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. [a] 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.