Illumination of Planets

This shows the intensity (e.g. watts/meter squared) at the surface of the planet. The color coding, as shown by the color bar at top left, is the ratio of the intensity to the maximum intensity. The planet has lines of latitude (parallels) and lines of longitude (meridians). When the number of meridians is chosen to be 24 then each meridian represents one hour of a 24 hour day. The planet tilt angle can be changed with its slider. The number of latitude lines and meridians can also be changed. For better viewing, the planet rasius can be changed. Of course, the radius of the Sun cannot be to scale but its position is correct for the views selected. To keep it simple I have chosen to make the planet orbit circular with the Sun at the center of the orbit. The number of solar days per orbit is adustable with a slider. The default value of 10 solar days per orbit is a reasonable choice for plotting intensity. I have also chosen to make both the orbital and the axial motion counter clockwise when viewed from above the planet north pole. This agrees with the axial and orbital motion of our Earth. Since the sense of orbital and axial motion is the same, the planet axial motion with respect to the angularly fixed stars (sidereal rotation) is one rotation more per orbit than the number of solar days. I've also included a plot of the intensity at a chosen latitude and at longitude 0. The sphere facet to be plotted is marked in black. The abscissa of this plot is marked as the north or south season rather than in degrees of orbit traversed. The peaks in the plot are obviously the days and the flats at the bottom of the peaks are the nights where the intensity is zero.Note that the plot shows the change in length of nights and days during the 4 seasons. We have long days and short nights when the plotted latitude goes through summer. There are 3 radio buttons to select different view positions of the planet. There is an orbit edge view by a distant galaxy in the same plane as the orbit. There is a view from the Sun where the viewer at the Sun follows the angle of the planet. There is also a view from a distant galaxy looking down on the orbit along a line-height perpendicular to the orbit and along the nominally North Pole direction. Of course, the position and angle of the viewer make no difference in the intensity at the chosen sphere facet. The intensity is obtained by taking the vector dot or inner product of the sphere facet normal and the present direction of the Sun. As always, the learner is welcome to press the F12 key in Windows to view my Javasceipt source code.