Orbits and planetary motion
A planet is moving through space while gravity continually changes the direction of its motion. An orbit is this continuing curved journey, not a rail that holds the planet in place.
- Gravity changes direction
- Sideways motion continues
- The path curves
- An orbit repeats
Why planets keep going around the Sun
Imagine a moving object whose direction is continually bent toward a central body. Gravity supplies that change of direction. A planet has sideways motion as well as gravitational acceleration toward the Sun, so it can keep falling around the Sun rather than reaching its surface. The orbit depends on its velocity and position.
Rotation and revolution are different
Rotation is a body turning on its own axis. Revolution is its travel around another body. Earth’s rotation and its orbit occur together, but neither movement should be used as a substitute for the other. Following a surface landmark reveals rotation; following the planet’s center around the Sun reveals revolution.
A day depends on what you measure
A sidereal rotation is measured relative to distant stars. A solar day follows the Sun’s apparent return to a position in the sky. Because a planet also travels along its orbit, those intervals need not be identical. The explorer’s rotation values use the sidereal convention, so do not read them automatically as sunrise-to-sunrise times.
Farther planets take longer
For planets orbiting the Sun, a larger orbit generally corresponds to a longer orbital period. Real planetary paths are ellipses, and orbital speed changes along a path. The explorer simplifies the paths into circles and compresses periods so distant planets move visibly during a short visit.
Read the animation carefully
The model preserves the order of the planets and shows their common orbital direction when viewed from the north side of the orbital plane. Starting positions are illustrative. The animation is not a sky map for tonight, and its displayed speed does not reproduce the true ratio of every planet’s orbital period.
Try this observation
Watch Earth’s center and then a landmark on its surface. Describe which motion each reveals. Next compare Mercury and Neptune: which completes an orbit first? Use the facts panel for real periods instead of timing the animation.