Space Age: add a Note about various "year" values (#2173)

Author: IsaacG

exercises/space-age/description.md

@@ -16,4 +16,10 @@ be able to say that they're 31.69 Earth-years old.
 
 If you're wondering why Pluto didn't make the cut, go watch [this YouTube video][pluto-video].
 
+Note: The actual length of one complete orbit of the Earth around the sun is closer to 365.256 days (1 sidereal year).
+The Gregorian calendar has, on average, 365.2425 days.
+While not entirely accurate, 365.25 is the value used in this exercise.
+See [Year on Wikipedia][year] for more ways to measure a year.
+
 [pluto-video]: https://www.youtube.com/watch?v=Z_2gbGXzFbs
+[year]: https://en.wikipedia.org/wiki/Year#Summary