THE SKY

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Altitude-Azimuth coordinate system

• Zenith = point directly overhead


• Horizon = boundary between ground and sky


• Meridian = arc from north to south through the zenith


• Altitude = angle of a point above the horizon


• Azimuth = angle of a point from north


• Nadir = point directly down

Earth rotation

• Earth rotates


• Objects do not stay in same place. Earth rotates on its axis, which is an imaginary line running from the North Pole to the South Pole. This rotation is what gives us day and night.


• Earth takes about 24 hours to complete one full rotation. This is why we have a 24-hour day, with daylight and nighttime alternating as different parts of Earth face the Sun.