The waxing and waning Moon

The waxing and waning Moon

This is one of the most familiar features of the Moon as viewed from the Earth. What we are seeing is the Moon's monthly orbit around the Earth (aka the synodic revolution), which starts with the New Moon and takes 29 days, 12 hours and 44 minutes. When new, the Moon is positioned exactly between the Earth and the Sun – thus, the illuminated area of its surface is not visible from Earth. This phase is represented on the diagram below by a black circle. As the Moon progresses on its orbit, it reflects a crescent of light that expands until it is seen from Earth as a luminous disc – the Full Moon. In this phase, the Moon is on the opposite side of the Earth to the Sun. From that point on, the illuminated area decreases until the Moon renews its orbit once more.

In the night sky of the northern hemisphere, a quick glance will tell you if the Moon is waxing – the illuminated area is shaped like a crescent, which, if you were to add an imaginary line to the left of it, would resemble the letter ‘p’ (p as premier in the meaning of increasing). In the southern hemisphere,the illuminated area is also expanding but is seen the opposite way round, while in equatorial regions the crescent appears to be lying on its back.

In the sky of the northern hemisphere, the illuminated area of the waning Moon reverts to a crescent shape, but now if you were to add an imaginary line to the right of it, it would resemble the letter ‘d’. ( d as decreasing ). This phase lasts from the Full Moon to the next New Moon.

The Moon ‘waxes’ during the phase from New Moon to Full Moon – the illuminated area increases in size every day. 

 

 

The Moon ‘wanes’ during the phase between the Full Moon and the next New Moon. Every night the illuminated area becomes smaller.