Phases of the Moon and length of a Month

The Moon reflects about 7% of the Sun's light.

On average, the Moon rises 50 minutes later each day, or equivalently changes its location with respect to the Stars about 15 degrees every day (in a west to east direction).

The line separating the illuminated portion from that in darkness is known as the terminator.

Whatever 'appearance' the moon takes on its right hand side, is what is 'growing'.

• A bright crescent to the right is a waxing moon and is seen in the evenings.
• A dark crescent to the right is a waning moon and is seen during the mornings

When the moon is a crescent, it is often possible to see the unlit side shining faintly - this is Earthshine, light reflected from the Earth.

The time-lapse between rising times on successive nights is called the retardation. To modify what I say above, the retardation can vary considerably depending on the time of year. Sometimes it can be as much as an hour, but during late September, it can be as little as 15 minutes, approximately. This difference is caused by a change in the Moon's path relative to the Earth (due to the inclination between the moon's path and the ecliptic) - during the Autumn it makes a noticeably shallower angle with the Earth. This September moon is referred to popularly as the Harvest Moon.

Synodic Month

The mean time required for the Moon's phases to repeat itself - the synodic month or lunation - is 29.5 days (709 hours). This is different from its actual orbital period.

Full Moons

A Full Moon will occur (statistically, obviously) 12.37 times a year.

Months having two full moons occur every 2.72 years on average.

Once every 19 years, a year will have two months with two full moons, because February will have no full moon.

Full moons in Winter are higher up than those in Summer.

Perigee and apogee

Recurrence of Phases

If a paricular phase appears during the night in an 'even' lunation, it will recur during the day in succeeding 'even' lunations, and during the day in 'odd' lunations. The phases will recur about 90 minutes later than the in each lunation in comparison with its 'partner' two lunations previous. After 15 lunations, the phases will start re-occuring at about the same time (although with 'day' and 'night' swopped over if that makes any sense)

Metonic Cycle

19 years differs from 235 months by 1/12 day

so that

19 X 12 years produces a difference of 1 day.