The moon, Earth’s constant companion, illuminates our night sky and has captivated humanity for millennia. Often appearing large and prominent, especially on certain nights, it’s natural to wonder: how big is the moon compared to Earth? While it dominates our celestial view, the reality of the moon’s size relative to our planet is quite fascinating and reveals much about our place in the solar system.
Decoding the Moon’s Dimensions: Key Size Metrics
To truly understand the scale of the moon, let’s delve into some key measurements. According to NASA, the moon’s average radius is approximately 1,079.6 miles (1,737.5 kilometers). Its diameter, measuring across its widest point, is about 2,159.2 miles (3,475 km). The equatorial circumference, or the distance around the moon at its equator, is a substantial 6,783.5 miles (10,917 km).
Moon Size Stats at a Glance:
- 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)
While these numbers provide concrete data, grasping the moon’s size relative to Earth requires a direct comparison.
Earth vs. Moon: A Size Ratio Unveiled
When we compare the moon to Earth, the stark difference in scale becomes apparent. The moon’s width is less than one-third that of Earth. In fact, the moon is approximately 27% the size of Earth by diameter. This means you could fit roughly four moons side-by-side to match Earth’s diameter.
NASA offers a relatable analogy: “If Earth were the size of a nickel, the moon would be about as big as a coffee bean.” This visual comparison effectively illustrates the size disparity between these two celestial bodies.
Another perspective is surface area. The moon’s surface area totals about 14.6 million square miles (38 million square km). While vast, this is actually smaller than the continent of Asia, which spans approximately 17.2 million square miles (44.5 million square km).
Moon’s Mass, Density, and Gravity Compared to Earth
Beyond diameter and surface area, examining mass and density further clarifies the moon’s size relative to Earth. The moon’s mass is about 7.35 x 10^22 kg, a mere 1.2% of Earth’s mass. To put it another way, Earth is approximately 81 times more massive than the moon.
In terms of density, the moon registers 3.34 grams per cubic centimeter (3.34 g/cm3), about 60% of Earth’s density. This difference in density, along with mass, contributes to the significant difference in gravitational pull. The moon’s gravity is only about 16.6% of Earth’s gravity. This means you would weigh about six times less on the moon than on Earth. A person weighing 100 lbs on Earth would only weigh 16.6 lbs on the moon. Imagine the lunar long jump potential!
The Moon’s Size in the Solar System Context
Our moon is unique in its size relative to its planet. Among all the planets and their moons in our solar system, Earth’s moon has the largest size ratio compared to its host planet. Despite this, when considering absolute size, our moon ranks as the fifth-largest moon in the solar system.
Jupiter boasts the largest moon, Ganymede, which dwarfs our moon with an equatorial radius of 1,635 miles (2,631 km). To provide further context, here’s a list of the 10 largest moons in our solar system:
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 |
This table highlights that while our moon is substantial, it is outsized by several moons orbiting the gas giants of our solar system.
Supermoon: When the Moon Appears Larger
Have you ever noticed the moon appearing exceptionally large in the night sky? This phenomenon is often referred to as a “supermoon.” The moon’s orbit around Earth is not perfectly circular; it’s elliptical. This means there are points in its orbit where the moon is closer to Earth (perigee) and farther away (apogee).
When a full moon coincides with the moon being at or near its perigee – its closest point to Earth (approximately 226,000 miles or 363,300 km) – we experience a supermoon. During a supermoon, the moon can appear up to 14% larger and 30% 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, where “syzygy” denotes the alignment of three celestial bodies – in this case, the Sun, Earth, and Moon. Supermoons occur roughly every 414 days, though some years, like 2016, can have multiple supermoons.
The Moon Illusion: Horizon Size Deception
Another instance where the moon might seem unusually large is when it’s rising or setting near the horizon. This is known as the “moon illusion.” It’s an optical effect that makes the moon appear significantly bigger when viewed near the horizon compared to when it’s high in the sky.
Despite being observed for centuries, the exact cause of the moon illusion remains debated. One theory suggests that our brains compare the moon to objects on the horizon, like trees and buildings, making it seem larger in comparison. Another theory involves how our brains process the curvature of the sky.
Regardless of the cause, the moon illusion is purely an optical trick. You can test this by holding your thumb up to the moon at the horizon and then again when it’s higher in the sky – the moon will appear the same size relative to your thumb in both instances.
Explore Further: Resources for Moon Enthusiasts
Want to delve deeper into lunar facts? The European Space Agency (ESA) offers educational infographics with quick moon facts. To explore how your weight would change on the moon and other celestial bodies, check out the Exploratorium’s interactive weight calculator. For more on the fascinating Ponzo illusion, explore The Illusion Index from the University of Glasgow.
Additional Resources:
- ESA Moon Fast Facts: https://www.esa.int/Education/Teach_with_the_Moon/Moon_fast_facts
- Exploratorium Weight on Other Worlds: https://www.exploratorium.edu/ronh/weight/
- The Illusion Index – Ponzo Illusion: https://www.illusionsindex.org/i/ponzo-illusion
Bibliography
Space.com articles referenced throughout the text.
The Nine Planets. https://nineplanets.org/questions/how-big-is-the-moon/
NASA. https://moon.nasa.gov/ & https://solarsystem.nasa.gov/
The Illusion Index. https://www.illusionsindex.org/
ESA. https://www.esa.int/
Exploratorium. https://www.exploratorium.edu/
