Basic physical astronomy — lunar orbital period used in civil calendars In practical (civil) astronomy, the Moon's apparent revolution around the Earth that determines the interval between successive new moons is approximately how many days?

Introduction / Context:
Lunar months used in calendars and tide predictions are based on the Moon’s synodic period, the time between successive phases (e.g., new moon to new moon). This differs slightly from the sidereal period because Earth is moving around the Sun while the Moon orbits Earth.

Given Data / Assumptions:

• The question concerns the commonly used month for phases (synodic month), not the sidereal orbital period.
• Mean values are accepted; small fluctuations due to orbital eccentricity are ignored.

Concept / Approach:

The synodic month is the cycle of lunar phases observed from Earth. Because Earth advances in its orbit during a lunar orbit, the Moon must travel a bit more than one sidereal revolution to catch up with the Sun–Earth line, lengthening the observed period to about 29.53 days.

Step-by-Step Solution:

Verification / Alternative check:

Almanacs and astronomical constants list mean synodic month ≈ 29.5306 days.

Why Other Options Are Wrong:

• 29, 29.35, and 30 days are approximations; they do not match the widely accepted mean value.

Common Pitfalls:

• Confusing the synodic month (≈29.53 d) with the sidereal month (≈27.32 d).

Final Answer:

29.53 days.