6 surprising facts about the moon

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Photo by Gregory H. Revera

There is no dark side of the moon, really. As a matter of fact, it’s all dark. The only thing that makes it look light is the sun. Gerry Driscoll, Pink Floyd, “Eclipse”, The Dark Side of the Moon.

The moon is the only natural satellite of the planet we call home. It is much smaller (27% the diameter of the earth) and less dense (60% of the density of the earth) which makes it just 1.2% of the earth’s mass. It is also the second brightest object in the sky, after the sun. Here are six interesting facts about it including what colour is the moon, who landed on the moon first and why we left mirrors on it.

1. There is more than one type of lunar month

The moon is tidally locked to the earth (in synchronous rotation) which means that it takes the same time to complete an orbit around the earth as it takes for it to make a full rotation of its axis. As a consequence, the face it shows towards the earth is always the same. In fact, humans had never seen the other side of the moon until 1959, when the russian mission Luna 3 took pictures of it.

The phase of the moon is determined by its position in respect to the earth and the sun – Image by Wikipedia user Orion 8

Historically calendars have been based on the lunar month: the time in-between two full moons (or any other two same phases of the moon: two new moons, two crescent moons, etc.), which takes 29 days, 12 hours, 44 minutes and 2.8 seconds. However, this measures the time period between two same phases of the moon and the phase of the moon is determined by its position relative to the earth AND the sun. Since the earth is also rotating around the sun during the month, this time period is slightly longer than the amount of time it takes for the moon to complete an orbit. This type of lunar month is called the synodic month.

The sidereal month measures the length of time it takes for the moon to complete an orbit and it clocks in at slightly less, 27 days, 7 hours, 43 minutes and 11.5 seconds. There are three more types of lunar month that are measured slightly differently. You can learn more here and here.

2. The moon is red and blue?

What colour is the moon? Look up into the night sky during a full moon and you might be tempted to say that it’s whitish/gray. However, one has to consider that the night sky is very very dark and the moon appears a lot brighter and whiter than it actually is.

Albedo (whiteness in latin), also known as the reflection coefficient, measures the amount of light that reflects from a surface. A completely white surface has an albedo of 1.0 and a completely black surface (which does not reflect any light at all) has an albedo of 0. The surface of the moon has an overall albedo of 0.12 (same albedo as worn asphalt) which makes it closer to black than to white. To give some more context the average albedo of the earth is about 0.3 . So we’ve concluded that the moon is definitely not white. But is it just grey?

Colour composite of the moon’s surface. to show the subtle colour variations of the surface, the light in the visible spectrum has been divided in just 7 bands (from 320 nm to 689 nm) – Credit: NASA/GSFC/Arizona State University – Click here for more info

Scale of saturation for red – Author: Lefty

The colour of the moon is determined by the mineral composition of its soil. Most of the moon’s surface contains iron oxide which gives it a reddish-brown colour. Some of the maria (the darker areas of the moon),  contain titanium dioxide which shifts the colour of the surface to blue. Pyroclastic glasses of various colours (including bright green) can also be found on the moon’s surface.

So why don’t we see these colours when we look up to the moon? Because of the low albedo, the colours are not very saturated and appear to be grey to the human eye.

3. The moon is leaving?

Even though the moon is clearly smaller than the earth it is still surprisingly large. At a quarter of the diameter of the earth it is the biggest moon in the solar system relative to the planet it orbits (in absolute terms it is the fifth biggest).

The mass of the moon is approximately 1/81 of that of the earth and the gravitational attraction between the two causes the earth to bulge (and vice versa). This is mostly noticeable on the earth’s oceans as the sea is pulled up by the moon’s gravity: the moon is responsible for tides (the gravitational attraction from the sun is also responsible, the interaction between the two forces causes spring and neap tides).

Because the earth rotates much faster than the moon orbits, this tidal bulge is slightly ahead of the moon. The extra mass of the bulge means that the moon is pulled forward in its orbit a little: there is torque between the earth and the moon. This torque increases the orbital velocity of the moon and decreases the rotational speed of the earth. As the orbital velocity of the moon increases so does its orbit.

The first photo ever taken of the moon and the earth in the same frame. Taken on September 18, 1977 by Voyager 1. – Source: NASA – More info here

In fact, the moon moves away about 3.8 centimetres every year. The earth’s day gets longer every year by 15 microseconds. Eventually an earth day and a a moon orbit will take the same amount of time and the earth will be tidally locked to the moon (always showing the same face) just as the moon is already tidally locked to the Earth. This process would take about 50 billion years and the length of an earth day would have increased to 47 of our current days.

Well, except that, by the time this would happen, the sun will have already turned into a red giant and engulfed both the earth and the moon (in about 5 billion years).

4.The first to land on the moon were the Soviets?

There have been a grand total of 12 people who have landed on the moon, all American males (8 of whom are still alive). Everyone is familiar with Neil Armstrong, Buzz Aldrin and with the “one small step for a man” quote (though barely anyone remembers the other 10 astronauts, I guess that’s what you get from not coming first or second).

