syzygy & quadrature

When two bodies are aligned with the Sun such that the three form a straight line, this is called syzygy. At full moon and new moon the Earth and Moon are at syzygy.

If the line which connects the three bodies forms a right angle the alignment is called quadrature. When the Earth and Moon are at quadrature, the Moon's phase will be first or last quarter, and it will appear as a half disc.

Inconstant Moon