### On Mercury you'd get 2 years older every day

Really! Here's why.

First, Mercury's year, the time it takes to make one trip around the Sun, is short. As the great astronomer Johannes Kepler explained almost 500 years ago, the closer a planet is to the Sun the faster it revolves around it. So Mercury, the closest planet, has the shortest year, only 88 days compared to our 365 days.

The other part of the explanation is the surprising length of Mercury’s day. As you know, a planet's day is the time it takes to rotate once with respect to the Sun; the time between one noon and the next. The tidal pull of the nearby Sun is so strong it has slowed Mercury’s rotation to a snail’s pace--slow enough that a day on Mercury is 176 Earth-days long, twice as long as its year of 88 days! Doing a little arithmetic, we divide Mercury's day (176) by its year (88) and confirm that, voilà, during one of its leisurely days Mercury would zip around the Sun twice ... two years.

PB

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