Natural science describes and predicts natural phenomena through empirical evidence. Science uses formal tools like mathematics and logic to explain nature. Modern science succeeded classical approaches to natural philosophy
Eyepiece is a lens closest to the eye for observing optical devices. Magnification depends on eyepiece focal length and objective focal length. Eyepieces consist of lens elements in housing with barrel
Arabs conquered North Africa and Iberian Peninsula by mid-century. Byzantine Empire defeated Arabs at Siege of Constantinople in 717-718. Battle of Tours in 732 halted Muslim advance into Western Europe. Abbasid Caliphate established in Baghdad in 750
Parsec is the distance to an object with one arcsecond parallax angle. Named by Herbert Hall Turner in 1913 to simplify astronomical calculations. Defined as 648,000/π astronomical units or 308,56775814913673 meters
Earth's axis is tilted 23.4 degrees from its orbital plane. North Pole always points toward Polaris, South Pole to Polaris Australis. Earth's tilt remains constant throughout the year
Moon completes one revolution around Earth in 29.5 days. Moon phases show how much of its illuminated side is visible. Moon doesn't emit light, reflects sunlight