Because there is no turning of the surface of the Earth (sense of rotation) underneath a horizontally and freely moving object at the equator, there is no curving of the object's path as measured relative to Earth's surface. The object's path is straight, that is, there is no Coriolis effect.

The Coriolis force is an inertial force that appears to act on objects in motion within a frame of reference that rotates, such as the Earth. Its strength is determined by the formula F=2m(ω×v), where ω is the angular velocity of Earth and v is the velocity of the object. Mathematically, the horizontal component of this force is proportional to the sine of the latitude (sinϕ). At the equator, the latitude ϕ is 0°, and since sin(0°)=0, the horizontal Coriolis force becomes zero. Physically, this occurs because at the equator, the Earth's surface is parallel to its axis of rotation; there is no "twisting" motion relative to the ground for an object moving horizontally. As you move toward the poles, the Earth's surface becomes more perpendicular to the axis, increasing the "rotational" effect on moving air or water, which is why cyclones cannot form within roughly 5° of the equator.