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Planet YEAR Mercury 87.96 Earth days Venus 224.68 Earth days Earth 365.26 Earth days Mars 686.98 Earth days Jupiter 11.862 Earth years Saturn 29.456 Earth years Uranus 84.07 Earth years Neptune 164.81 Earth years Pluto 247.7 Earth years YEAR A planetary year is the length of time it takes that planet to revolve around the sun.

YELLOW DWARF Yellow dwarfs are small, main sequence stars. The Sun is a yellow dwarf.

YERKES OBSERVATORY University of Chicago's Yerkes Observatory is an astronomical observatory located at Williams Bay, Wisconsin, USA. It has a 40 inch (102 cm) refracting telescope; this is the largest refractor in the world. The Observatory was named for Charles Tyson Yerkes (1837-1905), a Chicago transportation tycoon who financed it. The observatory was designed by George E. Hale in 1892.

YLEM Ylem (or Yelm) is the name given to the hypothetical primordial matter from which all the elements have formed. George Gamow proposed the existence of ylem.