Earth’s moon, a constant companion in our night sky, has captivated humanity for millennia. While it appears large from our perspective, especially during a supermoon, how does the moon truly measure up against its home planet, Earth? Let’s delve into a detailed comparison of the moon and Earth, exploring their size, mass, density, gravity, and other intriguing aspects.
Size Matters: Diameter, Radius, and Surface Area
When we talk about size, several measurements come into play. The moon’s mean radius is approximately 1,079.6 miles (1,737.5 kilometers), while its mean diameter stretches to 2,159.2 miles (3,475 km). In contrast, Earth boasts a significantly larger mean radius and diameter. To put it into perspective, the moon’s width is less than one-third of Earth’s. Circling the moon at its equator would mean traveling 6,783.5 miles (10,917 km).
The seemingly large appearance of the moon is due to its proximity to us. It’s our closest celestial neighbor, averaging around 238,855 miles (384,400 km) away. However, in terms of sheer size, the moon is considerably smaller than Earth. Imagine Earth as a nickel; in that scale, the moon would be about the size of a coffee bean, according to NASA.
The moon’s surface area covers about 14.6 million square miles (38 million square km). While vast, this is still less than the entire continent of Asia, which spans 17.2 million square miles (44.5 million square km).
Here’s a quick look at key moon stats for a clear comparison to Earth:
Moon Statistics:
- Radius: 1,079.6 miles (1,737.5 km)
- Diameter: 2,159.2 miles (3,475 km)
- Surface area: 14.6 million square miles (38 million square km)
- Mass: 7.35 x 10^22 kg
- Density: 3.34 grams per cubic centimeter (3.34 g/cm3)
Mass, Density, and Gravity: A World of Difference
The mass of the moon is estimated to be 7.35 x 10^22 kg. This is only about 1.2% of Earth’s mass, meaning Earth is approximately 81 times heavier than the moon. The density of the moon is 3.34 grams per cubic centimeter (3.34 g/cm3), around 60% of Earth’s density.
Perhaps one of the most noticeable differences for a hypothetical visitor would be gravity. The moon’s gravitational force is only about 16.6% of Earth’s gravity. This means you would weigh about six times less on the moon. A person weighing 100 lbs on Earth would only weigh 16.6 lbs on the moon. Imagine the lunar Olympics – long jumps would be dramatically different, with a 10-foot jump on Earth translating to a nearly 60-foot leap on the moon!
Interestingly, the moon’s gravity isn’t uniform across its surface. NASA’s GRAIL mission in 2012 meticulously mapped the moon’s gravity, revealing variations linked to surface features like craters and mountains.
Did you know that the moon is the second densest moon in our solar system? Only Jupiter’s moon Io is denser.
Moon Compared to Other Moons: A Relative Giant
While significantly smaller than Earth, our moon is actually quite large when compared to other moons in our solar system. In fact, relative to the size of its planet, Earth’s moon is the largest in the solar system. Overall, it ranks as the fifth-largest moon in the solar system.
Jupiter’s moon Ganymede holds the title of the largest moon in the solar system, dwarfing our moon in size.
Here’s a comparison of the 10 largest moons in our solar system to give context to the moon’s size:
10 Largest Moons in the Solar System
| Rank | Moon | Equatorial radius | Parent planet |
|---|---|---|---|
| 1 | Ganymede | 1,635 miles (2,631 km) | Jupiter |
| 2 | Titan | 1,600 miles (2,575 km) | Saturn |
| 3 | Callisto | 1,497 miles (2,410.3 km) | Jupiter |
| 4 | Io | 1,131.7 miles (1,821.6 km) | Jupiter |
| 5 | The Moon | 1,079.6 miles (1,737.5 km) | Earth |
| 6 | Europa | 969.84 miles (1,560.8 km) | Jupiter |
| 7 | Triton | 840.96 miles (1,353.4 km) | Neptune |
| 8 | Titania | 490.19 miles (788.9 km) | Uranus |
| 9 | Rhea | 474.91 miles (764.3 km) | Saturn |
| 10 | Oberon | 473.11 miles (761.4 km) | Uranus |
Supermoon: When the Moon Appears “Bigger”
Have you ever noticed the moon appearing exceptionally large and bright? This phenomenon is often referred to as a “supermoon.” This occurs because the moon’s orbit around Earth is not perfectly circular; it’s elliptical. At its closest point to Earth, called perigee, the moon is approximately 226,000 miles (363,300 km) away.
When a full moon coincides with perigee, we experience a supermoon. During a supermoon, the moon can appear up to 14 percent larger and 30 percent brighter than an average full moon. The term “supermoon” was coined by astrologer Richard Nolle in 1979. Astronomers refer to this event as a perigee-syzygy moon, with “syzygy” describing the alignment of the Sun, Earth, and Moon. Supermoons happen roughly every 414 days.
The Moon Illusion: Horizon Size Trickery
Another interesting aspect of how we perceive the moon’s size is the “moon illusion.” Sometimes, the moon appears significantly larger when it’s near the horizon, rising or setting. This is an optical illusion that has puzzled observers for centuries, and there’s still no single, universally accepted explanation.
One theory suggests that our brains compare the moon to familiar objects on the horizon, like trees and buildings. Because these objects appear small at a distance, the moon, being in the same visual field, is perceived as much larger in comparison. Another theory involves our perception of the sky itself. We are used to seeing nearby clouds overhead, and distant clouds on the horizon. When a horizon cloud appears the same size as an overhead cloud, we know it must be much larger. Our brain might apply similar logic to the moon.
To test if it’s an illusion, try this: hold your thumb up to the moon when it’s near the horizon. Then, do the same when the moon is high in the sky. You’ll notice the moon’s size relative to your thumb remains the same, indicating it’s indeed an optical trick of the mind.
Conclusion
In conclusion, while the moon is Earth’s closest celestial neighbor and a prominent feature in our sky, it is significantly smaller than Earth. It’s about a quarter of Earth’s size in diameter and possesses a fraction of Earth’s mass and gravity. However, in the grand scale of the solar system’s moons, our moon is a notable size, ranking fifth largest overall and largest relative to its planet. Phenomena like supermoons and the moon illusion further enhance our perception of this fascinating celestial body, reminding us of the dynamic and intriguing nature of space.
Additional Resources:
- ESA Educational Infographics: Moon Fast Facts
- Exploratorium Interactive Weight Calculator: How Much Would You Weigh on Other Worlds?
- The Illusion Index: Ponzo Illusion
Bibliography
- NASA Moon Facts: https://moon.nasa.gov/about.cfm
- NASA Solar System Exploration: https://solarsystem.nasa.gov/moons/earths-moon/in-depth/#otp_size_and_distance
- Space.com: https://www.space.com/
- The Nine Planets: https://nineplanets.org/
- NASA GRAIL Mission: https://www.nasa.gov/mission_pages/grail/main/index.html
- NASA Supermoon Article: https://solarsystem.nasa.gov/news/922/what-is-a-supermoon/
- NASA Moon Illusion Article: https://solarsystem.nasa.gov/news/1191/the-moon-illusion-why-does-the-moon-look-so-big-sometimes/
