Question

what force keeps the moon orbiting the earth?
doppler shift
gravity
parallax
variable stars

Explanation:

• Doppler shift: It is related to the change in frequency of waves due to the relative motion of the source and observer. It has nothing to do with the moon's orbital motion.
• Gravity: According to Newton's law of universal gravitation, every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. The gravitational force between the Earth and the moon provides the centripetal force required for the moon to orbit the Earth.
• Parallax: It is a method used to measure distances to nearby stars. It is not relevant to the force keeping the moon in orbit.
• Variable stars: These are stars whose brightness varies over time. This has no connection to the moon's orbital force.

Answer:

Gravity