Learn About Moon Distance and Orbital Changes

Understanding the Moon's Distance From Earth

The Moon orbits Earth at a varying distance that changes throughout each lunar month. The average distance between Earth and the Moon is approximately 238,855 miles or about 384,400 kilometers. However, this distance is not constant. The Moon's orbit is elliptical rather than perfectly circular, which means it moves closer to and farther from Earth in a predictable pattern.

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When the Moon reaches its closest point to Earth, called perigee, it is about 225,623 miles away (363,228 kilometers). During perigee, the Moon appears noticeably larger in the sky to observers on Earth. This occurs because the Moon is simply nearer to us, making it look bigger even though its actual size hasn't changed. Perigee happens roughly once each month as the Moon completes its orbit.

When the Moon reaches its farthest point from Earth, called apogee, it is approximately 252,088 miles away (405,696 kilometers). At apogee, the Moon appears slightly smaller than when it is at perigee. The difference in apparent size between perigee and apogee is subtle but measurable by astronomers and can be noticed by careful observers with binoculars or telescopes.

The variation between perigee and apogee distance amounts to roughly 26,465 miles or 42,600 kilometers. While this might sound like a small percentage compared to the overall distance, it represents a significant difference in the Moon's gravitational effects on Earth. This distance variation affects ocean tides, which are influenced by the Moon's gravitational pull.

Practical takeaway: When viewing the Moon, remember that its apparent size changes slightly each month due to its elliptical orbit. Photographing the Moon at perigee versus apogee over several months reveals this difference clearly.

How the Moon's Orbit Works and Changes Over Time

The Moon completes one full orbit around Earth approximately every 27.3 days. This period, called the sidereal month, describes the time it takes for the Moon to return to the same position relative to distant stars. During this orbital journey, the Moon passes through all four phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent.

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The Moon's orbit is not perfectly stable. Over very long timescales—thousands of years—the Moon's orbital characteristics gradually change. One significant change involves the Moon's distance from Earth. Currently, the Moon is moving away from Earth at a rate of approximately 1.5 inches or 3.8 centimeters per year. Scientists measure this change using laser reflectors that astronauts left on the lunar surface during the Apollo missions. By bouncing laser beams off these reflectors, researchers can calculate the exact distance to the Moon with extraordinary precision.

This gradual movement away from Earth occurs because of tidal friction. The Moon's gravity pulls on Earth's oceans, creating tides. These tides create friction that gradually slows Earth's rotation and transfers some of Earth's rotational energy to the Moon, causing it to spiral outward. This process has been occurring for billions of years and will continue into the distant future.

As the Moon moves farther away, its orbital speed decreases slightly, and its orbital period lengthens. In the distant past, the Moon was much closer to Earth, and days were shorter. Over millions of years, days have gradually become longer because the Moon has been moving away. Geologists studying ancient rock formations and fossil records have found evidence of this change in ancient coral rings and tidal patterns preserved in sedimentary rocks.

Practical takeaway: Track the Moon's phases on a calendar to become familiar with its 27.3-day orbital cycle. Noting when full moons occur helps predict when they will happen in future months.

The Perigee Full Moon (Super Moon) Explained

When a full moon occurs at or near the Moon's perigee—its closest approach to Earth—astronomers call this event a "perigee full moon." The popular media term "supermoon" refers to this same phenomenon. During a supermoon, the Moon appears larger and brighter than during a typical full moon because it is physically closer to Earth.

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The apparent size increase during a supermoon compared to a full moon at apogee is approximately 14 percent larger and roughly 30 percent brighter. While this might not sound dramatic, the difference is noticeable to observers, especially when comparing photographs of a supermoon to images of a distant full moon. Many people report feeling impressed by the Moon's size during these events, even though the actual difference is modest.

Supermoons occur roughly once or twice per year on average, though the exact timing varies. Sometimes the full moon occurs several days before or after perigee, so not every full moon at perigee qualifies as a supermoon. Astronomers debate precise definitions, with some using strict criteria and others applying looser standards. Generally, if the full moon occurs within about one day of perigee, observers recognize it as a supermoon.

Several supermoons occurred consecutively in 2020, with three in a row from January through March. This sequence happened because the Moon's perigee and the full moon dates aligned favorably during those months. Other years may have fewer supermoons or only one in the entire year, depending on how the lunar calendar aligns with the Moon's orbital position.

Practical takeaway: Check astronomical calendars a few months in advance to identify when supermoons will occur in your area. Plan viewing sessions during clear nights when the Moon is high in the sky for the best viewing experience.

Measuring and Tracking Lunar Distance Changes

Scientists measure the Moon's distance from Earth using a technique called laser ranging. During the Apollo missions between 1969 and 1972, astronauts placed reflectors on the lunar surface. These reflectors, called retroreflectors, contain corner cubes that bounce light beams directly back toward their source. By sending laser pulses from Earth-based stations toward these reflectors and measuring how long the light takes to return, scientists calculate the distance with precision of a few centimeters.

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The International Lunar Laser Ranging Observation (ILLRO) project coordinates observations from multiple stations around the world, including facilities in New Mexico, France, Germany, Australia, and China. Teams collect data night after night when weather permits and the Moon is above the horizon. Over decades, this accumulated data has revealed that the Moon moves away from Earth at about 3.8 centimeters annually—roughly the rate at which fingernails grow.

Researchers also track the Moon's changing distance by analyzing historical astronomical observations. Ancient eclipse records, some dating back thousands of years, provide clues about the Moon's past distance. Eclipse timing depends on the Moon's size in the sky, which correlates with its distance. By studying eclipse records from various ancient civilizations and comparing them with modern eclipse predictions, scientists can infer how the Moon's distance has changed over millennia.

Another tracking method involves monitoring how the Moon's gravity affects Earth's rotation. The Moon's gravitational pull creates tides that gradually slow Earth's spin. By measuring changes in day length using atomic clocks and comparing historical records of eclipse timings, scientists determine how much the Moon must have moved outward to produce the observed changes.

Practical takeaway: Data about lunar distance is continuously updated by international scientific organizations. Websites from organizations like NASA and the Lunar Reconnaissance Orbiter project provide current information about the Moon's distance and orbital parameters.

How Lunar Distance Affects Tides and Earth's Climate

The Moon's gravity creates Earth's tides by pulling on the planet's oceans. When the Moon is at perigee—its closest point—its gravitational pull is strongest, creating higher high tides and lower low tides. This phenomenon is called a king tide or spring tide when combined with the Sun's gravitational effects. During these periods, tidal ranges are more extreme than average, with water rising higher than usual and receding lower than normal.

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Conversely, when the Moon is at apogee—its farthest point—its gravitational pull is weaker, producing more modest tidal ranges called neap tides. The timing of supermoons (perigee full moons) can intensify coastal flooding in areas already prone to tidal issues. Some researchers study whether supermoons combined with seasonal weather patterns contribute to increased nuisance flooding in coastal cities