However, the USSR were first to pass by the moon (Luna 1 mission in 1957), the first to crash-land on the moon (Luna 2, 1958), the first to take pictures of the far side of the moon (Luna 3, 1959), the first to soft-land a probe on the moon and take close-up pictures of its surface (Luna 9, 1966) and the first to put an object in orbit around the moon (Luna 10 became the first artificial satellite of the moon in 1966).

Buzz Aldrin on the moon – 20 July 1969 – Source: NASA

Let’s remind ourselves of the other 10 men who walked on the moon: Pete Conrad, Alan Bean, Alan Shepard, Edgar Mitchell, David Scott, James Irwin, John W. Young, Charles Duke, Eugene Cernan and Harrison Schmitt.

The last of the 12, Harrison Schmitt, was the only geologist to land on the moon. As he was getting back on the lunar lander after his third EVA he accidentally kicked up some dirt and got very very excited. Because the dirt was orange.

5. Moon volcanoes

Harrison Schmitt got very excited because he recognised that the pale orange soil was volcanic rock, providing the first direct evidence that the moon had, at some point in the past, active volcanoes.

In fact, since then, we have not only discovered that the moon used to be volcanic but that it also has a metal core much like the earth. The core of the moon is thought to be mostly composed of metallic iron with traces of sulphur and nickel. The inside of the core is solid, surrounded by a liquid layer. Outside the core there is a small layer of liquid mantle. Unlike the earth, the moon’s mantle is mostly solidified. The core is thought to be quite small, just 330 km across, approximately 20% of the moon’s radius. Its temperature is likely between 1600-1700 Kelvin.

Artist’s rendering of the moon’s internal structure – Source: NASA

In comparison, the earth’s core temperature ranges from around 4000 K at the outer edge of the liquid core to 6000 K in the solid core (about the same temperature as the surface of the sun). It’s also much bigger, in absolute and relative terms. It stretches out to about 50% of the earth’s radius (3400 km).

The darker spots on the moon are called maria (latin for seas) as in ancient times they were thought to be large bodies of water. We now know that they are actually vast deposits of basaltic rock (rock formed by cooling lava). The maria are almost all found on the near side of the moon. Thirty one percent of the near side features maria while just 2% of the far side does.

6. Moon mirrors

Family picture left behind by Apollo 16 astronaut Charlie Duke – Source: NASA

The twelve brave men who got to walk on the moon didn’t leave behind just footsteps, a flag and a family picture but also several nifty pieces of equipment. Seismographs left on the moon by the Apollo astronauts measured moonquakes up to 1977. The data produced is still being analysed this day and it’s being used to determine the specifics of the moon’s core.

The astronauts also left behind several arrays of retroreflectors. Retroreflectors are pretty cool, they are mirrors constructed in such a way that they always reflect light straight back at the source. Shine a light on them at any angle and it will always be reflected straight at you.

Retroreflector -Image: Chetvorno

Arrays of half-cube retroreflectors (basically three mirror surfaces perpendicular to each other) were left on the moon by Apollo 11, 14 and 15. Two unmanned russian lunar landers (Lunokhod 1 and 2) also carried similar, but smaller, arrays of retroreflectors. The Lunokhod 2 array is routinely used by scientists.

But what are these reflectors for? As you can imagine, the astronauts did not leave behind the mirrors just because they wanted to check out how pretty they looked. The reflectors are part of an ongoing experiment called the LLRE (Lunar Laser Ranging Experiment). Basically, scientists (with the use of a telescope) can beam laser pulses to the moon and receive the beam back. Because laser is light and light travels at the speed of light (duh) by measuring the amount of time it takes for the pulses to come back we can measure the distance from the earth to the moon.

Lunokhod 1, the coolest lander? – Source: NASA

The distance of the moon is approximately calculated as the speed of light times the time taken divided by two. I say approximately because other effects have to be taken into account such as the relative motion of the moon and the earth, lunar libration, weather, relativity and more.

Several facts given in the article (including the distance from the earth to the moon, the first indications that the moon has a core and that the moon increases its orbit by 3.8 cm every year) were discovered using this experiment.

Apollo 14 retroreflector – Source: NASA

It is actually quite hard to hit the reflector arrays. They are very tiny and by the time the laser pulse has travelled to the moon it has expanded to a few km wide. And if the scientists do manage to hit them, only a photon every few seconds comes back. Nevertheless, the distance between the earth and the moon is being calculated with increasing accuracy every year. In fact, the LLRE is one of the highest accuracy measurements of distance ever.

The distance to the moon can be calculated to a few cm of accuracy. NASA compares it to measuring the distance between New York and LA to one hundredth of an inch (0.254 mm)!

Other findings of the LLRE are that gravity is very stable (the gravitational constant G has changed by less that 1 part in 100 billion since 1969) and that the orbit measurements match the predictions given by Einstein’s theory of general relativity.

And, of course, it’s also major proof that humans did indeed land on the moon.

Bonus fact

The size of a “supermoon” has been greatly exaggerated. Click here to read my article about it.

-Francesco

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