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How to Calculate a Lunar Month

Astronomers can calculate the lunar month using several definitions, one of the reasons being that the Earth isn̵7;t stationary. Usually what people mean by a ̶0;lunar month̶1; is the time between two new moons or two full moons. This, however, is not the only way to calculate it. A few methods are accessible to anyone with little more than a pair of good binoculars.

Things You'll Need

Binoculars or telescope

Instructions

- 1
  Calculate a synodic month by measuring how long it takes the Moon to go from New Moon to New Moon. This is about 29.5 days.
- 2
  Calculate a sidereal month by measuring how long it takes the Moon to pass a particular star. This is about 27.3 days. The reason for the 2.2-day difference is that one depends on the position of the sun and the other depends on the position of the background stars. From the Earth̵7;s perspective, the sun varies in position relative to the background stars.
- 3
  Calculate a tropical month as the time between the Moon̵7;s crossing overhead a particular longitude of the Earth̵7;s longitude-latitude system.

Astronomy