Have you ever gazed up at the Moon and wondered, “How Far From Earth Is The Moon?” It seems so close, yet it’s a celestial body residing hundreds of thousands of kilometers away. The distance isn’t constant; the Moon’s orbit is not a perfect circle, leading to variations in its proximity to our planet. Understanding the distance to the Moon is key to grasping its influence on Earth, from tides to eclipses, and the incredible journeys humans have undertaken to reach it.
Understanding the Moon’s Elliptical Orbit
Like all orbits in space, the Moon’s path around Earth isn’t a perfect circle but an ellipse. This means it’s an oval shape, causing the distance between the Earth and Moon to change throughout its monthly journey. Astronomers use a measure called ‘eccentricity’ to describe how much an orbit deviates from a perfect circle.
Eccentricity is expressed as a number between 0 and 1. An eccentricity of 0 represents a perfect circle. The closer to 0, the more circular the orbit. For example, Venus has the least eccentric and most circular orbit in our solar system at 0.007, while Mercury’s orbit is the most elliptical among planets, with an eccentricity of 0.2.
The eccentricity of the Moon’s orbit is 0.05. This relatively low number indicates that while elliptical, the Moon’s orbit is not wildly stretched. Adding to this, Earth isn’t exactly in the center of the Moon’s orbital path. Instead, it resides at one of the foci of this ellipse, meaning Earth is closer to one side of the Moon’s orbit than the other.
Apogee, Perigee, and Average Distance: Key Lunar Distances
When discussing the distance between Earth and the Moon, astronomers often refer to three key measurements to account for its elliptical orbit: apogee, perigee, and average distance.
At its apogee, the Moon is at its furthest point from Earth. The word ‘apogee’ itself comes from Greek roots meaning ‘away from Earth’. At apogee, the Moon is approximately 405,696 kilometers (252,088 miles) distant.
Conversely, perigee marks the Moon’s closest approach to Earth. ‘Perigee’ also has Greek origins, signifying ‘near Earth’. When the Moon reaches perigee, it is only about 363,104 kilometers (225,623 miles) away.
The difference between apogee and perigee is a significant 42,592 kilometers (26,465 miles). To put this into perspective, this distance is more than three times the diameter of the Earth itself!
Considering these varying distances, the average distance between the Earth and the Moon is 384,400 kilometers (238,855 miles). This average figure is often cited when answering the question “how far from earth is the moon” in general terms and provides a good benchmark for understanding lunar distance.
Diagram illustrating the Moon’s elliptical orbit around the Earth, highlighting apogee as the farthest point and perigee as the closest point in its orbit.
Supermoon and Micromoon: The Visual Difference
Do these variations in distance – apogee and perigee – have any noticeable effects for us on Earth? While the Moon’s actual size doesn’t change, its apparent size in our sky does fluctuate slightly depending on where it is in its orbit.
When a full moon occurs around the time the Moon is at perigee, it’s often called a supermoon. Because it’s closer to Earth, a supermoon appears slightly larger and brighter than an average full moon.
Conversely, a full moon occurring near apogee is sometimes referred to as a micromoon. At its furthest point, the Moon appears a bit smaller in the sky.
While these terms are popular, the visual difference between a supermoon and a micromoon is subtle to the naked eye. You might not easily notice the size difference just by looking up. The difference, about 14% in size and 30% in brightness, is more apparent in side-by-side photographic comparisons.
A visual comparison showing a micromoon appearing smaller and a supermoon appearing larger, emphasizing the difference in apparent size due to varying Earth-Moon distances.
The Moon’s Influence: Tides on Earth
One of the most significant ways the Moon affects Earth is through our tides. Tides are primarily caused by the gravitational pull of the Moon and the Sun, combined with the Earth’s rotation. The Moon’s gravitational force is the dominant factor due to its proximity.
When the Sun, Earth, and Moon align – during a full moon or new moon – their gravitational forces combine. This results in higher high tides and lower low tides, known as spring tides. Despite the name, spring tides have nothing to do with the spring season; they ‘spring forth’ in terms of tidal range.
When the Sun and Moon are at right angles to each other relative to Earth (during quarter moons), their gravitational forces partially cancel each other out, leading to smaller tidal ranges called neap tides.
Interestingly, the Moon’s perigee and apogee also have a minor influence on tides. At perigee, the Moon’s slightly stronger gravitational pull can increase the tidal range by about 5cm. Conversely, at apogee, the tidal range might be about 5cm smaller than average. While these are not dramatic changes, they demonstrate how the varying distance to the Moon subtly affects Earth’s oceans.
Diagram illustrating the alignment of the Sun, Earth, and Moon during spring tides (stronger tides) and neap tides (weaker tides), explaining the gravitational influences on ocean levels.
Distance to the Sun – Shared Journey
While we focus on “how far from earth is the moon,” it’s also important to consider the Moon’s distance from the Sun. Since the Moon orbits the Earth, and the Earth orbits the Sun, both celestial bodies are, on average, at a similar distance from the Sun.
The Earth and Moon are both approximately 150 million kilometers (93 million miles) from the Sun on average. This vast distance is so immense that light from the Sun, traveling at an incredible speed of 300,000 kilometers per second, takes about eight minutes to reach us. This means if the Sun were to suddenly disappear, we wouldn’t know about it for another eight minutes!
