Perigee and Apogee – Definition, Moon Distance, and Effects

Perigee and apogee describe the closest and farthest points, respectively, in the Moon’s elliptical orbit around Earth. These positions are part of a broader class of orbital points called apsides, which occur in any elliptical orbit. Because the Moon’s distance from Earth changes continuously, perigee and apogee influence how large the Moon appears in the sky, how strong ocean tides become, and how astronomers describe orbital motion. Understanding these terms connects everyday observations, such as “supermoons,” to fundamental principles of orbital mechanics.

Key Takeaways: Perigee and Apogee

• Perigee is the point where the Moon is closest to Earth, apogee is where it is farthest.
• The Moon’s distance ranges from about 356,400 km at perigee to about 406,700 km at apogee.
• The average Earth–Moon distance is about 384,400 km.
• A full Moon near perigee appears larger and brighter, often called a “supermoon”.
• A full Moon near apogee appears smaller and dimmer, sometimes called a “micromoon”.
• Perigee and apogee occur about once each lunar orbit, roughly every 27.55 days.
• These points are examples of apsides, which apply to all elliptical orbits.
• Perigee strengthens tides, while apogee weakens them slightly.

What Are Perigee and Apogee?

Perigee and apogee are specific points in the Moon’s orbit:

• Perigee: the point at which the Moon is closest to Earth
• Apogee: the point at which the Moon is farthest from Earth

The Moon does not orbit Earth in a perfect circle. Instead, it follows an ellipse, meaning its distance changes throughout the month. These two points mark the extremes of that distance.

Historical Terminology and Etymology

The terms perigee and apogee come from Greek roots that describe position relative to Earth.

• Peri- means near
• Apo- means away from
• -gee comes from Gaia, meaning Earth

These terms belong to a broader naming system used in astronomy. For example:

• Perihelion and aphelion describe distances from the Sun
• Periastron and apoastron describe distances in star systems

This consistent terminology allows scientists to describe orbits clearly across different contexts.

Moon Distance at Perigee and Apogee

The Moon’s distance varies within a predictable range:

These values vary slightly because the Moon’s orbit is influenced by the Sun’s gravity and other perturbations.

The Moon’s orbit is elliptical, so its distance from Earth changes continuously. Over time, the distance ranges from about 356,400 km at perigee to about 406,700 km at apogee, with an average distance of about 384,400 km.

Appearance of the Moon: Perigee vs Apogee

The Moon’s changing distance affects how it looks in the sky.

Perigee Moon (Supermoon)

When a full Moon occurs near perigee, it appears:

• About 10–14% larger in diameter than at apogee
• Up to 30% brighter than a dim apogee full Moon
• More visually striking, especially near the horizon

This is commonly called a supermoon, although that term is informal.

Apogee Moon (Micromoon)

When a full Moon occurs near apogee:

• It appears smaller and dimmer
• The difference is subtle without direct comparison
• This is sometimes called a micromoon

Supermoon vs Micromoon

Although the difference in size and brightness is real, it is often subtle without direct comparison or careful observation.

Effects of Perigee and Apogee

Perigee and apogee do more than mark positions in the Moon’s orbit, they influence how the Earth–Moon system behaves. As the Moon’s distance changes, its gravitational pull, apparent size, brightness, and orbital speed all vary in predictable ways. These effects are subtle on a day-to-day basis, but they become noticeable in phenomena such as tides and the appearance of so-called supermoons and micromoons.

Tides

The Moon’s gravitational pull varies with distance:

• Perigee produces slightly stronger tides, called perigean spring tides when aligned with the Sun
• Apogee produces weaker tides

These differences are measurable but usually modest compared to weather-driven coastal effects.

Orbital Speed

According to Kepler’s laws:

• The Moon moves faster at perigee
• The Moon moves slower at apogee

This occurs because gravitational attraction is stronger when the Moon is closer to Earth.

Earth–Moon System Dynamics

• Perigee and apogee shift over time due to gravitational interactions
• The line connecting them rotates (apsidal precession) with a period of about 8.85 years

How Often Do Perigee and Apogee Occur?

The Moon completes one cycle from perigee to perigee in about 27.55 days (the anomalistic month). Therefore:

• A perigee occurs roughly once every 27.55 days
• An apogee occurs roughly halfway between perigees

Lunar Perigee and Apogee Calendar

For example, here are the perigee and apogee dates for 2026:

Some months contain two perigees or apogees rather than one.

Perigee and Apogee as Types of Apsis

Perigee and apogee are specific examples of a broader concept called an apsis (plural: apsides).

• Periapsis: closest point in an orbit
• Apoapsis: farthest point

The names change depending on the central body:

This naming system helps astronomers describe orbits consistently across different systems.

Perigee and Apogee vs Perihelion and Aphelion

It is important to distinguish between Earth–Moon and Earth–Sun distances:

• Perigee/apogee refer to the Moon orbiting Earth
• Perihelion/aphelion refer to Earth orbiting the Sun

For example:

• Earth is closest to the Sun at perihelion (early January)
• The Moon is closest to Earth at perigee (about every 27.55 days)

These are separate orbital systems with different cycles.

Perigee and Apogee in Other Systems

Perigee and apogee are specific terms for objects orbiting Earth, but the same concept applies to all elliptical orbits. In general, the closest and farthest points are called periapsis and apoapsis.

