Here’s a quick answer: Neptune is approximately 4.4 billion km away from the Earth.
Have you ever wondered how far Neptune is from the Earth? It’s actually a bit of a trick question. That’s because the distance between Neptune and Earth is constantly changing, depending on where they are in their orbits around the Sun.
It gets even more complicated, because both Earth and Neptune follow the Sun on elliptical orbits. They both have a closest point to the Sun, called perihelion, and a distant point, called aphelion.
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So, the closest point occurs when Earth and Neptune are on the same side of the Sun. Earth is at its most distant point from the Sun, and Neptune is at its closest. At this point, Earth and Neptune are separated by only 4.301 billion kilometers.
At their most distant point, Earth and Neptune are on opposite sides of the Sun, at the most distant points on their orbits. At this point, Earth and Neptune are separated by 4.553 billion kilometers.
If you’re interested, here’s an article that lists how far away all the planets are from the Sun and Earth.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.