Observational Mathematics

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Across
  1. 6. Responsible for the slight changes in equatorial coordinates over time (10)
  2. 8. Calculated by subtracting the declination from 90 (5, 8)
  3. 10. A measure of the angular distance from the First Point of Aries (5, 9)
  4. 13. The time since an object last crossed your meridian (4,5)
  5. 14. A star does this as it passes through the meridian on its upper transit (9)
  6. 16. The point directly over your head (6)
  7. 17. The bearing from north of the object (7)
  8. 19. The line passing directly overhead from north to south (8)
Down
  1. 1. The path of the Sun as it appears in our sky (8)
  2. 2. A type of day designed to keep the stars in a fixed location (8)
  3. 3. The angle between the celestial equator and the object (11)
  4. 4. A user-friendly coordinate system (10)
  5. 5. A precise coordinate system (10)
  6. 7. The unit for measuring right ascension (5)
  7. 9. Unit for measuring declination (7)
  8. 11. Meaning 'closeness', the reason Solar System objects have continuously changing coordinates (9)
  9. 12. The Greek letter used as a symbol for declination (5)
  10. 14. An object that never 'rises' or 'sets' (11)
  11. 15. The angle between the observer's horizon and the object (8)
  12. 18. The kind of motion that makes the stars appear to move (7)