phase

In astronomy, apparent shape of the Moon or a planet when all or part of its illuminated hemisphere is facing the Earth.

As the Moon orbits the Earth its appearance from Earth changes as different amounts of its surface are illuminated by the Sun. During one orbit of the Earth (29.5 days – a lunar month) the Moon undergoes a full cycle of phases from new, to first quarter, to full, to last quarter.

The Moon does not reflect sunlight onto the Earth when it is between the Earth and the Sun, and the Moon is not seen. This is the new Moon phase. As the Moon orbits the Earth, part of the Moon reflects sunlight onto the Earth and a crescent is seen, starting from the right-hand side of the Moon. This is the waxing crescent phase. The half Moon phase occurs when half of the Moon's disc is illuminated as seen from Earth. As the Moon continues its orbit, gradually more of the Moon's surface becomes illuminated. At the waxing gibbous phase, three-quarters of the Moon's disc is visible. The full Moon phase occurs when the Earth is between the Moon and the Sun, and the Moon's disc is fully illuminated.

After the full Moon, gradually less of the Moon's surface is illuminated as it continues its orbit. It passes through the waning gibbous phase where three-quarters of its disc is visible, to the third quarter where half is seen, followed by the waning crescent, and finally the new Moon phase again.

The planets whose orbits lie within that of the Earth can also undergo a full cycle of phases, as can an asteroid passing inside the Earth's orbit.

Mars can appear gibbous at quadrature (when it is at right angles to the Sun). The gibbous appearance of Jupiter is barely noticeable.

phase

In physics, a stage in an oscillatory motion, such as a wave motion: two waves are in phase when their peaks and their troughs coincide. Otherwise, there is a phase difference, which has consequences in interference phenomena and alternating current electricity.