Mercury, Venus, Earth, and Mars — the four rocky terrestrial planets — occupy the inner Solar System out to about 1.5 AU. Mercury, closest to the Sun, completes an orbit in 88 days and is locked in a 3:2 spin-orbit resonance. Venus rotates retrograde with a crushing 92-bar CO₂ atmosphere and surface temperatures near 735 K. Earth is the only body with stable liquid water on its surface. Mars, half Earth's diameter, retains evidence of ancient rivers and lakes now frozen or lost. Together these worlds illustrate how distance from the Sun, mass, and atmospheric retention sculpt radically different planetary environments from similar starting materials.