Synchronous Nature of the Moon's Orbit

Synchronous Nature of the Moon’s Orbit

Synchronous Rotation of the Moon

  • The Moon orbits around the Earth in a manner known as synchronous rotation. This means it takes the same amount of time for the Moon to orbit the Earth as it does to rotate once on its axis.

  • This synchronous rotation is why we always see the same side of the Moon from Earth, commonly referred to as the near side.

  • The side of the Moon we never see from Earth is known as the far side or the dark side of the Moon. The term “dark side” is a misnomer - it receives just as much sunlight as the near side, but it was unseen and therefore unknown until the space age.

  • Synchronous rotation is a result of tidal locking, a gravitational interaction between the Earth and the Moon.

  • Over time, the Earth’s gravity has slowed down the rotation of the Moon to the point where the same gravitational force always pulls on the same area of the Moon.

  • Despite this, the Moon isn’t perfectly synchronous.Libration allows us to see up to 59% of the moon’s surface over a period of time instead of just 50%.

Orbit of the Moon

  • The moon’s orbit is not a perfect circle but an ellipse. As a result, the Moon’s distance from Earth varies throughout its orbit.

  • At perigee the moon is closest to the Earth, and at apogee it is the furthest.

  • This variation in distance leads to a small variation in the Moon’s apparent size as seen from Earth. This is why we sometimes see a supermoon - when a full moon coincides with the perigee.

  • It takes the Moon about 27.3 days to complete one orbit around the Earth, a period known as a sidereal month.

  • However, because the Earth is moving around the Sun, it takes about 29.5 days for the Moon to return to the same phase (e.g., full moon to full moon), a period known as a synodic month or lunar month. This is the basis for our months in the modern calendar.

Understanding the nature of the Moon’s orbit can help with understanding many of the Moon’s other behaviours, including its phases, eclipses and the tides on Earth. It also provides a basis for understanding orbits in the wider Universe.