If there were no atmosphere, the sky would be all black, except for the area directly around the sun, which you would not be able to look anywhere near because it would be so bright, as there would be no atmosphere to block its rays.
Contagion and contagiousness refers to how easily the spread of Genetic Disease is possible from one person to another. Other words for contagion include infection, infectiousness, transmission or transmissability. Contagiousness has nothing to do with genetics or inheriting diseases from parents.
contagious disease (also called a communicable disease) is an infectious disease that is capable of being transmitted from one person or species to another. Apparently they are the same. Not true. Contagious diseases are most often communicable enough to warrant quarantine, whereas communicable diseases (such as the flu) are not.