The correct answer is A) Doppler effect.
Scientists use the Doppler effect to determine the motion of objects in the universe, particularly through the changes in light or sound waves emitted by these objects. When an object moves towards an observer, its waves are compressed, resulting in a shift towards the blue end of the spectrum (blue shift). Conversely, when it moves away, the waves are stretched, leading to a shift towards the red end (red shift). This principle helps astronomers understand whether celestial objects are moving closer to or farther away from Earth, and at what speed.
The other options relate to different concepts in astronomy:
- B) Geocentric theory: An outdated model stating that the Earth is at the center of the universe.
- C) Big Bang theory: A cosmological model explaining the origin of the universe but not specifically about determining motion.
- D) Heliocentric theory: The model proposing that the Sun is at the center of the solar system, primarily affecting our understanding of planetary motion rather than determining motion in a broader context.