Tonight: Our Moon at its perigee for this lunar cycle

Tonight: Our Moon at its perigee for this lunar cycle <i>(Image: Sophie Allen)</i>
Tonight: Our Moon at its perigee for this lunar cycle (Image: Sophie Allen)

The moon is currently at its waxing crescent phase, meaning only 9% of the moon's near side is visible tonight. But over the next few weeks, it will progress into the full moon phase, while also getting further away due to its eliptical 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 Earth. The moon tonight is about 362,826km away, which is quite far away, but relative to its median distance for this month (around 383,874km), it is quite close. During the next set of the lunar cycle, the moon will be at its perigee on 24th December at 08:27 GMT, and it will be 358,270km away from Earth.

Perigee is important during astronomical viewing because lunar features such as the Maria are slightly easier to see. Observers with binoculas could even see some of the bigger craters, like Tycho and Copernicus. These are the main features of the lunar surface, and their history is incredibly interesting to learn about. Viewers with telescopes may not notice much more detail than usual, as telescopes are extremely powerful, but some may see the dark side of the terminator (the apparent line seperating the light and dark half).

Like most celestial objects, the moon doesn't have a perfectly circular orbit. Instead, it has an eliptical orbit. This means that there is a point where the moon is at its closest to Earth, and this point is called the perigee.

Contrary to popular belief, the moon doesn't orbit around the Earth 12 times per year (once a month), its orbital period is actually 27.3 days. Therefore, the moon orbits around the Earth 13 times per year. This is a sidereal month. The moon is also tidally locked with Earth, causing synchronous rotation. This is when the orbital period of the moon is equal to the rotational period. The rotational period is the amount of time it takes for the moon to rotate 360 degrees on its axis. It is because of synchronous rotation that we can always see the near side of the moon from Earth.

This doesn't mean we can only see 50% of the moon's surface. Lunar libration allows us to see around 59% of the moon's surface per cycle. Lunar libration occurs in latitude and longitude, and is why during animations of the phases, the moon appears to be nodding and shaking 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 different speeds of the moon's path, as the orbital path around Earth is ecliptic, and the flatter areas of it's 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 the furthest away from the Earth. The moon was last at its apogee on the 14th November, and it was 404,921km away. It is next at its apogee on the 12th December, at 00:28 GMT.

Our moon is responsible for our tides, and almost completely sheilds us from meteor impacts. The moon has an incredible history, which astronomers are learning about and constantly coming up with their own theories on how it was formed.