Journey to the Moon: How Long Does It Take?
Humanity’s fascination with the Moon has led to numerous missions to reach our celestial neighbor. So, how long does it take to get to the Moon from Earth? The answer isn’t fixed; it depends on various factors, including speed, trajectory, and mission objectives.
On average, a journey to the Moon takes about three days. However, this is just a general estimate.
For a flyby mission, where a spacecraft doesn’t need to slow down to orbit the Moon, the travel time can be significantly shorter. For missions aiming to orbit or land, more time is needed for deceleration and orbital insertion.
Here are some examples of travel times for different lunar missions:
-
Fastest Journey: The record for the quickest journey to the Moon is held by the New Horizons spacecraft, which achieved a flyby in just 8 hours and 35 minutes. However, New Horizons was not intended to stop at the Moon; it was on its way to Pluto and beyond.
-
Early Lunar Missions: The Soviet Union’s Luna 1, the first spacecraft to attempt reaching the Moon in 1959, took 34 hours (1 day 10 hours) to reach the vicinity of the Moon. It didn’t enter orbit, but it demonstrated a relatively rapid transit.
-
Fuel-Efficient but Slower: The European Space Agency’s SMART 1 spacecraft, launched in 2003 and powered by a fuel-efficient ion engine, took a much longer 13.5 months to reach the Moon. Its priority was fuel efficiency, not speed.
-
Apollo Missions (Human Spaceflight): The Apollo missions, which carried humans to the Moon, generally took longer than robotic flybys but were still relatively quick. On average, crewed Apollo missions took just over 78 hours (3 days 6 hours) to enter lunar orbit. Apollo 8 was the fastest at 2 days, 21 hours, and 8 minutes, while Apollo 17 was the longest at 3 days, 14 hours, and 41 minutes.
For a more relatable perspective, consider this:
- Driving Analogy: If you could drive to the Moon at a constant speed of 40 miles per hour (about 64 km/h), it would take approximately 5,791.375 hours, or over 241 days! Of course, this is a purely hypothetical scenario, requiring a very special rocket car!
Lunar Orbit and Day Length
Understanding “how far from earth is the moon” also connects to the Moon’s orbital period and its rotation. The lunar phase cycle, from new moon to new moon, takes approximately 29.5 days. This cycle is determined by the changing relative positions of the Sun, Earth, and Moon.
However, the time it takes for the Moon to complete one full orbit around the Earth, relative to distant stars, is slightly shorter: 27.3 days. This is known as the Moon’s sidereal period.
The difference arises because as the Moon orbits Earth, Earth is also moving in its orbit around the Sun. To complete a full cycle of phases, the Moon needs a little extra time to ‘catch up’ to the same position relative to the Sun.
Another fascinating aspect is the length of a day on the Moon. The Moon rotates on its axis at almost the same rate it orbits Earth. This synchronous rotation is why we always see the same ‘near side’ of the Moon from Earth. The ‘far side’ remained unseen until space exploration.
A lunar day, from midday to midday on the Moon, lasts about 29.5 Earth days. This results in approximately two weeks of daylight followed by two weeks of night on the Moon. Coupled with the lack of a significant atmosphere, this leads to extreme temperature variations, ranging from over 100°C (212°F) during the lunar day to around -150°C (-238°F) during the lunar night.
The Moon’s Recession: Moving Away from Earth
It might be surprising to learn that the answer to “how far from earth is the moon” is constantly changing, albeit very slowly. Astronomical measurements have revealed that the Moon is gradually moving away from the Earth at a rate of about 3.8 centimeters (around 1.5 inches) per year!
This subtle recession has been precisely measured using Lunar Laser Ranging. Astronauts on Apollo missions and Soviet lunar rovers left retroreflector mirrors on the Moon’s surface. Scientists on Earth can bounce laser beams off these mirrors and measure the time it takes for the laser light to return. Knowing the speed of light allows for highly accurate distance calculations.
Image of the retroreflector mirror placed on the Moon by Apollo 11 astronauts, used for the Lunar Laser Ranging Experiment to precisely measure the Earth-Moon distance.
This gradual increase in lunar distance has long-term implications. In the distant future, total solar eclipses, as we know them, will become a thing of the past. As the Moon moves further away, it will appear smaller in the sky, eventually becoming too small to completely block out the Sun’s disk.
However, this is a process that will take billions of years. Theory suggests the Moon will eventually stop receding from Earth in about 50 billion years. But long before that, in approximately 5 billion years, the Sun will enter its red giant phase, expanding and dramatically altering the solar system. During this phase, the Sun’s expansion is expected to push the Moon back towards Earth, eventually causing it to disintegrate due to tidal forces.
In conclusion, “how far from earth is the moon” is not a simple, static number. It’s a dynamic distance that varies due to the Moon’s elliptical orbit and is slowly increasing over vast timescales. Understanding these lunar distances provides crucial insights into the Earth-Moon system, tidal forces, space travel, and the long-term evolution of our celestial neighborhood.
This article has been written by an astronomer at the Royal Observatory, Greenwich
01/06/2018: Affelia Wibisono
