<Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (September 2013) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (September 2013) (Learn how and when to remove this template message) </Td> </Tr> <P> Before the days of modern, inexpensive, 3D computer graphics, a heliodon (precisely - movable light source) was used to show the angle of the sun on a physical model of a proposed building . Today, mathematical computer models calculate location - specific solar gain (shading) and seasonal thermal performance, with the ability to rotate and animate a 3D color graphic model of a proposed building design . </P> <P> Heating and cooling issues in passive solar building design can be counterintuitive . Precise performance calculations and simulations are essential to avoid reinventing the wheel and duplicating expensive experimental design errors, such as skylights that turn a building into a solar furnace in summer (see Daylighting #Sawtooth roof). </P>

When does the sun shine vertically on the equator