Earth’s moon, a constant companion in our night sky, shines brightly as the most prominent celestial object after the Sun. It has captivated humanity for millennia, inspiring myths, legends, and scientific inquiry. But when we gaze up at the moon, a fundamental question arises: how big is the moon compared to Earth? The answer might surprise you. While it appears large in our sky, especially during a full moon, our lunar neighbor is significantly smaller than our home planet.
The moon, depicted in a photograph taken from space, is shown to be considerably smaller than the Earth, highlighting the size difference between these two celestial bodies.
To put it into perspective, the moon’s average radius measures about 1,079.6 miles (1,737.5 kilometers), and its diameter spans 2,159.2 miles (3,475 km). According to NASA, this means the moon’s width is less than one-third of Earth’s. More precisely, the moon’s equatorial circumference stretches 6,783.5 miles (10,917 km). While these numbers might seem substantial, comparing them to Earth truly reveals the moon’s size relative to our planet.
Initially, the moon might seem quite large to us earthlings. This perception is mainly due to its proximity to Earth. As our closest celestial neighbor, the moon resides at an average distance of approximately 238,855 miles (384,400 km). This comparatively short distance in cosmic terms makes the moon appear larger in our field of vision.
Let’s delve into some key lunar statistics to further understand its size:
Moon 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)
The Moon’s Size Compared to Earth: A Matter of Proportions
When we directly compare the moon’s dimensions to Earth, the size difference becomes strikingly clear. The moon is just a little over one-quarter, or about 27%, of Earth’s size. This size ratio, approximately 1:4, is notably larger than the ratio between most other planets and their moons in our solar system. Despite its smaller stature compared to Earth, our moon holds the position of the fifth largest moon in the entire solar system.
To visualize this size difference more intuitively, NASA offers a helpful analogy: “If Earth were the size of a nickel, the moon would be about as big as a coffee bean.” This relatable comparison vividly illustrates the scale difference between the two celestial bodies.
Another interesting comparison involves surface area. The moon’s surface area totals about 14.6 million square miles (38 million square km). While vast, this is actually less than the total surface area of the continent of Asia, which covers approximately 17.2 million square miles (44.5 million square km). This comparison helps to ground the moon’s size in a more familiar terrestrial context.
Related Reading: Does the Moon Rotate? Unveiling Lunar Rotation
Lunar Mass, Density, and Gravity: How They Compare to Earth
Beyond mere size, comparing the moon’s mass, density, and gravity to Earth provides a more complete understanding of their differences. The moon’s mass is approximately 7.35 x 10^22 kg, which is about 1.2% of Earth’s mass. In simpler terms, Earth is roughly 81 times more massive than the moon.
Density is another key differentiating factor. The moon’s density is 3.34 grams per cubic centimeter (3.34 g/cm3), about 60% of Earth’s density. This lower density contributes to the moon’s lower gravitational pull.
The moon’s gravitational force is only about 16.6% of Earth’s gravity. This significant difference means that a person would weigh approximately six times less on the moon than on Earth. Imagine the implications for athletic feats! A person who can jump 10 feet high on Earth could potentially jump nearly 60 feet on the moon due to the reduced gravity.
It’s also important to note that lunar gravity isn’t uniform across the moon’s surface. Like most celestial bodies, the moon’s gravity varies based on its surface features. NASA’s GRAIL mission in 2012 meticulously mapped these gravitational variations, providing unprecedented detail about the moon’s gravitational field.
Fun Fact: Did you know that the moon is the second densest moon in our solar system? Only Jupiter’s moon Io is denser, boasting a density of 3.53 g/cm3.
According to Maria Zuber, the principal investigator for the GRAIL mission at MIT, “What this map tells us is that more than any other celestial body we know of, the moon wears its gravity on its sleeve. When we see a notable change in the gravity field, we can sync up this change with surface topography features such as craters, rilles or mountains.”
Noah Petro, the deputy project scientist for NASA’s Lunar Reconnaissance Orbiter (LRO) mission, emphasizes the moon’s importance in understanding the solar system, stating, “The moon is the Rosetta Stone by which we understand the rest of the solar system.” Even with extensive study, the moon continues to reveal valuable insights into planetary science.
