Ever looked up at a full moon and thought, "Wow, that thing looks huge tonight"? You aren't imagining things. It’s not just a trick of the light or that weird "moon illusion" where it looks bigger near the horizon. Sometimes, the moon is actually physically closer to us. This specific point in its orbit is called perigee.
Space is rarely a perfect circle. Most people picture the moon orbiting Earth in a nice, even ring, but that’s not how gravity works. The moon follows an elliptical path—sort of an oval shape. Because of this, there’s a point where it’s furthest away (apogee) and a point where it’s closest (perigee). When it hits that close point, everything from the tides in our oceans to the size of the moon in your backyard telescope changes.
The Mechanics of Perigee
So, how close are we talking? On average, the moon sits about 238,855 miles away. But at perigee, it can pull into roughly 225,623 miles. That’s a difference of about 13,000 miles. Think about that for a second. That is more than the entire diameter of the Earth. It’s a massive shift in cosmic terms.
Johannes Kepler was the guy who figured this out back in the 17th century. He realized planets and moons don't move in circles. They move in ellipses. This was a huge deal because it explained why the moon seemed to speed up and slow down. Newton later backed this up with his laws of universal gravitation. Basically, as the moon gets closer to Earth at perigee, Earth's gravity pulls on it a bit harder, and the moon has to move faster to keep from falling in. It’s like a cosmic game of tetherball.
Why the Orbit Isn't a Circle
You might wonder why the moon doesn't just settle into a perfect circle. Well, blame the Sun. And Jupiter. And even the slight bulge of the Earth at its equator. These are what astronomers call "perturbations." The Sun is constantly trying to tug the moon away from us, while Earth is trying to keep it. This tug-of-war stretches the orbit. It’s never static. In fact, the distance of perigee changes every month because the orbit itself is "precessing," or rotating, in space.
Perigee vs. The Supermoon
You’ve definitely heard the term "Supermoon." It’s all over the news every few months. But here’s the thing: "Supermoon" isn't a technical astronomical term. It was actually coined by an astrologer named Richard Nolle in 1979. Astronomers prefer the much more mouthful-sounding term: perigee-syzygy.
Syzygy is just a fancy word for three celestial bodies lining up. When the Sun, Earth, and Moon align (a full moon or a new moon) at the exact same time the moon is at perigee, you get a Supermoon.
Does it actually look different?
Yes. But it’s subtle.
A perigee full moon can look about 14% larger and 30% brighter than an apogee full moon (the "micromoon"). If you’re just glancing at the sky, you might not notice the 14% size increase. However, the brightness is usually pretty obvious. On a clear night during perigee, the moonlight can be intense enough to cast sharp shadows even in areas with some light pollution.
The Impact on Earth: More Than Just a View
Perigee isn't just for stargazers. It has real-world physical effects on our planet. The most obvious one is the tides.
Gravity follows the inverse-square law. This means that if you decrease the distance between two objects, the gravitational pull increases significantly. When the moon is at perigee, its tidal pulling power is at its peak. This leads to what we call "perigean spring tides."
Don't let the name fool you; it has nothing to do with the season. "Spring" refers to the water "springing forth." During these times, high tides are several inches higher than usual, and low tides are lower. If a big storm or a hurricane happens to hit during a perigee tide, the flooding can be way worse. Coastal cities like Miami or Charleston often deal with "sunny day flooding" during these perigee events because the ocean is just physically pushed higher onto the land.
Perigee in the Solar System
Earth isn't the only thing with a perigee. Technically, any object orbiting Earth has a perigee. This includes the thousands of man-made satellites and the International Space Station (ISS).
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For satellite operators, perigee is a constant concern. If a satellite's perigee is too low, it starts brushing against the upper layers of Earth’s atmosphere. This causes "drag." It slows the satellite down, which—counterintuitively—makes it drop even lower. If they don't use onboard thrusters to kick the perigee back up, the satellite will eventually burn up in the atmosphere.
Interestingly, we use different names for this "closest point" depending on what is being orbited:
- Perihelion: Closest point to the Sun (Earth hits this in early January).
- Periastron: Closest point to a star.
- Perijove: Closest point to Jupiter (NASA’s Juno probe does high-speed perijove passes to take those incredible cloud photos).
Common Misconceptions
People get perigee wrong all the time. One of the biggest myths is that perigee causes natural disasters. You’ll see "doom-scrolling" articles claiming a Supermoon will trigger earthquakes or volcanic eruptions.
Honestly? There is almost no evidence for this.
While the moon's gravity does slightly flex the Earth's crust (called terrestrial tides), the effect is tiny compared to the tectonic forces already at play. Geologists at the USGS have looked into this extensively, and the consensus is that while there might be a very slight statistical uptick in micro-quakes, perigee isn't going to cause the "Big One."
Another misconception is that the moon stays at perigee for a long time. It doesn't. Because the moon moves faster at perigee, it sweeps through that part of its orbit quickly. The "peak" perigee only lasts for an instant, though the moon looks relatively large for a day or two on either side.
How to Track It Yourself
You don't need a PhD to track perigee. In fact, if you’re a fisherman, a surfer, or a photographer, you probably should keep an eye on it.
- Check a Lunar Calendar: Most weather apps or astronomy sites like TimeandDate.com list the monthly perigee and apogee dates.
- Watch the Tides: If you live near the coast, look at your local tide tables. Look for the days with the highest predicted swells—those often line up with perigee.
- Photography Tips: If you want to capture that "huge moon" look, catch the perigee moon right as it’s rising. This combines the physical closeness of perigee with the "moon illusion," where your brain compares the moon to trees or buildings on the horizon. Use a telephoto lens (at least 300mm) to compress the image and make the moon look gargantuan behind a foreground object.
Why Perigee Matters for the Future
As we head back to the moon with the Artemis missions, perigee is becoming a logistical factor again. Navigating a spacecraft from Earth to the moon requires precise calculations of the moon's fluctuating distance. Fuel is expensive. Weight is limited. If you can timing a launch to take advantage of the moon’s position, you can optimize your orbital insertion.
Basically, perigee is a reminder that we live in a dynamic, shifting system. Nothing in space is static. The ground beneath your feet is being pulled by a rock 225,000 miles away, and for a few days every month, that pull gets just a little bit stronger.
Next time you see a massive, glowing moon hanging over the trees, take a second to appreciate the orbital dance. It’s not just a pretty view; it’s a physical manifestation of gravity at work.
Your Next Steps for Moon Watching:
- Download a "Moon Phase" app to get alerts for when the moon reaches its perigee each month.
- If you're near a coast, visit the beach during a perigee tide to see how much further the water reaches compared to a normal week.
- For the photographers: Plan your next shoot for a "Supermoon" date, but set up your tripod at least a mile away from your foreground subject (like a lighthouse or a statue) to maximize the scale of the moon.