There are three ways for heat to travel from one place to another. The most immediate is conduction. When you hold a warm object in your hand, heat passes from the object to your hand, making its surface temperature rise. This is conduction. It requires that two bodies at different temperatures be placed in direct contact with each other. Convection works by gravity, and heat can be transferred by this means only through fluids. Contact with a warm surface raises the temperature at the base of a mass of fluid. The warmed fluid expands, reducing its density, and cooler, denser fluid sinks beneath it, pushing it upward and then being warmed in its turn. This is how heat is transferred from one region of the atmosphere to another.
Neither of these mechanisms allows heat to travel from the Sun to the Earth, because the two bodies are not in direct physical contact and there is no fluid medium separating them. The warmth we receive from the Sun arrives as radiant heat, which is a form of electromagnetic radiation. Unlike conduction and convection, radiation requires no medium through which to travel. It traverses empty space.
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