By Chris Deziel | Updated Mar 24, 2022
Gravity is the invisible force that keeps everything—from your phone to the stars—tethered to Earth. While the gravitational pull between two people is negligible, the influence of massive bodies like planets and stars shapes the cosmos.
The universal law of gravitation states that the force between two masses is proportional to the product of their masses and inversely proportional to the square of the distance separating them:
F_g = G \frac{m_1 m_2}{R^2}
where G is the gravitational constant. When one mass dwarfs the other—such as Earth versus an object on its surface—the planet’s mass dominates, and every object experiences a pull toward the planet’s center proportional to its own mass. This is why “whatever goes up must come down” holds true for objects that don’t achieve escape velocity.
Every planet exerts a similar gravitational force on objects on its surface, but the magnitude varies with both mass and density. A denser planet has more mass underfoot, increasing the pull you feel.
On Earth, the acceleration due to gravity is 9.8 m/s², defined as 1 g. Expressing other planets’ surface gravity as a fraction of Earth’s simplifies comparison:
Notably, Jupiter’s immense mass makes it the strongest gravitational field in the system, while Mercury’s high density gives it a surprisingly similar pull to Mars, despite its smaller size. Conversely, Saturn’s lower density offsets its large mass, resulting in a surface gravity close to Earth’s.
All planets exert a tug on Earth, but the effect is dominated by the Moon and, to a lesser extent, the Sun, due to their proximity. The Moon’s pull is the strongest; relative to it, the other bodies’ tidal influences are tiny:
These forces manifest most noticeably as tides. While the Sun’s effect is substantial, the Moon’s closer distance makes it the primary driver of tidal cycles.
Planetary positions are not fixed; as they orbit, their distance from Earth changes, leading to fluctuations in gravitational pull that can vary by up to an order of magnitude. Understanding these variations is essential for precise orbital calculations and has historically intrigued observers seeking patterns between planetary alignments and terrestrial events.
Whether you’re a space enthusiast or a curious mind, grasping the nuances of planetary gravity enriches our appreciation of the forces that shape our everyday world and the broader universe.
