This program models the transit / occultation in a star + planet system, and ant the same time the spectral shift of an emission line from the planet's atmosphere. The planet is moving around the star following a circular orbit, whose inclination can be changed in real time (the scaling for the inclination is such that 100 corresponds to 90 degrees). Also the planet and stellar sizes can be changed in real time by moving the cursors with the mouse (in this case their scalings are such that 100 corresponds to half the orbital size). It is also possible to slow down the animation, by setting the Slow Motion parameter to larger values: this may be useful to inspect in greater detail the initial and final phases of the occultation . The lower section of the screen shows the evolution of the stellar observed flux, namely of the fraction of the star that is not covered by the planet. On the same screen one may see the spectral shift of the planet's line (for this, the horizontal axis indicates the line observed wavelength, in arbitrary units). One may also notice that the line intensity decreases proportionally to the fraction of the planet covered by the star. Even if the relative scales are physically unreasonable, the program allows one to set a planet size even larger than than of the star, so that one can experience a full range of conditions like, for instance, partial or even total eclipses, as well as full of grazing occultations. The core of the program is the routine for calculating the intersecting area between two circles.
Graphic cartoon by Barbara Olmi
