Earth, Moon, and Sun.

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Across
  1. 3. the border between light and dark on the Moon
  2. 5. an object, diagram, or computer program that helps us understand something by making it simpler or easier to see
  3. 6. the sun, the planets that orbit the sun, and other objects that orbit the sun
  4. 7. the relative size of things
  5. 8. moon: a moon phase that looks like more than half a circle from Earth
  6. 10. when the Moon is completely dark
  7. 13. a rocky sphere that travels around a planet
  8. 14. a moon phase in which the Moon is not visible from Earth
  9. 17. the nearly circular path a smaller object (like the Moon) travels around a larger object (like Earth).
  10. 18. a planet outside our solar system
  11. 19. the amount of time it takes for a planet to complete one orbit around a star or stars
Down
  1. 1. a moon phase that looks like a half-circle from Earth
  2. 2. a moon phase that looks like less than half a circle from Earth
  3. 4. a moon phase that looks like a full circle from Earth
  4. 9. to shine light on an object and make it visible
  5. 11. a moon outside our solar system
  6. 12. something we observe to be similar over and over again
  7. 15. the star that is the main source of light at the center of our solar system
  8. 16. the shape of the illuminated part of the Moon as it appears from Earth