Examples include:

• Artificial satellites around Earth, which also have perigee and apogee
• Planets orbiting stars, described using perihelion and aphelion
• Binary star systems, where the terms periastron and apoastron apply

This broader framework allows astronomers to describe orbital motion consistently across many different systems.

Observing and Photographing Perigee vs Apogee

The difference between a perigee Moon and an apogee Moon is measurable but not always obvious to casual observers. The apparent size change is only about 10 to 14 percent, which can be difficult to notice without a reference.

To observe the difference:

• Compare photos taken at the same focal length on different dates
• Use foreground objects, such as buildings or trees, for scale
• Observe multiple full Moons over time rather than relying on a single event

The Moon often appears much larger near the horizon, but this is due to the Moon illusion, a psychological effect unrelated to its actual distance.

Orbital Speed and Energy

The Moon’s speed changes as it moves along its elliptical orbit. It travels fastest at perigee and slowest at apogee. This variation follows directly from Kepler’s second law, which states that an orbiting body sweeps out equal areas in equal times.

From an energy perspective, the Moon has:

• Higher kinetic energy at perigee
• Lower kinetic energy at apogee

As the Moon moves closer to Earth, gravitational potential energy decreases while kinetic energy increases. The opposite occurs as it moves toward apogee. The total energy of the orbit remains constant.

Perigee–Syzygy: When Perigee Aligns With the Moon’s Phase

Perigee becomes especially significant when it coincides with specific Moon phases, particularly when the Sun, Earth, and Moon align in a configuration called syzygy. This alignment produces the most noticeable effects of the Moon’s varying distance.

When a full Moon occurs near perigee, it is commonly called a supermoon. At this time, the Moon appears slightly larger and brighter than average because it is both fully illuminated and closer to Earth. Conversely, when a full Moon occurs near apogee, it appears smaller and dimmer, sometimes called a micromoon.

Perigee also affects tides during both full and new Moon phases. When perigee aligns with syzygy, the gravitational forces of the Moon and Sun combine to produce especially high and low tides, known as perigean spring tides. These tides are stronger than typical spring tides but are still part of the normal range of tidal variation.

Why the Moon’s Orbit Is Elliptical

The Moon’s orbit is elliptical rather than perfectly circular because of how it formed and how it interacts gravitationally with other bodies. The Earth–Moon system likely originated from a giant impact early in Earth’s history, and the resulting debris formed a system that naturally settled into an elliptical orbit.

Gravitational interactions, especially with the Sun, continually perturb the Moon’s orbit. These perturbations maintain its slight eccentricity, about 0.055, which is enough to create measurable differences between perigee and apogee distances.

The orientation of the ellipse also changes over time. The line connecting perigee and apogee slowly rotates in a process called apsidal precession, completing one cycle approximately every 8.85 years. This means the locations of perigee and apogee shift gradually relative to Earth and the stars.

Measuring the Moon’s Distance

Astronomers measure the distance to the Moon with remarkable precision. Today, the most accurate method uses laser ranging, in which lasers are fired from Earth toward retroreflectors placed on the Moon during the Apollo missions.

By measuring the time it takes for the laser pulse to travel to the Moon and back, scientists determine the distance to within a few centimeters. This technique confirms the variation between perigee and apogee and also reveals that the Moon is slowly moving away from Earth at a rate of about 3.8 cm per year.

Before laser ranging, astronomers estimated the Moon’s distance using parallax, observing its position against background stars from different locations on Earth. These earlier methods were less precise but still demonstrated that the Moon’s orbit is not perfectly circular.

Common Misconceptions

The supermoon is dramatically larger than a normal Moon.
The difference is real but modest. Most people notice it only when comparing images.

Perigee causes major natural disasters.
There is no strong scientific evidence linking perigee to earthquakes or volcanic eruptions.

The Moon’s orbit is extremely stretched.
The orbit is only slightly elliptical, with a small eccentricity (~0.055).

Apogee means the Moon is far away in space.
Even at apogee, the Moon remains gravitationally bound and relatively close on a cosmic scale.

FAQs

How much does the Moon’s distance change?
About 50,000 km (31,000 miles) between perigee and apogee.

Does a supermoon affect people or behavior?
There is no consistent scientific evidence that it affects human behavior.

Why does the Moon look huge near the horizon?
This is the Moon illusion, a psychological effect, not related to perigee.

How often do supermoons occur?
Several times per year when a full Moon aligns closely with perigee.

Is the Moon brightest at perigee?
Yes, a full Moon near perigee is the brightest full Moon of the year.

Do satellites have perigee and apogee?
Yes, any object orbiting Earth, including artificial satellites, has perigee and apogee.

References and Further Reading

• D’Eliseo, Maurizio M.; Mironov, Sergey V. (2009). “The Gravitational Ellipse”. Journal of Mathematical Physics. 50 (2): 022901. doi:10.1063/1.3078419
• Michelsen, Neil F. (1982). The American Ephemeris for the 21st Century – 2001 to 2100 at Midnight. Astro Computing Services. ISBN 0-917086-50-3.
• Spencer, David B.; Conte, Davide (2023). Interplanetary Astrodynamics. CRC Press. ISBN 9781000859744.
• Swerdlow, Noel M.; Neugebauer, Otto (1984). Mathematical astronomy in Copernicus’s De revolutionibus. Vol. 1. Springer-Verlag. ISBN 0-387-90939-7.