The Moon

Young Astronomers Blog, Volume 29, Number 3.

On many nights when you look up into the night sky you can see a bright object known simply as the Moon. Twelve humans from the planet Earth have walked on the Moon, although none since 1972. Last year was the 50^th anniversary of Apollo 13, the flight that almost didn’t make it home. February 2021, marks the 50^th anniversary of Apollo 14, the mission that got NASA back on track. Oh, and yes, it was the first time an astronaut went golfing on the Moon.

The Moon is around ¼ the width of the Earth and is a quarter of a million miles away. It takes about 27 days for the Moon to complete one orbit. The Moon’s orbit is not fully circular, it is elliptical with a slight eccentricity, so the Moon’s distance varies as it orbits the Earth.

The Moon is covered with craters, most from a time called the “Late Heavy Bombardment”, which occurred some four billion years ago. The Moon’s surface is composed of two types of terrain.

• Highlands are older and lighter colored areas.
• Maria/seas are newer, darker, and smoother areas that were covered with lava a few billion years ago.

Many features on the Moon are easily visible from the Earth. These include several Maria (e.g., the Sea of Tranquility, Serenity, and Rain, along with the Ocean of Storms). With binoculars or a telescope, you might spot a few large craters (e.g., Tycho, Copernicus, and Plato). Notice the “rays” coming out of Tyco near the Moon’s southern pole. The Planetary Society has a nice map showing some of these features.

The composition of the Moon is somewhat like that of the Earth. There is smallish iron core at the center surrounded by a liquid core and possibly a molten layer. The bulk of the Moon’s interior is a mantle over which there is a thin crust. The crust is thicker on the far side than it is on the near side. This could explain why there are smooth Maria on the near side (due to lava flow), but not on the far side. On top of the crust is a layer of “lunar regolith”, which has a texture similar to concrete.

The Moon is tidally locked with the Earth. This means that it always keeps one side facing the Earth. Therefore, the Moon has a “near” side and a “far” side. The far side is not the “dark” side as we will see below. It also means that the Moon’s rotation is the same as its orbital period (27 days). However, we can see 59% of the Moon’s surface. This is due to something called libration. The Moon wobbles a bit, so although we can only see 50% during a full moon, the visible portion changes revealing a slightly different part of the Moon’s surface each time. Even with this, a good portion of the Moon’s far side is never visible from the Earth. Images from various spacecraft show that the far side is mostly older lighter highlands with few dark Mare as seen on the near side.

The Sun always illuminates ½ the Moon at any given time, albeit a changing half. Therefore, the Moon reflects sunlight in such a way that we see it go through phases. The Moon completes one set of phases every 29 ½ days. It takes longer than its 27-day orbit because the Moon and Earth are also moving along an orbit about the Sun. Each year, the folks at NASA’s Scientific Visualization Studio create a simulation of the Moon’s orbit and corresponding phases.

• A new moon is when the far side of the moon is completely illuminated by the Sun.
• The moon is waxing as the light appears after a new moon.
• A  1^st (first) quarter (waxing) moon is when half of the near side and half of the far side is illuminated.
• A full moon is when the near side is fully illuminated by the Sun.
• The moon is waning as the light disappears after a full moon.
• A  3^rd (last) quarter (waning) moon is when half the near side and half the far side is illuminated.
• We are back to a new moon.

There two additional phases, each can be waxing or waning.

• A crescent is when the sunlight is just visible before or after a new moon.
• A gibbous is between a half and full moon.

Find out about the current phases of the Moon for the Winston-Salem area in the Old Farmer’s Almanac.

While the phases change as the Moon orbits the Earth, the position of the Moon in the evening sky also changes. If you watch the Moon from night to night, you will notice that it slowly moves from west to east across the sky while the hourly movement is from east to west as the Earth rotates.

Full Moons are given various names. Three are the Wolf Moon in January, Snow Moon in February, and the Harvest Moon in the fall (the full moon closest to the autumnal equinox). From time to time, we experience a Blue Moon, which is the second full moon within the same calendar month (or more traditionally the same season). When a full moon coincides with the Moon’s closest approach to the Earth (remember the Moon’s elliptical orbit), it is called a ‘Supermoon”.

The Moon is just far enough away that the apparent size is the same as the apparent size of the Sun when viewed from the Earth. As such, an eclipse occurs from time to time. The Moon’s orbit is slightly tilted to the plane of the Solar System. Therefore, eclipses are somewhat rare and occur only during a full or new moon and when the Moon’s orbit intersects with the Solar System’s plane. There are two types of eclipses, which you can remember by the object that gets dark.

• Lunar Eclipse is when the Earth blocks the Sun’s light, and the Moon turns dark.
• Solar Eclipse is when the Moon blocks the Sun’s light, and the Sun turns dark.

The Moon is the primary reason we have tides. The Moon’s (and the Sun’s) gravity pulls on the Earth’s oceans causing them to rise and fall as the Earth rotates. The Sun does play a role.

• The stronger Spring tide occurs when the Moon and Sun are lined up together during a full or new moon.
• The weaker Neap tide is when the Sun is perpendicular to the Moon during a 1^st or 3^rd quarter moon.

Gravity falls off by the square of the distance (remember Newton and his law of gravity?). Tidal forces, however, fall off by the cube of the distance, which is why the Sun’s tidal force is less than the Moon’s. It’s also interesting that tides pull on both the near side of the Earth facing the Moon and the far side facing away from the Moon. So, high tide occurs twice a day.

Of course, humans travelled to the Moon during the Apollo program in the late 1960s and early 1970s. There have been several robotic missions as well. The Ranger program literally crashed spacecraft into the Moon in the 1960s and the Surveyor program followed with several soft landings. NASA is planning on returning humans to the Moon in 2024 with the Artemis program.

There have been several missions to the Moon in the last year or so. Notable are the Chinese Chang’e 4, which landed on the Moon’s far side, and Chang’e 5, which brought back the first lunar samples since 1976.

Still, the Earth’s Moon shouldn’t be there. There wasn’t enough material left over from the formation of small planets like the Earth to form a large moon. The favorite theory for the formation of the Moon is The Giant Impact Theory. It says that a Mars sized object (Theia) hit the Earth soon after the Earth formed, and the resulting debris eventually coalesced into the Moon.