Perigee is the point in an orbiting body’s path where it comes closest to Earth, and for most people, the term matters because of the Moon. The lunar perigee occurs roughly once every 27.5 days, bringing the Moon about 50,000 kilometers closer than its farthest point. That difference is large enough to visibly affect the Moon’s size and brightness, shift tidal patterns, and even influence animal behavior in the ocean. But perigee is not just a lunar concept; it applies to any object orbiting Earth, from satellites to the International Space Station to passing asteroids.
Why the Moon’s Distance Keeps Changing
The Moon does not orbit Earth in a perfect circle. Its path is an ellipse, meaning the distance between the two bodies stretches and compresses over the course of each orbit. At perigee, the Moon sits roughly 363,000 kilometers away. At apogee, the opposite extreme, it is about 405,000 kilometers away. That gap of around 42,000 kilometers is comparable to the circumference of Earth itself, which gives you a sense of just how pronounced the variation is.
This elliptical shape is not static. Gravitational tugging from the Sun, the slight oblateness of Earth, and other factors cause the orientation of the Moon’s ellipse to rotate slowly over time. The line connecting perigee and apogee, known as the line of apsides, completes a full rotation roughly every 8.85 years. That means the calendar date on which perigee falls drifts gradually, and the exact perigee distance varies from month to month. Some perigees bring the Moon closer than others, with the tightest approaches occasionally dipping below 357,000 kilometers.
Supermoons and What You Actually See
When a full moon coincides with a particularly close perigee, it earns the popular label “supermoon.” The term was coined in astrology circles and adopted by the media, but the underlying phenomenon is real, if sometimes overhyped. A full moon at perigee appears about 14 percent larger in diameter and roughly 29 percent brighter than a full moon at apogee, sometimes called a “micromoon.”1arXiv. Micro moon versus macro moon: Brightness and size Those numbers come from straightforward geometry and the inverse-square law governing light intensity.
In practice, the size increase is hard to notice with the naked eye unless you are comparing photographs side by side. The Moon has no fixed reference frame in the sky, so your brain does not register a 14 percent change in angular diameter the way it would if you held two different coins next to each other. The brightness boost is more perceptible, particularly if you are outdoors on a clear night. Moonlit landscapes during a supermoon can cast noticeably sharper shadows, and photographers often notice the difference when shooting long exposures.
The flip side gets less press. A full moon at apogee is dimmer and smaller, and these “micromoons” happen just as regularly. The difference between the brightest supermoon and the faintest micromoon in a given year amounts to a meaningful change in ambient nighttime light levels, something that matters for wildlife as much as for stargazers.
How Perigee Shapes the Tides
Tides are driven by the gravitational pull of the Moon and, to a lesser extent, the Sun. Because gravitational force strengthens with proximity, perigee amplifies the Moon’s tidal influence. The tidal force varies roughly as the inverse cube of distance, which means even a modest percentage change in the Moon’s distance translates to a measurable difference in tidal range. At perigee, tidal forces can be about 20 percent stronger than at apogee.
This effect matters most when perigee lines up with a new or full moon, the phases when the Sun and Moon pull in roughly the same direction to produce spring tides. A perigean spring tide can push water levels several centimeters higher than a typical spring tide, flooding low-lying coastal roads and amplifying wave action along shorelines. Coastal communities already prone to tidal flooding, particularly in flat terrain like parts of the southeastern United States or the Netherlands, feel these perigean events most acutely. When a strong onshore wind or storm surge arrives at the same time, the combination can turn what would have been a modest flooding event into a serious one.
Over longer timescales, the slow rotation of the Moon’s orbital ellipse creates an 8.85-year cycle in how perigee aligns with the lunar phases. Coastal planners and tidal researchers track this cycle because it modulates the frequency of unusually high tides over roughly a decade. There is also a longer 18.61-year cycle related to the tilt of the Moon’s orbit, which interacts with the perigee cycle to produce complex patterns in extreme tidal levels.
Measuring the Moon’s Distance to Millimeter Precision
Our ability to pin down perigee distances has improved dramatically since the Apollo program. Between 1969 and 1972, astronauts placed five retroreflector arrays on the lunar surface. Observatories on Earth fire short laser pulses at these arrays and time the round trip, a technique called lunar laser ranging. By dividing the travel time by the known speed of light, scientists calculate the Earth-Moon distance with extraordinary precision.
