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Planet Tracker

David Chandler and Michael Zeilik

(System requirements: DOS 3.0 or higher. Printouts require an HP Laserjet printer.)

Planet Tracker is available for download under the name PLTVER42.ZIP from the astronomy software archive at SEDS (in their solar system subdirectory). It is shareware, but we ask that you register if you make regular use of this program.

Planet Tracker (for MS DOS computers) is an innovative presentation of planetary motion. The software design is by David Chandler and the accompanying text materials are by David Chandler and astronomy textbook author Michael Zeilik, both active astronomy teachers. Planet Tracker differs from general purpose planetarium software in its ease of use by students and its specific conceptual focus.

Planet Tracker is built in three modules:

  • Animations of planetary motion along the ecliptic.
  • Split-screen animations comparing five different frames of reference.
  • Worksheet generator for classrooms without computers.

Motion Along the Ecliptic displays the motions of the sun and any or all of the naked-eye planets against the background of the Zodiac stars. Running counters display the date and Julian Day, allowing quantitative questions to be explored. The animation can be single stepped or run in continuous mode, optional trails can be drawn, and vertical exaggeration can be introduced to display the retrograde loops in more detail. At any time the animation can be frozen and the planets can be identified by name.

Motion Along the Ecliptic: In this illustration Mercury is selected and its motion is traced over a long period of time. A vertical exaggeration factor of 3 is used to highlight the changing patterns of the retrograde loop. Note that the shape of the loop depends on the sky background when Mercury is in retrograde. (See the Planet Tracker notes for further discussion of this and other interesting phenomena that can be displayed.)


Split Screen Animation displays planetary motion from five different points of view, allowing any two to be compared side-by-side. The five viewpoints are: the apparent dome of the sky for a fixed time of day, the zodiac as seen from the Earth, heliocentric solar system, geocentric solar system, and a special rotating frame of reference holding both the earth and sun stationary. At any time during the animations, rays can be drawn from the earth to each planet, showing that the essential geometry is preserved despite the apparent differences in the viewpoints.

Split-Sreen Animation: Earth-centered view of the dome of the sky compared with the heliocentric orbital positions of the planets for the same date.


Split-Sreen Animation: Heliocentric vs. Ptolemaic (Tychonic) models of planetary motion. Note that despite the vast differences in the apparent paths traced out, the relative positions of the planets in the two systems is identical at every instant.


Basic Worksheets produces heliocentric and geocentric orbit diagrams and time lines for any given starting date. In situations where computers are not available for direct access by students the worksheets provide a solid core of material for in-class activities that teach planetary motion concepts. The worksheets are the basis of many of the lab activities in the on-disk lab manual. They are also useful for making overhead transparency masters and observing aids to be taken outside at night. (A standard dot matrix or HP Laserjet printer is required. Unlike Deep Space, Planet Tracker has not been converted to Postscript.)

Zodiac Time Line: For any day of the year project the planet positions on that date up to the ecliptic, which runs along the center of the star chart strip at the top of the page. Note that Mars in near opposition and in the middle of its retrograde motion about June 10.

Solar Time Line: This shows the same information as the previous chart, except the positions of the planets are plotted relative to the sun rather than relative to the stars. Note the high degree of symmetry of the orbits relative to the sun. (This is especially apparent if plots for several years are printed out and taped together!) A plot like this could (in principle) have been plotted by Copernicus based on observations and helped lead to his heliocentric theory of planetary motion.

An extensive content-oriented help file and on-disk lab manual (ASCII files) will help you understand the capabilities of the program and its immediate usefulness for your introductory astronomy classes.

P.O. Box 642, Eastlake, CO  80614 / Voice: 800-516-9756 / Fax: 720-285-1924