60-Second Adventures in Astronomy. Number
four: a Day on Mercury. No two planets act exactly the same whether it’s Jupiter
spinning in only 10 hours, Venus spinning backwards or Uranus tilting to one side.
But Mercury is particularly strange, it takes nearly 59 Earth days to rotate,
which might make for a pretty long day but at least you’d have time to get
things done. But while the days are long on Mercury, the years are relatively
short, it travels around the Sun in just 88 Earth days. Now until 1965 we thought
Mercury span exactly once per orbit, which would mean that one side of it was
always facing the Sun. If it span twice every orbit its day would be the same
length as its year, which would at least make calendars nice and simple. But it
actually spins three times for every two orbits which means each Mercury day
lasts for two Mercury years, so while you might get a bit bored waiting for the
evening at least you’ll be able to celebrate your birthday twice a day. Even
if you had to share it with everyone else.