The Fossil Bulge: Why the Moon is Shaped Like a Lemon

While it looks like a perfect silver coin in the night sky, the Moon is technically an oblate spheroid—a sphere that is slightly flattened at the poles and bulged at the center.

However, planetary scientists often go a step further and describe it as lemon-shaped because it has two distinct bulges: one on the side facing Earth and another on the far side.

1. The Physical Dimensions

To the naked eye, the Moon looks round, but the measurements reveal its true eccentricity.

• Equatorial Diameter: ~3,476 km
• Polar Diameter: ~3,472 km
• The Difference: The Moon is about 4.3 kilometers (2.7 miles) wider at its equator than it is from pole to pole.
• The “Lemon” Axis: The “long” axis of the Moon (the tips of the lemon) points directly toward Earth and away from it.

2. Why is it shaped this way?

The Moon’s shape is essentially a “fossil” of its early history. Over 4 billion years ago, the Moon was much closer to Earth and still in a molten, liquid state. Two main forces sculpted it:

• Tidal Stretching: Because the Moon was so close, Earth’s massive gravity pulled on the Moon’s molten rock like taffy. This created a permanent “tidal bulge” on the side facing Earth and a corresponding bulge on the opposite side (much like how the Moon creates tides in Earth’s oceans today).
• Rotational Flattening: At the same time, the Moon was spinning much faster than it does now. This centrifugal force caused it to flatten slightly at the poles and widen at its equator.
• The “Fossil Bulge”: As the Moon cooled and moved further away from Earth, its outer crust solidified, “freezing” these bulges in place. Scientists call this a Fossil Bulge because it represents a shape the Moon should have had billions of years ago, not what it would have if it formed today.

3. The “Lumpy” Reality

Beyond the general lemon shape, the Moon is also quite uneven due to tidal heating.

• Crust Thickness: The crust is actually thinner at the poles and thicker at the regions facing Earth. This is because early tidal forces generated heat that warped the crust as it was forming.
• The 34-Degree Shift: Interestingly, recent studies from researchers at UC Santa Cruz found that the Moon’s current orientation is slightly “off.” The long axis of the lemon shape is tilted about 34 degrees away from where it originally formed. This suggests that as large craters were formed by asteroid impacts, the Moon’s mass shifted, causing the entire satellite to tip over slightly over millions of years.

4. Why don’t we see it?

If you look at a lemon, the shape is obvious. If you look at the Moon, it looks perfectly round. This is because the distortion is extremely subtle relative to its size.

On a scale of a basketball, the Moon’s bulges would be thinner than a few sheets of paper. Additionally, the surface is covered in massive impact basins and mountains that act like “noise,” masking the underlying geometric shape from our eyes.