Delving into the cosmos, one of the most fundamental questions about our celestial neighbor, the Moon, is simply: how far away is it? The answer, surprisingly, isn’t a single number. The distance between Earth and the Moon is a dynamic figure, constantly changing as the Moon journeys around our planet in an elliptical orbit. Let’s embark on a journey to explore the fascinating variations in lunar distance and understand the captivating effects they produce.
The Moon’s Elliptical Path Around Earth
Imagine the Moon’s orbit around Earth not as a perfect circle, but as a slightly stretched circle, an ellipse. This elliptical shape is a common characteristic of orbits in space. To quantify how much an orbit deviates from a perfect circle, astronomers use a measure called ‘eccentricity’. This value ranges from 0 to 1, where 0 represents a perfect circle. Venus boasts the most circular orbit in our solar system, with an eccentricity near 0, while Mercury’s orbit is more elliptical.
The Moon’s orbit has an eccentricity of 0.05, indicating a subtle but significant elliptical path. Adding to this complexity, Earth isn’t perfectly centered within this lunar orbit. Instead, Earth resides at one of the foci of the ellipse, meaning it’s positioned closer to one side of the Moon’s orbital path than the other.
Apogee, Perigee, and Average Distance: Decoding Moon’s Distance
When astronomers discuss the distance to the Moon, they often refer to three key measurements: apogee, perigee, and average distance.
At its apogee, the Moon reaches its farthest point from Earth, approximately 405,696 kilometers (252,088 miles) away. The term ‘apogee’ itself originates from Greek, where ‘apo’ signifies ‘away from’.
Conversely, at perigee, the Moon is at its closest point to Earth, a mere 363,104 kilometers (225,623 miles) distant. ‘Perigee’ also has Greek roots, with ‘peri’ meaning ‘near’.
The difference between apogee and perigee is a substantial 42,592 km (26,465 miles), exceeding Earth’s diameter by more than threefold! To simplify discussions, astronomers also use an average Earth-Moon distance of 384,400 km (238,855 miles).
Diagram depicting the Moon’s elliptical orbit with labels for apogee and perigee, highlighting the varying distances from Earth. The eccentricity is exaggerated for clarity.
Supermoon and Micromoon: Visualizing Distance Variations
Do these variations in distance have noticeable effects? Indeed, they do, subtly influencing the appearance of the full Moon. When a full Moon coincides with perigee, we witness a supermoon. Conversely, a full Moon at apogee is termed a micromoon.
A supermoon appears slightly larger and brighter in our sky compared to an average full moon, while a micromoon appears somewhat smaller. However, these differences are subtle and often require careful observation or photographic comparison to appreciate fully. Supermoons can appear up to 14% larger and 30% brighter than micromoons.
A side-by-side comparison of a micromoon and a supermoon as viewed from Earth, illustrating the apparent size and brightness difference caused by varying Earth-Moon distances.
Lunar Distance and Tidal Forces
The Moon’s gravitational pull, along with the Sun’s gravity and Earth’s rotation, are the primary drivers of our ocean tides. The strongest tides, known as spring tides, occur during full and new moons when the gravitational forces of the Sun and Moon align and reinforce each other. Interestingly, spring tides have no connection to the spring season; the name refers to the way tides “spring” higher and recede lower.
When the Moon is at perigee, its gravitational influence on Earth is slightly stronger, leading to slightly larger tidal ranges – the difference between high and low tides. Conversely, at apogee, the tidal range is marginally smaller. However, these distance-related variations in tidal height are relatively minor, typically around 5cm.
Diagram illustrating the positions of the Sun, Earth, and Moon during spring tides (aligned) and neap tides (perpendicular), explaining the gravitational forces and their effect on tidal ranges.
Moon’s Distance from the Sun: A Shared Journey
Since the Moon orbits the Earth, and Earth orbits the Sun, both celestial bodies share a similar average distance from our star. On average, both Earth and Moon are approximately 150 million kilometers (93 million miles) from the Sun. This immense distance means that sunlight takes about eight minutes to reach both Earth and the Moon. Imagine, if the Sun were to suddenly cease shining, we wouldn’t know about it for a full eight minutes!
Journey Time to the Moon: How Long Does It Take?
The time it takes to travel to the Moon isn’t fixed; it depends on the speed of travel and the chosen trajectory. A simple flyby mission, not requiring lunar orbit insertion, can be much faster.
