The moon, Earth’s constant companion, is the brightest and most prominent object in our night sky. It has captivated humanity for millennia, sparking curiosity and wonder. When we gaze up at the moon, a common question arises: how big is the moon compared to Earth? While it appears large and luminous in our sky, the reality is that the moon is significantly smaller than our home planet. Let’s delve into the fascinating details of the size difference between Earth and the moon.
Key Size Statistics: Moon vs. Earth
To truly grasp the size disparity, let’s look at some key measurements. The moon’s average radius is approximately 1,079.6 miles (1,737.5 kilometers). Its mean diameter stretches across 2,159.2 miles (3,475 km). In contrast, Earth dwarfs its lunar companion. Our planet boasts a mean radius of 3,959 miles (6,371 kilometers) and a mean diameter of 7,918 miles (12,742 km). The moon’s equatorial circumference measures 6,783.5 miles (10,917 km).
When we compare these figures directly, a clear picture emerges. The moon’s width is less than a third of Earth’s. Specifically, the moon is just over one-quarter, or about 27%, the size of Earth, according to NASA. This size ratio (approximately 1:4) is actually quite remarkable and larger than the ratio seen between most other planets and their moons in our solar system.
Visualizing the Size Difference
Numbers can sometimes be abstract. To better visualize the size difference between the Earth Size Compared To Moon, consider this analogy from NASA: “If Earth were the size of a nickel, the moon would be about as big as a coffee bean.” This simple comparison effectively illustrates the significant size gap between the two celestial bodies.
Another way to understand the scale is to compare surface area. The moon’s surface area covers about 14.6 million square miles (38 million square km). While this is a vast area, it’s actually less than the total surface area of the continent of Asia, which spans approximately 17.2 million square miles (44.5 million square km). Imagine Asia – a massive landmass on Earth – and realize that the entire surface of the moon is smaller than that single continent.
Despite its smaller size, the moon can appear quite large in our sky. This is primarily due to its proximity to Earth. The moon is our closest celestial neighbor, situated at an average distance of about 238,855 miles (384,400 km) away. This relatively close distance makes it appear larger to us than more distant, albeit larger, objects in space.
Other Key Comparisons: Mass, Density, Gravity
Beyond diameter and radius, other characteristics highlight the differences between Earth and the moon. The moon’s mass is estimated to be 7.35 x 10^22 kg, which is only about 1.2% of Earth’s mass. To put it another way, Earth weighs approximately 81 times more than the moon.
In terms of density, the moon has a density of 3.34 grams per cubic centimeter (3.34 g/cm3). Earth is considerably denser, with a density of about 5.51 g/cm3. This means the moon’s density is roughly 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 approximately six times less on the moon than you do on Earth. For example, a person weighing 100 pounds on Earth would only weigh about 16.6 pounds on the moon. Imagine the possibilities for lunar athletics – a 10-foot jump on Earth could become a nearly 60-foot leap on the moon!
Moon’s Size in the Solar System Context
While the moon is significantly smaller than Earth, it holds its own in the wider solar system. Our moon is the fifth largest moon in the solar system overall. However, when considering the size of moons relative to their parent planets, Earth’s moon stands out. It is the largest moon in the solar system relative to the size of its planet. This makes the Earth-moon system somewhat unique.
To put the moon’s size into perspective against other moons, consider the largest moon in our solar system: Jupiter’s moon Ganymede.
| 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 Phenomenon
Have you ever noticed nights when the moon appears exceptionally large and bright? This is often a supermoon. The moon’s orbit around Earth is not perfectly circular; it’s elliptical. This means there are times when the moon is closer to Earth (perigee) and times when it’s farther away (apogee).
A supermoon occurs when a full moon coincides with the moon being at or near its perigee, its closest point to Earth. At perigee, the moon is approximately 226,000 miles (363,300 km) from Earth. During a supermoon, the moon can appear up to 14 percent larger and 30 percent brighter than an average full moon. While the term “supermoon” gained popularity relatively recently, the astronomical term for this phenomenon is perigee-syzygy moon, referring to the alignment of the Sun, Earth, and Moon. Supermoons are not incredibly rare, occurring roughly every 414 days.
The Moon Illusion: Why It Looks Bigger on the Horizon
Sometimes, you might observe the moon looking particularly large when it’s rising or setting near the horizon. This is known as the moon illusion, an optical effect that makes the moon appear bigger when viewed near the horizon compared to when it’s high in the sky.
Despite centuries of observation, there is no single, universally accepted explanation for the moon illusion. One prominent theory suggests it’s related to how our brains process distance and size in relation to the horizon. Our brains may use terrestrial objects on the horizon, like trees and mountains, as points of reference. When the moon is near these familiar objects, we subconsciously compare its size to them, leading to the perception that it is larger. Conversely, when the moon is high in the vast expanse of the sky, we lack these familiar reference points, and it appears smaller.
Another theory involves the perceived distance of the horizon versus the sky above. We tend to perceive the horizon as being farther away than the sky directly overhead. If we see the moon at the horizon and subconsciously perceive it as being very distant, our brain may automatically scale up its perceived size to compensate for the assumed distance.
Regardless of the exact cause, the moon illusion is indeed an illusion. You can easily test this by holding your thumb up at arm’s length to compare it to the moon’s size when it’s on the horizon and again when it’s higher in the sky. You’ll find that the moon’s actual angular size remains the same; only our perception changes.
Conclusion
In conclusion, while the moon dominates our night sky and plays a significant role in Earth’s environment, it is considerably smaller than our planet. Understanding the earth size compared to moon highlights the vastness of space and the fascinating relationships between celestial bodies. Despite its smaller stature, the moon remains a crucial object of study, offering invaluable insights into the formation and evolution of our solar system. Its continued exploration promises to unlock even more secrets of our cosmic neighborhood.
Additional resources
For more quick facts about the moon, explore these educational infographics by ESA: Educational infographics by ESA
Want to know how much you’d weigh on the moon or other celestial bodies? Try this interactive tool from the Exploratorium: science museum Exploratorium
To delve deeper into the fascinating world of optical illusions, including the Ponzo illusion, visit The Illusion Index: The Illusion Index
Bibliography
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Daisy Dobrijevic
Reference Editor
Daisy Dobrijevic joined Space.com in February 2022 having previously worked for our sister publication All About Space magazine as a staff writer. Before joining us, Daisy completed an editorial internship with the BBC Sky at Night Magazine and worked at the National Space Centre in Leicester, U.K., where she enjoyed communicating space science to the public. In 2021, Daisy completed a PhD in plant physiology and also holds a Master’s in Environmental Science, she is currently based in Nottingham, U.K. Daisy is passionate about all things space, with a penchant for solar activity and space weather. She has a strong interest in astrotourism and loves nothing more than a good northern lights chase!
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