The approximate period between two consecutive new moons (a synodic month) is:

The synodic month, also called the lunar month, is the time between two consecutive new moons (or two consecutive full moons). It is about 29.53 days (29 days, 12 hours and 44 minutes). This is the period of the complete cycle of the phases of the Moon, as seen from the Earth.

The Moon's phases occur because, as the Moon revolves around the Earth, we see different portions of its sunlit half:

New Moon (Amavasya): the Moon is between the Earth and the Sun; its sunlit side faces away from us, so it is not visible.

Waxing crescent, first quarter, waxing gibbous: the visible part grows (the bright fortnight, Shukla Paksha).

Full Moon (Purnima): the Earth is between the Sun and the Moon; we see the whole sunlit face.

Waning gibbous, last quarter, waning crescent: the visible part shrinks (the dark fortnight, Krishna Paksha).

The sidereal month is the time the Moon takes to complete one revolution around the Earth relative to the distant stars. It is about 27.3 days. The synodic month is about two days longer because, during this time, the Earth has also moved along its orbit around the Sun, so the Moon has to travel a little further to return to the same position relative to the Sun and the Earth.

The Moon also rotates on its axis in about 27.3 days, the same as its orbital period. This is called synchronous rotation (tidal locking), and it is why we always see the same side of the Moon from the Earth. The far side was first photographed by the Soviet Luna 3 in 1959.

Lunar calendars, such as the Islamic Hijri calendar and many Hindu calendars, are based on the synodic month. Twelve lunar months make about 354 days, about 11 days shorter than a solar year; Hindu calendars add an extra month (Adhik Maas) about every three years to keep in step with the seasons.

Eclipses occur only at new moon (solar eclipse) or full moon (lunar eclipse), when the Sun, Earth and Moon are aligned.

Why not the other options?

• ✗27.3 days is the sidereal month (and the Moon's rotation period).
• ✗30.5 days and 28 days are incorrect.