The APOLLO station at Apache Point Observatory in New Mexico has been running since 2006 and achieves a median nightly accuracy of 1.7 millimeters.2Publications of the Astronomical Society of the Pacific. Fifteen Years of Millimeter Accuracy Lunar Laser Ranging with APOLLO: Data Set Characterization That is astonishing given that the target is nearly 400,000 kilometers away. The precision is limited partly by the design of the reflectors themselves: because each Apollo-era array contains many small corner cubes spread across a flat panel, slight tilting of the Moon causes the reflected photons to arrive back at slightly different times, blurring the measurement. Next-generation reflectors now being built use a single large corner cube instead, eliminating that spread and promising even sharper range data in the future.3The Planetary Science Journal. Next-generation Laser Ranging at Lunar Geophysical Network and Commercial Lander Payload Service Sites
This millimeter-level ranging does far more than just tell us the perigee distance for a given month. It feeds into tests of general relativity, measurements of how Earth’s core and mantle respond to tidal forces, and a precise record of how quickly the Moon is receding from Earth over time.
The Moon Is Slowly Moving Away
Each perigee is ever so slightly farther than the one before it. Tidal interactions between Earth and the Moon transfer energy from Earth’s rotation into the Moon’s orbit, gradually pushing the Moon outward. Lunar laser ranging over more than four decades has pinned down this recession rate at about 38 millimeters per year.4PubMed Central. The past and present Earth-Moon system: the speed of light stays steady as tides evolve That is roughly the rate at which your fingernails grow.
But this present-day rate has not been constant throughout Earth’s history. Geological evidence, including ancient tidal deposits called rhythmites, preserves a record of past tidal cycles. Rhythmites from roughly 620 million years ago indicate that the Moon was only about 3.5 percent closer than it is today, and the average recession rate over that span was about 22 millimeters per year, little more than half the current rate.5Reviews of Geophysics. Geological constraints on the Precambrian history of Earth’s rotation and the Moon’s orbit The difference is mostly explained by changes in the shape of Earth’s ocean basins. Continental drift reshapes coastlines and seafloor topography over hundreds of millions of years, altering how tidal energy dissipates and, in turn, how efficiently the Moon is pushed outward.
This slowdown also means Earth’s rotation has been gradually decelerating. Those same ancient rhythmites indicate that 620 million years ago, a day lasted only about 22 hours, and the year contained around 400 solar days.6Reviews of Geophysics. Geological constraints on the Precambrian history of Earth’s rotation and the Moon’s orbit In a very real sense, perigee has been slowly stretching for billions of years, carrying the Moon away from Earth and lengthening our days in the process.
Perigee for Satellites and Spacecraft
The term perigee is not reserved for the Moon. Any object orbiting Earth has a perigee if its orbit is elliptical. Satellites in highly elliptical orbits, such as Molniya orbits used for communications coverage over high latitudes, sweep in close to Earth at perigee and arc far out at apogee. The perigee altitude determines how quickly the satellite passes through the densest part of the atmosphere, how much atmospheric drag it encounters, and how rapidly its orbit decays without periodic boosts.
Spacecraft designers also exploit perigee strategically. A technique called aerobraking uses the thin upper atmosphere at perigee to slow a spacecraft down, gradually circularizing an initially elongated orbit without burning fuel. This has been successfully performed in eight missions at Earth, Venus, and Mars since its first demonstration in 1991.7Acta Astronautica. Aerobraking: Review of the state of the art and required developments Each pass through perigee skims the spacecraft through just enough atmosphere to bleed off orbital energy, lowering the apogee on each subsequent loop until the orbit is nearly circular. The process can take months and requires careful monitoring because dipping too deep at perigee risks overheating or even losing the spacecraft.
For objects in low Earth orbit, like the International Space Station, perigee altitude matters for a more mundane reason: atmospheric drag. Even at altitudes above 400 kilometers, there is enough residual atmosphere to slow the station gradually. At perigee, where the station passes through slightly denser air, the drag is greatest. Without regular reboosts from visiting spacecraft, the ISS would lose altitude and eventually re-enter the atmosphere.
Beyond Earth’s Orbit
Astronomers use a family of related terms depending on what is being orbited. A close approach to the Sun is perihelion, a close approach to Jupiter is perijove, and a close approach to Saturn is perikrone (or sometimes pericrone). The physics is the same in every case: an elliptical orbit means the distance between two bodies oscillates, and the point of closest approach concentrates gravitational effects.