From the vantage point of the Deep Space Climate Observatory satellite, both the Earth and the moon are visible in space, visually confirming the moon’s smaller size relative to Earth as they orbit.
How Does Our Moon Compare to Other Moons in the Solar System?
While significantly smaller than Earth, our moon holds its own when compared to other moons in our solar system. According to The Nine Planets website, our moon is the largest moon in the solar system relative to the size of its planet. In terms of absolute size, it ranks as the fifth largest moon overall. Jupiter’s moon Ganymede claims the title of the largest moon in the solar system.
The 10 Largest Moons in Our 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 Phenomenon: When the Moon Appears Larger
A supermoon is seen rising above the skyline of Washington D.C., as captured in a photograph, illustrating how the moon can appear larger and brighter in the night sky during this astronomical event.
Have you ever noticed that the moon sometimes looks exceptionally large? This often happens during a phenomenon known as a supermoon. The moon’s orbit around Earth is not perfectly circular; it’s elliptical. This means the distance between the Earth and moon varies throughout the month. The point in the moon’s orbit when it is closest to Earth is called perigee, 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 initially coined by astrologer Richard Nolle in 1979. Astronomically, this phenomenon is known as a perigee-syzygy moon, referring to the alignment of the Sun, Earth, and Moon. However, “supermoon” is certainly more catchy and widely used.
Despite the hype, the difference in size and brightness isn’t always dramatically noticeable. NASA points out that a 30 percent increase in brightness can easily be masked by cloud cover or urban light pollution.
Related Explorations:
- What is a Blood Moon? Decoding Lunar Eclipses
- What is a Blue Moon? Understanding Rare Lunar Events
- How was the Moon Formed? Unraveling Lunar Origins
Noah Petro from NASA explains that the moon’s elliptical orbit is due to the gravitational influences of Earth, the Sun, and other planets in our solar system. These forces constantly “pulling and pushing on the moon,” creating opportunities for close passes like those resulting in supermoons.
Supermoons occur roughly every 414 days on average. Interestingly, 2016 had three supermoons. The closest supermoon of recent times occurred in November 2016, and the moon won’t be as close to Earth again until November 25, 2034.
The Moon Illusion: Why It Looks Bigger Near the Horizon
The moon appears exceptionally large as it rises behind the Cerro Armazones mountain in Chile, demonstrating the moon illusion, an optical phenomenon where the moon seems larger when near the horizon.
Have you ever felt that the moon appears much larger when it’s rising or setting near the horizon? This is a well-known optical illusion called the moon illusion or Ponzo illusion. This effect, observed since antiquity, makes the moon seem bigger when viewed near objects on the horizon compared to when it’s high in the sky.
Despite being a long-observed phenomenon, there’s still no universally accepted scientific explanation for the moon illusion. NASA suggests one theory involving our perception of clouds. We typically see clouds overhead at relatively close distances, while horizon clouds can be much further away. If a horizon cloud appears the same size as an overhead cloud, our brain interprets it as being much larger due to its distance. This same logic may be applied to the moon near the horizon, making it appear larger.
Another hypothesis proposes that the illusion arises from comparing the moon’s size to terrestrial objects like trees and buildings on the horizon. When contrasted with these nearby objects, the moon can seem to “loom large.” However, when viewed high in the vastness of the open sky, without such points of reference, it appears smaller.
You can test if it’s truly an illusion. Hold your thumb up to the moon when it’s near the horizon and then again when it’s higher in the sky. You’ll find that the moon’s size relative to your thumbnail remains consistent, demonstrating that the perceived size difference is indeed an illusion created by our perception.
Additional Resources for Lunar Exploration
For more quick facts about the moon, explore these educational infographics by the ESA: ESA Moon Fast Facts. To discover how much you would weigh on the moon and other celestial bodies, check out this interactive tool from the Exploratorium science museum: Exploratorium Weight on Other Worlds. For more on the Ponzo illusion, The Illusion Index at the University of Glasgow offers a comprehensive collection of illusions: The Illusion Index – Ponzo Illusion.
Bibliography
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Daisy Dobrijevic
Reference Editor
Daisy Dobrijevic is a Reference Editor at Space.com.
With contributions from Space.com staff.
