What is meant by first contact?
Basically it's the start of the eclipse. The point where the eclipsing body first touches the primary star's image and starts a decrease in the primary star's brightness as seen from Earth. But it's not so simple. With epsilon Aurigae, first contact appears to be wavelength dependent. This means that the longer (red and visual bands) wavelengths will start to show an eclipse before the shorter (blue and ultraviolet bands) wavelengths. Read more