<P> In Western culture , the four principal phases of the Moon are new moon , first quarter , full moon , and third quarter ( also known as last quarter ) . These are the instances when the Moon 's ecliptic longitude and the Sun 's ecliptic longitude differ by 0 ° , 90 ° , 180 ° , and 270 ° , respectively . Each of these phases occur at slightly different times when viewed from different points on Earth . During the intervals between principal phases , the Moon 's apparent shape is either crescent or gibbous . These shapes , and the periods when the Moon shows them , are called the intermediate phases and last one - quarter of a synodic month , or 7.38 days , on average . However , their durations vary slightly because the Moon 's orbit is rather elliptical , so the satellite 's orbital speed is not constant . The descriptor waxing is used for an intermediate phase when the Moon 's apparent shape is thickening , from new to full moon , and waning when the shape is thinning . </P>
when do the different phases of the moon occur
when the Moon 's ecliptic longitude and the Sun 's ecliptic longitude differ by 0 ° , 90 ° , 180 ° , and 270 ° , respectively