HEAT GAIN 

DIRECT GAIN

Sometimes it's desirable to store heat inside a greenhouse even though the heat losses at night may be great.  Temperature swings are moderated with thermal mass.
This is a simple solar greenhouse made with a 2x3 frame held together with plywood.  It would also be a good idea to glue solid insulation on the inside and paint everything black before installing the glazing.  You should be able to remove a side panel to have access to the inside.

Place the greenhouse in bright sunlight on a clear, cold day.  Get out your digital thermometer  and take temperature readings every minute over the period of an hour.  is the temperature different at different locations of the greenhouse. Why? Why not?  

Place a row of blackened, glass bottles, filled with water in the back of the solar greenhouse and repeat this experiment on a similar  bright sunny  cold, clear day. The more temperature readings you take the better. You may repeat this experiment as many times as you like with different glazings and different kinds of thermal mass. You may decide to use bricks instead of bottles of water. Be sure to keep an accurate log of all your temperature readings. Also record weather conditions and ambient temperatures. 

Actually the best way to run this kind of experiment would be to have two solar greenhouses side by side. One greenhouse would have nothing in it and the other would have thermal mass, like bottles of water or  perhaps some bricks. It would also be nice to have a data data logger  capable of sampling the inside temperature of both greenhouses and the ambient (outside temperature) simultaneously. We don't always have the best tools to work with, but if we're careful and persistent we should notice a pattern of temperature moderation in the greenhouse with thermal mass. Please take the time to post your observations and calculations on the HEAT GAIN forum  Bulletin Board.