I created a driver for calculating the position of the sun using the SunCalc.js library. I use this to automatically close my roller shades on the west side of the house when the sun starts to come through the windows.
The driver uses your hub's lat/long to calculate the position of the sun at the current time. You can choose how often you wish to update. I've found that the altitude doesn't change more than a degree in 5 minutes.
Hopefully this is helpful to someone!
Altitude is the angle up from the horizon. Zero degrees altitude means exactly on your local horizon, and 90 degrees is "straight up". Hence, "directly underfoot" is -90 degrees altitude.
Azimuth is the angle along the horizon, with zero degrees corresponding to North, and increasing in a clockwise fashion. Thus, 90 degrees is East, 180 degrees is South, and 270 degrees is West.
Using these two angles, one can describe the apparent position of an object (such as the Sun at a given time).