By Sabine Stanley Ph.D., John Hopkins University
The Moon is the beautiful bright object decorating our night sky with its different phases, and creating phenomena like the eclipses. However, it is much more than a celestial ornament. In fact, our survival has depended heavily on this lifeless object.
The Moon is an extraordinarily important element in our world. Without Moon, Earth’s axis would change so much that it could make life impossible. Mars’s axis oscillates by about 20° every hundred thousand years since it has no large moon. On the other hand, Earth’s axis moves only 2° over thousands of years. Venus is also orbiting upside down, perhaps due to the lack of a strong moon. Even without knowing these, people always valued the Moon.
Learn more about Humans on the Moon: A never-Ending story.
Culture, Religious Deities, and Myths
Many gods and goddesses all over the world originate from the Moon: the Greek goddess Selene, the Roman goddess Luna, the Hindu god Chandra, the Inuit god Igaluk, and the Chinese goddess of the Moon Chang’e. Chang’e inspired the name of Chinese Lunar Exploration missions. In myths, Chang’e has a pet: a jade rabbit called Yutu. Yutu was also the name of the first rover to cross the far side of the Moon in 2019. The fact that the Moon and the Sun appear about the same size in our sky also has had interesting cultural consequences. However, this does not mean they have the same distance from the Earth.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
The Earth-Moon Distance
The Moon is at a distance 30 times Earth’s diameter, far less than Earth’s distance to the Sun, and far smaller than the Sun itself. The Earth-Moon distance would easily fit three times inside the Sun.
Earth’s diameter is 31⁄2 times larger than the Moon’s, which means our planet has an unusually big moon. What is the Moon made of?
Learn more about Jupiter’s planetlike system of moons.
The Moon’s Composition and Gravity
The Moon and Earth have very similar composition, but the Moon has significantly less iron and, consequently, a smaller core. Thus, it is much less dense and has about 1% of Earth’s mass. However, the Moon is smaller, and its surface is closer to its core, resulting in a bit more surface gravity.
Moon’s surface gravity is 17% of Earth’s, making the astronauts float and bounce on the surface, even though their suits triple their mass.
Learn more about Near-Earth Asteroids and the Asteroid Belt.
Phases of the Moon
Moonlight is, in fact, a reflection of the Sun’s light, shining on only one side of the Moon. Hence, there are lunar phases. A full orbit around the Earth takes Moon 27 days (a sidereal period), but the one that reveals all the phases is a synodic period about 291⁄2 days.
The new Moon is when the Moon is directly between the Earth and Sun, with the Sun shining only at the side of the Moon facing away from the Earth. However, the Moon moves and starts getting visible first with the Waxing Crescent Moon. When the Earth-Moon line is perpendicular to the Earth-Sun line, the Moon reaches the First Quarter. Here, the total surface can be seen.
The phases continue with an outward-curving shape: the Waxing Gibbous Moon. When the Moon gets directly on the opposite side of the Earth from the Sun, a full moon is created. After the full moon phase, there is the Waning Gibbous phase up to the Last Quarter, then a Waning Crescent phase. In the end, the Moon gets in between the Earth and Sun once again to repeat the whole cycle.
How do Eclipses Occur?
The Moon does not orbit the Earth in the same plane that the Earth orbits the Sun, or there would be a solar eclipse at every new moon, and a lunar eclipse at every full Moon. The Earth-Moon orbital plane is tilted by about 5° from the Sun-Earth orbital plane.
Eclipses happen when the Moon is at the new or full phase and also in the same plane as the Earth-Sun orbit. The condition is called ‘syzygy’, and a total eclipse happens when the Moon is perfectly aligned. The rest of the eclipses are partial solar eclipses as the Moon is not in the same plane.
Lunar eclipses happen when the Moon is in Earth’s shadow zone. When it is fully in the shadow and receives no sunlight, the Earth’s atmosphere bends the light, and the longer wavelength, i.e., red, hits the Moon and creates the blood moon.
The Moon Illusion
A full Moon can appear 12% bigger at the horizon (perigee) compared to when it reaches the farthest point from Earth (apogee). The difference is an optical effect called the Moon illusion, and the Moon’s measured size does not change at night. However, during a solar eclipse, the Moon at apogee cannot fully block the Sun, creating an Annular eclipse where the Sun will appear like a ring around the Moon. The Moon at perigee can cause a total solar eclipse.
In conclusion, the Moon is a vital element in explaining many phenomena and also a leading reason for our Earth’s stability.
Common Questions about the Moon
The Moon and its gravity have vital effects on the Earth, its rotation, its speed, the phenomena on Earth, and human life. Without the Moon, the Earth’s axial orbit would have changed drastically, and living conditions could have failed long ago.
Apart from tidal waves and its role in cultures and literature, the Moon regulates some important conditions on Earth and keeps its rotational axis from changing drastically. If it had not been for the Moon, Earth would have lost its living conditions.
No. We cannot survive without the Moon and the effects of its gravity. Without the Moon, the Earth could not preserve life and conditions related to it.
The Moon always faces the Earth with the same side since it is tidally locked to the Earth. Earth has slowed down the Moon up to the point that the Moons’ rotation period got equal to its orbital period.