The time between successive new moons — one full cycle of the Moon's phases (about 29.5 days).
Symbols
Tsyn
synodic period — from new moon to new moon
[d]
Where it comes from
Longer than the sidereal month, because while the Moon orbits, the Earth itself moves along its own orbit around the Sun: 1/T_syn = 1/T_syd − 1/T_year.
When to use it
For working out the dates of new and full moons, the lunar calendar and the Saros cycle (223·T_syn).