95. _The Aspects of the Moon._--As the moon revolves around the earth, she comes into different positions with reference to the earth and sun. These different positions of the moon are called the _aspects_ of the moon. The four chief aspects of the moon are shown in Fig. 108. When the moon is at _M_, she appears in the opposite part of the heavens to the sun, and is said to be in _opposition_; when at _M'_ and at _M'''_, she appears ninety degrees away from the sun, and is said to be in _quadrature_; when at _M''_, she appears in the same part of the heavens as the sun, and is said to be in _conjunction_. 96. _The Sidereal and Synodical Periods of the Moon._--The _sidereal period_ of the moon is the time it takes her to pass around from a star to that star again, or the time it takes her to _make a complete revolution around the earth_. This is a period of about twenty-seven days and a third. It is sometimes called the _sidereal month_. The _synodical period_ of the moon is the time that it takes the moon to _pass from one aspect around to the same aspect again_. This is a period of about twenty-nine days and a half, and it is sometimes called the _synodical month_. [Illustration: Fig. 109.] The reason why the synodical period is longer than the sidereal period will appear from Fig. 109. _S_ represents the position of the sun, _E_ that of the earth, and the small circle the orbit of the moon around the earth. The arrow in the small circle represents the direction the moon is revolving around the earth, and the arrow in the arc between _E_ and _E'_ indicates the direction of the earth's motion in its orbit. When the moon is at _M_{1}_, she is in conjunction. As the moon revolves around the earth, the earth moves forward in its orbit. When the moon has come round to _m_{1}_, so that _m_{3}m_{1}_ is parallel with _M_{3}M_{1}_, she will have made a complete or _sidereal_ revolution around the earth; but she will not be in conjunction again till she has come round to _M_, so as again to be between the earth and sun. That is to say, the moon must make more than a complete revolution in a synodical period. [Illustration: Fig. 110.] The greater length of the synodical period is also evident from Fig. 110. _T_ represents the earth, and _L_ the moon. The arrows indicate the direction in which each is moving. When the earth is at _T_, and the moon at _L_, the latter is in conjunction. When the earth has reached _T'_, and the moon _L'_, the latter has made a sidereal revolution; but she will not be in conjunction again till the earth has reached _T''_, and the moon _L''_.