Version 3.033 of the SilverLining Sky, 3D Cloud, and Weather SDK features an overhauled ephemeris model – this means that the positions of the sun, moon, planets, and stars for a given time and location are now more accurate than ever. The sun position should be accurate to within one arc-minute, and the moon within eight arc-minutes between the years 1500 – 2500.

The position of the sun and moon would sometimes be off when using geocentric coordinates with SilverLining, especially if you were using an unusual coordinate system such as the Y-axis pointing up through the North pole. Our model of precession, or the wobbling of Earth’s axis, also wasn’t quite right, and this led to errors that seemed most pronounced in the Eastern and Southern hemispheres. Things should behave properly now.

Remember that your scene still needs to correspond to our conventions about which axis points in which direction, however. The vectors you pass to Atmosphere::SetUpVector() and Atmosphere::SetRightVector() will determine which ways are “up” and “East” in a flat coordinate system. In a geocentric coordinate system, we assume a standard ECEF system where Z points from the center of the Earth through the North pole, and X points from the center of the Earth to the prime meridian at the equator. If you set the geocentric-z-is-up setting in SilverLining.config to “no,” we’ll assume that Y points up through the North pole instead of Z, but still in a right-handed system.

SilverLining’s astronomical model was one of the most interesting things to develop. You’ll find a lot of attention to detail in it that reflects our interest in the subject. Not only are the positions of the sun, moon, and stars accurate – but so is the phase of the moon. We also model the effect of atmospheric refraction on the apparent position of the sun when it’s near the horizon, and calculate the correct color and brightness of the stars as well as the visible planets. We even model the drift of star positions over time, so you can set the date in SilverLining to the distant past or future, and see how the shapes of the constellations change.