I want to clarify the relation between sun, calcium, and vitamin D. It is true that the UVB rays of the sun hitting the skin generates vitamin D (and not calcium). However, the body can not absorb calcium (whether it is from food or supplements) if it does not have vitamin D.
Since the sun is the primary means for the body to get vitamin D, in that sense, the sun is necessary for calcium absorption by the body. Vitamin D is woefully lacking in our diet, unless you are taking vitamin D supplements -- which you should as vitamin D is one of the most important vitamin to take as mentioned here: http://hubpages.com/t/22237a
That is why many calcium supplements will add vitamin D along with it so that calcium can be better absorbed.