Earth science i really need with
18. the apparent backward movement of a planet is called
a. heliocentric motion.
b. retrograde motion.
c. eccentricity.
d. kepler's first law of motion.
19. who first introduced the idea that the sun is the center of the universe?
a. nicolaus copernicus
b. johannes kepler
c. albert einstein
d. edwin hubble
20. the four giant gas planets that are made primarily of lightweight elements are
a. venus, saturn, uranus, and neptune.
b. jupiter, saturn, uranus, and neptune.
c. earth, saturn, neptune, and venus.
d. mercury, venus, earth, and neptune.

18.) Apparent retrograde motion is the apparent motion of a planet in a direction opposite to that of other bodies within its system, as observed from a particular vantage point. Thus option B is the correct answer. 19) Nicolas Copernicus developed the heliocentric model in the 16th century, which led to the believe that the sun is the center of the Universe, with the planets (including Earth) and stars orbiting it. 20.) A gas giant is a large planet composed mostly of gases, such as hydrogen and helium, with a relatively small rocky core. The gas giants of our solar system are Jupiter, Saturn, Uranus and Neptune.