Motions of the Earth

Rotation: Earth spinning on its own axis (an imaginary line through the North and South poles). Direction: west to east (counterclockwise when viewed from the North Pole). Causes day and night — the side facing the Sun has day; the opposite side has night.

Earth's axis is tilted at 23½° from the perpendicular to its orbital plane. This tilt is the cause of seasons.

Circle of Illumination: The boundary between the day and night sides of Earth — a great circle. Because of Earth's spherical shape, this boundary creates the curved shadow seen during lunar eclipses.

Why seasons occur:

Earth's axis is tilted 23½° from the perpendicular. As Earth revolves around the Sun (takes ~365.25 days = 1 year), different hemispheres are tilted toward or away from the Sun at different times of the year.

June 21 (Summer Solstice — Northern Hemisphere):

• Northern Hemisphere tilted toward Sun → more direct sunlight → summer
• Days are longest in the North; Sun rises earliest, sets latest
• At the North Pole: midnight sun (24 hours of daylight)
• At the South Pole: polar night (24 hours of darkness)
• Sun is directly overhead at the Tropic of Cancer (23½°N)

December 22 (Winter Solstice — Northern Hemisphere):

• Northern Hemisphere tilted away from Sun → less direct sunlight → winter
• Days shortest in the North; Sun rises latest, sets earliest
• Reverse for Southern Hemisphere — their summer
• Sun directly overhead at Tropic of Capricorn (23½°S)

March 21 and September 23 (Equinoxes):

• Neither hemisphere is tilted toward or away from the Sun
• Sun is directly overhead at the Equator
• Day and night are approximately equal everywhere on Earth (~12 hours each)
• "Equinox" = Latin for "equal night"

Leap Year: Earth takes 365 days, 5 hours, 48 minutes, and 46 seconds to complete one revolution. The calendar year is 365 days. The extra ~6 hours accumulate — every 4 years, one full extra day has accumulated → a Leap Year adds February 29.

Exception: Century years (1700, 1800, 1900) are NOT leap years — unless divisible by 400 (1600, 2000, 2400 ARE leap years). This correction makes the calendar accurate.

Why it matters: Without leap year correction, our calendar would slowly drift away from the seasons — eventually, December would be in summer! Astronomical accuracy matters for agriculture (planting seasons), religious festivals (Hindu festivals use solar/lunar calendar), and navigation.