Moon Phase Calculator

A Julian Day Number (JDN) is a continuous count of days since the beginning of the Julian Period on January 1, 4713 BC. It is widely used in astronomy because it avoids the complications of calendar systems and allows straightforward date arithmetic.

The algorithm is accurate to within about one day for most historical and near-future dates. It uses a simplified model based on the mean synodic period of the Moon. For precise predictions (such as exact times of moon phases), a more sophisticated astronomical model accounting for orbital perturbations is needed.

The synodic month is the time between two consecutive identical moon phases, such as new moon to new moon. It averages 29.53058867 days. This differs from the sidereal month (27.32 days), which is the time for the Moon to complete one orbit relative to distant stars.

Moon phases are caused by the changing angle between the Moon, Earth, and Sun. As the Moon orbits Earth, we see different portions of its sunlit face. The Moon does not produce its own light — it reflects sunlight.

Waxing means the illuminated portion is increasing (growing toward full moon). Waning means the illuminated portion is decreasing (shrinking toward new moon). The terms come from Old English words meaning to grow and to decrease.

Moon phases affect ocean tides, which are driven by the gravitational pull of the Moon (and Sun). Full and new moons produce stronger spring tides. There is some evidence of effects on animal behavior and reproduction cycles in certain species. Claims about effects on human mood or behavior are not well supported by scientific evidence.