Movement of Moon Around The Earth
This is my humble endeavor to put some light on the movement of moon around the earth and the various faces of the moon as we see from the earth.This article will give you an idea as to how do we see moon in different shapes and size each day.
MOVEMENT OF THE MOON
The moon revolves around the earth in a fixed path in direction which is variably constant. The term might seem to be a little tricky but the asymmetry of the rotation and revolution turns to be symmetric over a long duration of time.
Similar to that of the sun the moon may also be considered to move eastwards around the earth in an orbit known as lunar orbit. The plane of this orbit is inclined at about 5 degree 9 minutes to the plane of ecliptic. This inclination varies from between 5degree17.5 minutes and 5 degree over a period of time.
The points where the moon's path crosses the ecliptic( Ecliptic can be defined the sun's relative path as observed from the Earth's reference frame, considering earth as stationary. It is tilted at an angle of 23.5 degree from the earth's rotational axis.)are called Nodes . The point where the moon crosses ecliptic from south to north is called Ascending Node . and the point where the moon crosses the ecliptic from north to south at one point is called the Descending Node.
Regression of Nodes
The moon's orbit from the ascending node to the descending node takes 27.2122 mean solar days (The draconitic period). Measured relative to a fixed star the moon takes 27.3216 mean solar days to complete its orbit (The sidereal period).The movement of the nodes westwards along the ecliptic is called the regression of nodes. It is analogous to the precision of the equinoxes along the equator but is much faster and has a period of 18.61 years. This is equivalent to 27.3216-27.2122= 0.1094 days per orbit.
Perigee and Apogee
During the orbital period of the moon, which is about 271/2 mean days, the moon will ,like the sun, reach a position of maximum inclinatin and declination towards north and south respectively, it will also pass through a position of maximum distance from the earth called the perigee and position of minimum distance from the earth known as apogee.
The Lunar Day
The moon travels across the celestial sphere much faster than the sun, making about 13 complete orbits in one year. Since its motion is eastwards relative to the stars it will cross the observer's meridian rates each day. The length of lunar day is about 24hrs and 50min.
The Phases of the Moon
The terms new moon, full moon, first and last quarters are known as the phases of the moon.
At one point during its orbit the moon's right ascension must be the same as that of the sun. When the sun and moon's right ascension are the same the moon is said to be new. Its illuminated edge, which faces the sun, will appear as a very narrow crescent. The period between two successive new moon is called a lunation.
After new moon its illuminated area increases each succeeding night, until on about the seventh day the moon will appear as an illuminated half disc. Then the moon is said to be in its first quarter.
At one point during moon's orbit the moon's ascension must be 180 degree different from that of the sun. When the moon's right ascension is 180 degree different with respect to that of sun it is said to be full moon. The moon will be fully illuminated by the sun. Approximately 15 days later, after the full Moon, the Earth will be between the Sun and Moon.
After full moon its illuminates area decreases each succeeding night until on about the twenty second day only half the disc is illuminated then the moon is said to be in its last quarter.