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Across 3. tides and planetary orbits are caused by the 5. is the amount of matter in an object; it does not depend on forces acting on it. 7. galaxy that is spherical or a flattened disk 9. The Sun is a star in the _ galaxy located in a spiral arm about two-thirds of the way from the center of the galaxy. 11. Because distances in space are so great that conventional numbers are too large to work with, astronomers use a unit of measurement called _ to measure the distance to stars and galaxies in space. The distance in one _ is equal to the distance light travel in one year. 14. The day when the tilt of Earth causes the length of day and night to be equal, an _ occurs. This is the start of spring and autumn. 17. The Sun’s gravitational attraction, along with the planet’s inertia (continual forward motion), keeps the planets moving in _ (Earth’s orbit is slightly oval) and determines how fast they orbit. 22. Refracting and reflecting _ collect visible light, then use convex lenses or mirrors to focus the light producing larger, brighter images of distant objects in space. 23. the rise and fall of ocean water 25. The Sun also pulls on Earth and o can combine its force with the Moon causing even higher tides 26. Planets nearer the Sun move/orbit _ than planets farther from the Sun because the gravitational attraction is greater.

Down 1. contain instruments to collect data and travel out of Earth’s orbit to explore places that would be too dangerous for astronomers; the instruments that a probe contains depends upon the space mission. 2. The concept of _ can be used to explain the difference between mass and weight. 4. Mass is measured on a 6. collect the light from distant stars and separate that light into bands of different colors; by studying these bands, astronomers identify the elements in a star. 8. Weight is measured using a 10. is a measure of the pull of gravity on an object. 12. The _ being most massive object in the solar system has the greatest pull on objects, like planets, in the solar system. 13. receive radio waves emitted from objects in space, including waves from very distant stars and galaxies; it can receive information in any weather and during day or night. 15. The Sun also pulls on Earth and can be a right angles, pulling against the Moon’s pull, causing very little tidal change 16. The day when the tilt of Earth causes the length of day to be the longest or the shortest, a _ occurs. This is the start of summer and winter. 18. galaxy with no definite shape 19. are placed in orbit around Earth with special instruments and telescopes that collect information from space which is sent back to Earth where it is interpreted. 20. are made up of gas, dust, and billions of stars and have different shapes 21. galaxy with a nucleus of bright stars and two or more spiral arms 24. The Sun is a _ in the Milky Way galaxy located in a spiral arm about two-thirds of the way from the center of the galaxy.