visible in this position. After about 10.5 days of the conjunction, about three quarters of the moon is visible and this phase is called the waxing gibbous moon. Full moon is visible aer about 14.5 days of the conjunction. After this the illuminated part of the moon starts decreasing and the moon passes through waning gibbous, last quarter and waning ascent.