This matters profoundly for moons of the outer planets. Saturn’s moon Enceladus, for example, has a slightly elliptical orbit that brings it closer to and farther from Saturn with each revolution. The changing gravitational squeeze generates internal friction that heats the moon from within, a process called tidal dissipation. This internal heating is what keeps a global ocean of liquid water beneath Enceladus’s icy crust, feeding the dramatic water-ice geysers that the Cassini spacecraft photographed erupting from its south pole.8PubMed Central. Origin and Evolution of Enceladus’s Tidal Dissipation Without the orbital eccentricity that creates a meaningful difference between closest and farthest approach, there would be no tidal flexing, no internal heat, and likely no subsurface ocean.
Jupiter’s moon Io is an even more extreme example. Locked in an orbital resonance with Europa and Ganymede, Io’s eccentricity is continuously refreshed, and the resulting tidal heating makes it the most volcanically active body in the solar system. The mechanism is the same one that operates between Earth and the Moon, just scaled up by a much more massive central planet and a much closer orbit.
Animal Behavior and Perigean Tides
The biological world does not track perigee directly, but it responds to the tides that perigee intensifies. Many marine organisms time key life events, especially reproduction, to tidal rhythms. The Ucides cordatus mangrove crab along the Brazilian coast provides a well-studied example. These crabs emerge en masse for mating events called “andadas,” and researchers found that the timing of these mass emergences shifts between new and full moons depending on which phase produces the higher-amplitude tides.9Animal Behaviour. Effects of geophysical cycles on the rhythm of mass mate searching of a harvested mangrove crab The crabs appear to anticipate the highest spring tides, which come about a month before their larvae would need maximum tidal flushing to survive. This kind of anticipatory behavior likely relies on an internal biological clock rather than a direct response to water levels.
Perigean spring tides, being the most extreme, may serve as particularly strong cues for organisms that depend on peak tidal flushing for larval dispersal, spawning, or foraging access. Coral spawning on some reefs, the migration of grunion onto California beaches, and the egg-laying cycles of horseshoe crabs have all been linked to spring tide timing, and the influence of perigee on those tides adds another layer of variation that these organisms have to navigate. Research in this area is still patchy. The tidal signal is clear, but teasing apart the specific contribution of perigee from the broader rhythm of spring and neap tides requires long data sets and careful statistical work.
Common Misconceptions About Perigee
Several misunderstandings circulate widely enough to be worth addressing. The most persistent is that supermoons are dramatically larger than normal full moons. As noted, the actual size increase is about 14 percent in apparent diameter, which is subtle to the naked eye. The spectacular “giant moon” photos you see online are typically shot with a telephoto lens near the horizon, where the well-known Moon illusion, a trick of perception unrelated to orbital distance, makes the Moon appear enormous regardless of whether it is at perigee.
Another common claim is that perigee triggers earthquakes or volcanic eruptions. This idea resurfaces after every major seismic event that happens to fall near a perigee date. While the Moon’s gravity does exert a tiny tidal stress on Earth’s crust, and some studies have found weak statistical associations between tidal forcing and very small tremors, the effect is far too small to trigger significant earthquakes. Major quakes release energy many orders of magnitude beyond what tidal stresses can add, and there is no credible pattern linking perigee dates to large seismic events.
A subtler misconception is that the Moon’s recession means perigee is getting dramatically farther in a human lifetime. At 38 millimeters per year, the change over a century is less than 4 meters. That is meaningful over geological time, but completely imperceptible to anyone observing the Moon today. The supermoons you see this decade are, for all practical purposes, the same as those your great-grandparents saw.
Watching Perigee Yourself
You do not need a telescope to appreciate perigee, but you do need realistic expectations. The best way to notice the size difference is to photograph the full moon at perigee and again at apogee, about six months apart, using the same lens and settings, then compare the images. The brightness difference is easier to perceive in real time, especially if you spend time outdoors regularly under moonlight and develop a feel for how bright a “normal” full moon appears.
Astronomical almanacs and many smartphone apps list perigee dates and distances for each month. If you want the most dramatic supermoon, look for months when perigee falls within a few hours of the exact full moon, which happens only a few times per year. The Moon’s distance at those events can be 10,000 or more kilometers closer than an average perigee, compounding the visual effect. Conversely, if you are an amateur astronomer trying to observe faint deep-sky objects, a micromoon full moon at apogee gives you the least sky-brightening interference. Timing your observing sessions to avoid perigee full moons can noticeably improve what you see through a telescope.