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Across 4. The ___ is the star at the center of the Solar System 5. The ___ of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle (2 Words) 7. ___ , in the best-known case, occurs when an orbiting astronomical body always has the same face toward the object it is orbiting (2 Words) 8. ___ on Earth is the illumination of the lower atmosphere when the Sun is not directly visible because it is below the horizon 9. A ___ is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and – according to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals 11. In physics, a ___ consists of an abstract coordinate system and the set of physical reference points that uniquely fix the coordinate system and standardize measurements within that frame (3 Words) 13. Earth orbits the Sun at an average distance of 149 million miles (2 Words) 15. ___ is the second planet from the Sun 16. ___ occurs at the darkest stage of twilight, or at the very end of astronomical twilight after sunset and just before night 17. The ___ is the gravitationally bound system of the Sun and the objects that orbit it, either directly or indirectly (2 Words)

Down 1. In the Solar System, a planet is said to be inferior or interior with respect to another planet if its orbit lies inside the other planet's orbit around the Sun (4 Words) 2. In astronomy, ___, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane (2 Words) 3. In celestial mechanics, ___ occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers (2 Words) 4. The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars (2 Words) 6. ___ is the time that marks the beginning of twilight before sunrise 10. A ___ is a certain portion of a planet's area that reflects sunlight as viewed from a given vantage point, as well as the period of time during which it occurs (2 Words) 12. The ___ compose the background of astronomical objects that appear to not move relative to each other in the night sky compared to the foreground of Solar System objects that do (2 Words) 14. ___ is the closest planet to the Sun