The EJS Herschel's Star Gages Model illustrates William Herschel's methods of "star gages" by which he attempted to map out the shape of our galaxy in 1785.  Herschel's star gages (sic) relied on two important assumptions: that Herschel's telescope (his "large 20 foot" with an 18.5 inch aperture) could see to the ends of the galaxy, and that within the galactic system stars are distributed uniformly.  If the first assumption holds then the stars seen in the telescope all lie within a conical region of space with the apex at the telescope and the base at the edge of the galaxy.  If the second assumption holds then the number of stars seen in the telescope is proportional to the volume of this cone.  Since the volume of the cone is proportional to the cube of its height, the distance to the galactic edge in any direction is proportional to the cube root of the number of stars seen in that direction.

This simulation allows the user to use Herschel's method of star gages to map out the shape of an artificial "star system" for which Herschel's assumptions are valid.  One window shows the view through a telescope, with a slider to change the telescopes direction (around a single fixed axis).  Another window shows a 3D view of the star system, showing either all of the stars in the system or only those stars visible through the telescope.  A third window shows a plot of the star gages.  Plotting star gages for many different directions maps out a cross-section of the star system. An optional slider allows the user to decrease the distance to at which stars are no longer visible, and a menu allows the user to select a star system in which the stars are not distributed uniformly.  These options let the user explore how violations of Herschel's two fundamental assumptions invalidate his star gage method.

Please note that this resource requires at least version 1.5 of Java (JRE).