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Gravity is a universal attraction between masses. When one body, such as a star, vastly outweighs another, the lighter body is drawn toward the heavier. A planet feels a pull toward its star. If they were initially at rest relative to each other, the planet would simply fall straight toward the star, mirroring the everyday experience of gravity on Earth.
In reality, a planet is never stationary with respect to its star; it moves at tremendous speed along its orbit. Earth, for instance, travels roughly 108,000 km/h (67,000 mi/h) around the Sun. This velocity is nearly perpendicular to the radial line to the Sun. Gravity pulls Earth inward, while its sideways speed keeps it from crashing, resulting in a stable orbit.
Any circular motion can be described by a centripetal force— a force that acts toward the center of the circle. In an orbit, gravity supplies this centripetal pull. An everyday analogy is a ball tied to a string and whirled around; the string provides the centripetal force, and the ball’s tangential speed keeps it moving in a circle. The physics of a planet orbiting a star follows the same principles.
Planetary orbits are usually close to circular because the processes that form planetary systems distribute angular momentum evenly. In a perfect circle, the planet’s velocity is always perpendicular to the line to the star. However, bodies such as comets often travel on highly elongated, elliptical trajectories. These orbits can still be fully described by gravity, though their analysis is more involved than for circular paths.
