Motions of the Earth
To understand how sundials work, we must first know something about how the Earth is moving. In fact, the Earth is moving in a dizzying variety of ways. First, the stars of the Milky Way Galaxy (including the Sun and its satellites) are all moving like a pinwheel around the center of the galaxy. Over thousands of years, the Earth varies its orbital path around the Sun (eccentricity) from more elliptical to more circular, changes the inclination of its tilt axis (obliquity) from about 22 to 24.5, and wobbles on its axis (precession) like a wobbling, spinning top. But there are two fundamental motions of the Earth that especially relate to time:
Rotation - the spin of the Earth. The Earth is rotating like a spinning top around an imaginary axis (spin axis) that runs through the Earth from the North Pole to the South Pole. The spin velocity is just over 1,000 mph at the Equator, and about 900 mph at Niceville, Florida. The Earth makes one full (360) rotation in about 24 hours.
Revolution - the orbit of the Earth around the Sun. Earth's orbit varies in speed (moving faster when closer to the Sun, and slower when further from the Sun), but its average velocity is about 66,600 mph, and it completes one revolution in about 365.25 days, making one year. An additional day, February 29th, is added to the calendar every 4 years to account for the annual 0.25 difference - this is a leap year.
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