I describe an interactive unit for teaching/learning planetary orbits at secondary level. The unit consists of a pre-test, activites related to the planetary orbits and a post-test. The pre-test is designed to modulate the activities to suit the needs of the students. The activities are (1) Trajectory of a particle in a gravitational field when the field is constant, and when it varies with distance (invoking a fictitious planet WonderX); (2) Plotting the orbit of a planet round the sun starting from select initial conditions and discovering Kepler’s laws; (3) Plotting orbits with force laws containing terms like 1/r3 and 1/r4 and discovering precession. Students work through graded steps. To help them, simple numerical algorithms and computer programs have been developed as they are not yet comfortable with calculus. A post-test gauges the progress of the students for a possible revisit to the topic. Such units can be very effective in teaching basic astronomy in developing countries.