Tonight: Our Moon is at its perigee for this lunar cycle (Image: Sophie Allen)
The moon is currently in its waxing waxing phase, meaning only 9% of the moon’s near side is visible tonight. But in the coming weeks it will move to the full moon phase, also moving away due to its elliptical orbit.
Tonight, at 01:31 GMT, the moon is at its perigee for this month. This means that the moon is at its closest point to the Earth. The moon tonight is about 362,826 km away, which is quite far, but compared to its median distance for this month (about 383,874 km), it is quite close. During the next set of the lunar cycle, the moon will be at its perigee on December 24 at 08:27 GMT and will be 358,270 km from Earth.
Perigee is important during astronomical observation because lunar features such as the Maria are slightly easier to see. Observers with binoculars could even see some of the larger craters, such as Tycho and Copernicus. These are the main features of the lunar surface and their history is incredibly interesting to know. Viewers with telescopes may not notice much more detail than usual, as the telescopes are extremely powerful, but some may see the dark side of the terminator (the apparent line that separates the light and dark halves).
Like most celestial objects, the moon doesn’t have a perfectly circular orbit. Instead, it has an elliptical orbit. This means that there is a point where the moon is closest to the Earth, and this point is called perigee.
Contrary to popular belief, the moon does not orbit the Earth 12 times a year (once a month), its orbital period is actually 27.3 days. Thus, the moon orbits the Earth 13 times a year. This is a sidereal month. The moon is also tidally locked with the Earth, causing it to lock in rotation. This is when the moon’s orbital period equals its rotation period. The rotation period is the time it takes for the moon to rotate 360 degrees on its axis. It is because of tidally locked rotation that we can always see the near side of the moon from Earth.
This does not mean that we can only see 50% of the lunar surface. Lunar libration allows us to see about 59% of the lunar surface per cycle. Lunar libration occurs in latitude and longitude, which is why during phase animations, the moon appears to nod and shake its “head” at the same time. Lunar libration in latitude is caused by the inclination of the moon’s orbital plane, which is 5.1 degrees above the ecliptic. Libration in longitude is caused by the different velocities of the moon’s path, since the orbital path around the Earth is ecliptic, and the flatter areas of its orbital path allow us to see slightly around the moon’s eastern limb.
The opposite of perigee is apogee. This is when the moon is at a point in its orbital period where it is farthest from Earth. The moon was last at its apogee on November 14, and was 404,921 km away. It reaches its apogee on December 12, at 00:28 GMT.
Our moon is responsible for our tides and almost completely protects us from meteor impacts. The moon has an incredible history, one that astronomers are learning about and constantly developing their own theories about how it formed.