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Simply without delving into the physics. It is about the relative efficiency of the transfer of energy by radiation, convection and conduction through the media of glass.
The sun shines through the glass which allows the heat rays to radiate through it virtually unrestricted. This heat is absorbed by the contents of the greenhouse which get hotter. This heat also gets re radiated but mostly it heats the air which circulates. However glass is a good insulator and it does not let the air convection give off its heat by conduction. As a result while the sun shines the heat accumulates in the greenhouse.
At night the greenhouse will lose most of its accumulated heat by radiation.
Barry
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