Why an orbit keeps missing the ground

Falling, with sideways motion

Imagine giving an object sideways speed while gravity pulls it inward. With appropriate conditions, the surface curves away as the object falls: the object remains in orbit. An orbiting astronaut is still affected by gravity; apparent weightlessness comes from being in free fall together with the spacecraft.

Ellipses, not perfect circles everywhere

In the ideal two-body problem, bound orbits are ellipses, with the center of mass at a focus of each body’s ellipse. A circle is a special case. Real trajectories are affected by other bodies, so a simple ellipse is an approximation.

An object in an elliptical orbit moves faster near its closest approach and slower farther away. Angular momentum connects these changing distances and speeds. The small experiment below deliberately uses circular orbits instead.

Farther out usually means a longer year

For a negligible-mass object orbiting a one-solar-mass star, Kepler’s third law gives approximately P² = a³ when P is in years and the semimajor axis a is in astronomical units. At four AU, the period is about eight years, not four.

Changing the central mass changes the relationship. The calculator is an educational model around a Sun-mass star, not a prediction for every planet or a way to plan a spacecraft mission.

A sky track is a view from a moving Earth

An object’s daily rise and set are largely shaped by Earth’s rotation. Planet tracks against the stars also reflect orbital motion and our changing viewpoint. A line drawn across Sky Orbit is an apparent sky path, not a scaled picture of an orbit around the Sun.