Who doesn’t love the sight of a silver Moon hanging in the night sky above? For the entire history of humanity, the Moon has featured prominently in our mythology, religion, and timekeeping. With the advent of astronomy, the Moon became one of the first studied objects.
In this video, we will learn more about Earth’s only natural satellite and begin to understand why humans have dedicated centuries of study to uncover its mysteries. Let’s get started.
When we look at the other planets in the solar system, we see that the Earth is not the only one with a moon. But the Earth’s Moon is unique nonetheless. With a diameter nearly one quarter the Earth’s diameter, it is one of the largest moons in the solar system. The other large moons all orbit large planets—mostly Jupiter and Saturn. Mars is the only other small planet that has moons. But Mars’s two moons are 8 and 20 kilometers across, respectively, compared to our Moon’s diameter of about 3,500 kilometers. Clearly, there is a big difference between Mars’s moons and the Earth’s Moon. But why is the Earth’s Moon so large? And how did it come to share this bit of space with the Earth?
Scientists have a few ideas about how the Moon might have formed. Perhaps it formed at the same time as the Earth, out of the same material? An analysis of the materials in the Moon shows that it is very similar to the Earth, but not identical. If these two objects formed at the same time, we would expect the Moon’s core to be as dense as the Earth’s, but it is not. Another idea is that the Moon was an asteroid that wandered too close to the Earth and was captured by the Earth’s gravity. That would explain why it has a different density–except that the density and chemical composition of the Moon is too close to the Earth’s to explain this. How can these objects be too similar to be from different places, but too different to be identical?
The current model is what’s known as the Giant Impact Theory. Simulations show that if something about the size of Mars were to hit the Earth, it could create the Moon. The idea is that the Mars-sized object hit the Earth, and the entire Earth’s surface melted, so there is no crater to be seen today. Some of the debris from that collision would have rejoined the Earth, to make the larger Earth we know today. The rest of the debris from this collision solidified in Earth’s orbit, forming the Moon. Since that debris would have been mostly from the crust of the Earth, that is why the Moon’s composition matches the Earth’s crust so well. The core of the debris might have been the leftover core of the impactor, explaining why the Moon’s core and the Earth’s don’t match in density.
So to summarize, the current model of the Moon’s formation has a Mars-sized object hitting a smaller Earth, and then together they solidified to make the current Earth and Moon. After this collision, the Moon stabilized in an orbit that takes it around the Earth every 27 days.
The Moon is very slowly moving away from the Earth at a rate close to 3.8 centimeters each year. This is also causing the Earth’s rotation to slow down, though only by less than two milliseconds every hundred years. The gravity between the Earth and the Moon also keeps the Moon from turning away from the Earth, so that the Moon always keeps the same side facing the Earth. This means the time it takes for the Moon to spin on its axis is the same amount of time the Moon takes to orbit the Earth. This is called being “tidally locked” and means there is half of the Moon we never see. This distant side of the Moon is aptly called the far side.
Another consequence of the gravity between the Earth and the Moon is the tides in the ocean. The reason for tides is that gravity is stronger when two objects are closer together. One side of the Earth is closer to the Moon than the other side, experiencing a stronger force of gravity. Water flows toward this side of the Earth. This is called the Earth’s tidal bulge. The Earth itself is pulled from its center more strongly than the water on the side of the Earth facing away from the Moon. This water on the far side of the Earth gets left behind as the Earth is pulled toward the Moon, creating a second tidal bulge on that far side.
The orbit of the Moon around the Earth combined with the orbit of the Earth around the Sun also produces the phases of the Moon. We see these phases as the Moon appears to change shape throughout a month. This is an optical illusion. The Moon is always a sphere and doesn’t really change shape. How does this optical illusion work? To understand it, you first must realize that the Moon doesn’t emit any of its own light, so the whiteness we see on the Moon is actually reflected sunlight. The Sun always shines on half of the Moon, but the lit half is not always the half we can see from the Earth. As the Sun’s light moves around the Moon’s surface, this looks to us on Earth as if the Moon is changing shape. For example, when the entire lit half of the Moon is facing away from the Earth, we can’t see the Moon at all. This is called the New Moon.
As the Moon moves around the Earth, we can begin to see portions of the lit side. The Moon orbits so that if you were looking down on the Earth from the North Pole, the Moon would go counter-clockwise around the Earth. This means that when we start to see the lit portion of the Moon, it begins to light up from the right side as a crescent. Over the next week, the crescent grows larger. When the lit part of the Moon is growing larger from one night to the next, this is called “waxing.” After about a week, half of the Moon will be visible to the Earth–the right half. This is the First Quarter moon.
