# How We Do It

Stars in full-sky maps and horizon views are plotted using “The Bright Star Catalog, 5th Revised Edition”, which contains position, magnitude, spectral type, and proper motion data for 9096 stars brighter than magnitude 6.5. It is the most widely used digital star database since it includes comprehensive information for all naked eye stars. The master versions of this catalog are distributed on the NASA Astronomical Data Center CD-ROM, were specially processed for use in Your Sky

The heliocentric coordinates of Mercury through Neptune are calculated using the algorithms given in Jean Meeus' Astronomical Algorithms, based upon the VSOP87 planetary theory of Bretagnon and Francou. The position is calculated, for dates between 1885 and 2099, using the method given in Chapter 36 of Astronomical Algorithms.

The specified orbital elements are used in conjunction with the general solutions for elliptic and parabolic motion given in Chapters 32 and 33 of Astronomical Algorithms. The orbit is plotted by calculating positions before and after the specified epoch until the orbit either closes (if elliptical) or reaches a preset distance from the Sun (if parabolic). The time step is calculated based on the rate of motion of the object, smaller near perihelion, larger at aphelion.

Once the orbit has been calculated, in heliocentric co-ordinates, a viewing transform is established, taking into account the heliocentric viewpoint and stereoscopic viewing options. The transform projects the three-dimensional orbit into the pixel space of the bitmap. A 64-sided polygon is drawn to approximate the elliptical orbit, using the Postage Stamp Rasteriser available from this site.

**How do you calculate the positions of the planets?**The heliocentric coordinates of Mercury through Neptune are calculated using the algorithms given in Jean Meeus' Astronomical Algorithms, based upon the VSOP87 planetary theory of Bretagnon and Francou. The position is calculated, for dates between 1885 and 2099, using the method given in Chapter 36 of Astronomical Algorithms.

**How do you calculate the orbits of comets and asteroids?**The specified orbital elements are used in conjunction with the general solutions for elliptic and parabolic motion given in Chapters 32 and 33 of Astronomical Algorithms. The orbit is plotted by calculating positions before and after the specified epoch until the orbit either closes (if elliptical) or reaches a preset distance from the Sun (if parabolic). The time step is calculated based on the rate of motion of the object, smaller near perihelion, larger at aphelion.

**How do you draw the orbits in the image?**Once the orbit has been calculated, in heliocentric co-ordinates, a viewing transform is established, taking into account the heliocentric viewpoint and stereoscopic viewing options. The transform projects the three-dimensional orbit into the pixel space of the bitmap. A 64-sided polygon is drawn to approximate the elliptical orbit, using the Postage Stamp Rasteriser available from this site.