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The equatorial radius of Neptune is 24,764 km.
That’s the quick answer. But you need to understand that things are a little more complicated. Like all of the planets in the Solar System, Neptune is spinning rapidly, completing a rotation in 16 hours and 6 minutes. This rapid rotation causes the planet to flatten out, so that the radius across the equator is bigger than the radius to the poles.
So here’s the more precise answer. The radius of Neptune, measured from the center to the equator is 24,764 km. And the radius of Neptune, measured from the center to either pole is 24,341 km. I’ll do the math for you. That means that the points on the equator are 423 km further away from the center of Neptune than either pole.
Need some comparison? Neptune’s radius is 3.9 times the radius of Earth. In other words, you could line up almost 4 Earths side by side to match the width of Neptune.
We have written many articles about Neptune for Universe Today. Here’s an article about the potential for liquid water deep down within Neptune. And here’s an article about how Neptune’s largest moon Triton might have been captured by Neptune’s gravity.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.