As the Moon’s lit side continues to grow each night, it will start to look like a deflated ball. This shape is called “gibbous,” which means round. Two weeks after the New Moon, the entire lit side of the Moon will be visible from the Earth. We call this the Full Moon.
After the Full Moon, the Moon’s orbit will take it back towards the Sun. The amount of the lit side that we can see will shrink each night, and the Moon will once again become gibbous. When the lit portion of the Moon gets smaller each night, this is called “waning.” Three weeks after the New Moon, the Moon will once again be half lit, but this time we will see the left half lit. This is the Last Quarter Moon or the Third Quarter Moon.
Finally, past this point, the Moon will be waning and appear as a crescent, until it returns to the New Moon part of its phase. Because the Earth is moving around the Sun as the Moon orbits the Earth, it takes 29.5 days for the Moon to return to the same appearance (New Moon to New Moon for instance), even though the Moon only takes 27 days to orbit the Earth.
This month-long play of light in the sky has taken on great meaning to cultures all over the globe. Many cultures traditionally kept lunar calendars, and some still do today. The modern Western calendar is based on the Sun, but also has divisions for the Moon—called months (originally “moonths”). Our months no longer correspond to the actual appearance of the Moon, but other cultures have kept their calendars linked to the current appearance of the Moon. In Islam, for instance, the month begins when someone first views the crescent Moon after a New Moon. Even in Western society, we still place importance on the nights of the Full Moon, or look for the rare blue Moon. A blue Moon is the second Full Moon in the same calendar month.
The Moon also has the unique distinction of being the only place humans have ever visited off the planet Earth. Between 1969 and 1972, six lunar landers, carrying two astronauts each, landed on the Moon. The astronauts took several days to get to the Moon, landed for about a day, and then took several days to return.
During the day on the Moon, astronauts conducted experiments and gathered rock samples. They placed instruments like a mirror and a seismograph. The mirror allows scientists to this day to use a laser to measure the distance between the Earth and Moon. The seismograph measures earthquakes—or in this case, moonquakes. The rock samples allowed tests of the chemical composition of the surface of the Moon.
These experiments are how we know the composition and density of the Moon’s surface as well as the density of the Moon’s core and the speed with which the Moon is moving away from the Earth. Here is a photograph taken by Neil Armstrong of Buzz Aldrin, both of Apollo 11, setting up the lunar seismograph
The first mission to land on the Moon, Apollo 11, landed in a region called the “Sea of Tranquility.” Despite this name, the Moon has no seas, oceans, or any liquid water. The surface of the Moon is both too cold for liquid water and too hot. How is this possible? Without an atmosphere, the temperature of the surface of the Moon depends on whether the surface is in the Sun or in the shade. In the Sun, the temperature of the Moon is about 260 degrees Fahrenheit. In the shade, it is closer to -300 degrees Fahrenheit. If sunlight hits frozen water on the Moon, it would evaporate pretty quickly. The Moon does have polar regions where ice is trapped in the bottom of craters that never see sunlight. Still, the Moon definitely has no liquid water. Instead, the Sea of Tranquility is one of the many lunar maria. Maria is the plural for mare, which is Latin for “sea.” The maria are huge impact craters that flooded with lava billions of years ago. This lava has since hardened into dark, relatively flat regions. These are what you see when you look up at the Moon and imagine a man, rabbit, frog, or any of the other shapes different cultures have seen in the dark shapes of the maria. The bright, rocky areas covered in dense craters are called the highlands.
A lot could be said about the Moon, but let’s review the important basics that we have covered. The Moon has a diameter about one-fourth of the Earth’s. Information obtained from the Apollo missions has convinced scientists that the Moon formed when a Mars-sized object impacted the Earth billions of years ago. Not long after that, additional craters on the Moon’s surface flooded with lava, creating the lunar maria. Now, we see the Moon in our sky as it orbits the Earth. Many students memorize the visible phases of the Moon: New Moon, waxing crescent, First Quarter, waxing gibbous, Full Moon, waning gibbous, Third (Last) Quarter, and waning crescent. It may be more important to understand that these phases are because the Earth can only see certain parts of the Moon’s lit side, while half of the Moon is always lit. There is no dark side of the Moon, but there is a far side.
That’s all for now! Thanks for watching, and happy studying!
Return to Astronomy Videos