EME 811
Solar Thermal Energy for Utilities and Industry

1.1. Space and Time

When you are courting a nice girl an hour seems like a second. When you sit on a red-hot cinder a second seems like an hour. That's relativity."

— Albert Einstein

Einstein, in the special theory of relativity, proved that different observers, in different states of motion, see different realities."

— Leonard Susskind

The location of the sun in the sky through time is relative to the location of the Earth with respect to the sun, the location of the observer on Earth, and the orientation of the collector at the observer’s location on Earth. The geometric relationships are defined by angles separated into three groups; 1) the relationship between the Earth and sun, as spheres, 2) the relationship between the sun and an observer (or point on the Earth), and 3) the relationship between a collector at some orientation and the sun. As the Earth rotates on its axis, the sun appears to traverse across Earth's sky. As the Earth orbits around the sun, the tilt of the axis moves the northern and southern hemispheres closer and further from the sun moving the sun's trajectory across Earth's sky higher and lower in the sky, creating seasons. The temporal and spatial relationships are inextricably linked.