Unit 9 Lesson 1

3. The largest object in the solar system that all other objects orbit.
5. A measurement that compares quantities using proportions rather than exact numbers.
10. The appearance of a planet moving backward across the sky.
11. The first object identified in the asteroid belt, now classified as a dwarf planet.
12. The scientist who first used a telescope in 1609 to observe Jupiter's moons.
13. Chunks of dust and ice that develop a glowing head and tail as they approach the sun.
15. Small circles planets move in while orbiting Earth in Ptolemy's model.
16. The assistant to Tycho Brahe who calculated that planetary orbits are not perfect circles.
18. A celestial body reclassified as a dwarf planet because it shares its orbit with other objects.
20. The scientist who proposed the sun-centered model to explain retrograde motion more simply.
21. A group of stars forming a recognized pattern or shape.

1. A type of measurement that determines an exact quantity, such as size or distance.
2. Bits of rocky or metallic space debris that enter Earth's atmosphere and generate heat and light.
4. A model of the universe with Earth at the center.
6. The four inner planets, including Mercury, Venus, Earth, and Mars.
7. The apparent shift in position of an object when viewed from different points.
8. A model of the solar system where the sun is at the center.
9. The force determined by an object's mass that holds planets in their orbits.
12. Giants The large outer planets made mostly of gas, specifically Jupiter and Saturn.
14. An instrument used to collect and concentrate light to make distant objects look larger.
17. The oval-shaped path that planets follow as they orbit the sun.
19. When a celestial object passes in front of another, like Venus across the sun.