The MKS (meter-kilogram-second) units is part of the SI (Systeme Internationale) that is the standard in much of the world and for scientific work.

Length

• The meter was originally defined as 1/10⁷ of the distance from the equator to the North pole measured along a longitude passing through Paris.
• In 1899 it was defined as the length between two marks on a particular platinum-iridium rod kept in Paris. Then in 1960 the meter was redefined as 1,650,763.73 wavelengths of a particular orange light emitted by krypton-86.
  
• Basic numbers:
  Radius of nucleus: 10^-15 m
  Radius of atom: 10^-10 m
  Radius of bacteria: 10^-4 m
  Length of your stride: 1 m
  Radius of earth: 6 x 10⁶ m
  Radius of earth's orbit: 10¹¹ m
  Distance to nearest star: 10¹⁶ m
  Radius of our galaxy: 10²¹ m
  Radius of the universe: 10²⁷ m
  

Mass

• The kilogram was originally defined as the mass of 1 liter of water. One kilogram is about 2.2 pounds (careful! kilogram is a mass unit, pounds is a force unit).
• The standard kilogram is a particular platinum-iridium cylinder at the International Bureau of Weights and Measures near Paris.
  
• Basic numbers:
  Mass of electron: 10^-30 kg
  Mass of Hydrogen atom: 10^-27 kg
  Mass of bacteria: 10^-15 kg
  Mass of one liter of water: 10⁰ kg
  Mass of large ship: 10⁸ kg
  Mass of earth: 6 x 10²⁴ kg
  Mass of sun: 10³⁰ kg
  Mass of galaxy: 10⁴¹ kg
  Mass of universe: 10⁵² kg
  

Time

• The second is defined as the time required for 9,192,631,770 periods of the radiation emitted from a particular transision of a cesium atom.
  
• Basic numbers:
  Time for electron to orbit an atom: 10^-18 sec
  One year: π x 10⁷ sec
  Human lifespan = 2 x 10⁹ sec
  Age of the Earth = 10¹⁷ sec
  Age of the universe = 10¹⁸ sec