Record-Breaking Speed: 8 Hours and 35 Minutes
The fastest journey to the Moon was achieved by the New Horizons spacecraft, completing the trip in an astonishing 8 hours and 35 minutes. However, this was a flyby mission, passing the Moon en route to Pluto.
Luna 1: A Near Miss in 34 Hours
The first attempt to reach the Moon was by the Soviet Union’s Luna 1 in 1959. While it didn’t achieve lunar orbit, it reached the Moon’s vicinity within 34 hours (1 day 10 hours), a significant milestone in space exploration.
SMART 1: A Fuel-Efficient 13.5-Month Journey
The European Space Agency’s SMART 1 spacecraft, launched in 2003, utilized an ion engine for exceptional fuel efficiency. This came at the cost of speed, with the journey taking 13.5 months.
Apollo Missions: Crewed Lunar Voyages in Around 3 Days
Human missions to the Moon, like the Apollo program, typically take longer than robotic missions. The nine crewed Apollo missions, including orbiters and landers, averaged just over 78 hours (3 days 6 hours) to reach lunar orbit. Apollo 8 achieved the quickest transit at 2 days, 21 hours, and 8 minutes, while Apollo 17 took the longest at 3 days, 14 hours, and 41 minutes, including time spent in Earth orbit.
Driving to the Moon: A Hypothetical Road Trip
For a fun perspective, imagine driving to the Moon at a constant speed of 40 mph. Such a journey would take approximately 5,791.375 hours, or roughly 241 days! Of course, this is purely hypothetical and doesn’t account for the need for a rocket-powered car and the challenges of space travel.
Lunar Orbit Duration: Month vs. Sidereal Period
The familiar lunar phase cycle, from new moon to new moon, takes approximately 29.5 days. This is often mistakenly considered the Moon’s orbital period. However, the actual time it takes for the Moon to complete one orbit around Earth, relative to distant stars, is shorter: 27.3 days.
This difference arises because we measure the lunar phase cycle relative to the Sun, while the true orbital period is measured against ‘fixed’ points in space – distant stars. As the Moon orbits Earth, Earth also moves in its orbit around the Sun. Therefore, the Moon needs a little extra time to “catch up” and return to the same position relative to the Sun, resulting in the longer 29.5-day lunar phase cycle.
Lunar Day Length: A Slow Rotation
Have you ever noticed that the same side of the Moon always faces Earth? This is because the Moon’s rotation period is synchronized with its orbital period – a phenomenon called synchronous rotation. With minor exceptions called librations, we consistently see the near side of the Moon. The far side remained a mystery until space exploration unveiled it, sometimes mistakenly referred to as the “dark side” not because it lacks sunlight, but because it was once unknown.
This synchronous rotation results in a very slow lunar day. It takes 29.5 Earth days for the Moon to complete one rotation, meaning a lunar day – from midday to midday – also lasts about 29.5 Earth days. Daylight on the Moon lasts approximately two Earth weeks, followed by another two weeks of night. Combined with the Moon’s lack of atmosphere, this leads to extreme temperature variations, from over 100°C during the day to around -150°C at night.
Is the Moon Receding? The Moon’s Slow Drift Away
Remarkably, the Moon is gradually moving away from Earth at a rate of about 3.8 centimeters per year. This subtle recession has been precisely measured using laser reflectors left on the Moon’s surface by Apollo astronauts and Soviet Lunokhod rovers. By bouncing laser beams off these mirrors and measuring the return time, scientists can accurately determine the Earth-Moon distance and track its subtle changes.
The Lunar Laser Ranging Retroreflector, deployed on the Moon by Apollo 11, used to precisely measure the Earth-Moon distance by reflecting laser beams back to Earth.
In the distant future, this lunar recession will have noticeable effects. Total solar eclipses, as we know them, will eventually become impossible as the Moon will appear too small to completely block the Sun. However, this is a very long-term process. While the Moon will theoretically stop receding in about 50 billion years, the Sun will have already evolved into a red giant in about 5 billion years, dramatically altering the solar system and likely causing the Moon to eventually disintegrate due to tidal forces as it spirals back towards Earth.
Article written by an astronomer at the Royal Observatory, Greenwich.
Updated: 01/06/2018 by Affelia Wibisono
