Space
Pluto has not completed a single orbit of the Sun since it was discovered in 1930.
One Pluto year lasts about 248 Earth years. Pluto will finish its first full orbit since discovery in the year 2178.
Why is this true?
Pluto's orbit is so stretched that for twenty years of each circuit it is closer to the Sun than Neptune is. It was last inside Neptune's orbit between 1979 and 1999.
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More about space
Space facts are mostly a lesson in scale. Days longer than years, storms wider than Earth, light that left its star before the pyramids existed — the numbers stop being intuitive very quickly. Every entry here comes from established astronomy and includes a short explanation of what causes the effect, so the fact holds up when someone asks the obvious follow-up question at the dinner table.