Scientists suspect that the moon was formed when the earth was impacted by a large planetary body shortly (in geological terms) after its formation.  That same impact likely knocked the earth's axis of rotation off from its plane of orbit around the sun and onto the angle we have today 23.5 degrees.

Lucky for us that it did.  Not only do we now have the wonderous vision of the moon in the night sky, but the earth's tilt also gives us the variations of the seasons.  A third fortuitous benefit is that as we travel around the sun, the tilt means that the path the sun travels through the sky on its course from east to west each day also changes over the seasons, getting higher in the sky in summer and lower in winter.  This in turn allows us to make an optimal sized eave that lets the sun under when it is lower in the sky in winter, but blocks it when it is higher in the sky in summer. 
The rule of thumb for designing northern eaves is called the 45% rule and works well for southern australia, up to a lattitude just above Sydney.  Simply measure the distance from the proposed window sill to the underside of the eave, and the eave should project 45% of that distance.  The second part of the rule asks you to move the window head down from the bottom of the eave so that it is no longer in that area that is always in shadow (and thus not an efficient location for glass.)

Applying this rule will give you shading that keeps out the sun in the hottest part of summer, lets a bit in during the shoulder seasons and give full sun penetration during winter. 

Be aware that as the sun rises and sets on the horizon, no eave will give complete shade all day.  Also be aware that sometimes during the House Energy Rating process eaves are one of the variable we play with to tweak, when improving each room.  If a room is a little cold, shortening the eave can often help a bit.  Just make sure though, that before moving it in from the 45% rule, that temperatures on hot days in summer are not going to be exacerbated.


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