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Rotation and the Length of a Day
Earth completes one rotation roughly every 24 hours
Explanation
The exact time it takes Earth to complete one full rotation (360°) on its axis is about 23 hours 56 minutes (a sidereal day), but a solar day — measured relative to the Sun — comes out to about 24 hours because of Earth’s orbital motion.
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Revolution and the Length of a Year
Earth orbits the Sun once roughly every 365.25 days
Explanation
The time it takes Earth to complete one orbit around the Sun isn’t exactly 365 days, but about 365.2422 days. Those extra roughly six hours accumulate every four years into February 29 — a leap day.
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How Leap Years Work
An extra day is added to February every four years
Explanation
To correct for the leftover time in Earth’s orbital period, a leap year is added every four years — but years divisible by 100 are common years, while those also divisible by 400 are leap years, a refined rule (for example, 2000 was a leap year, but 1900 was not).
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Earth’s Axial Tilt
Earth is tilted about 23.5 degrees
Explanation
Earth’s rotational axis is tilted about 23.5 degrees relative to its orbital plane, and this tilt is the fundamental reason the seasons exist.
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Why the Seasons Happen
The tilt — not the distance — causes the seasons
Explanation
Many people assume otherwise, but the seasons are caused not by a change in the distance between Earth and the Sun, but by the tilt of Earth’s axis. When the Northern Hemisphere tilts toward the Sun it’s summer there, and winter when it tilts away.
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Summer and Winter Solstice
The longest and shortest days of the year
Explanation
The summer solstice (around June 21) is the longest day of the year in the Northern Hemisphere, and the winter solstice (around December 21) is the shortest. The Southern Hemisphere is the exact opposite.
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Spring and Autumn Equinox
The days when daytime and nighttime are nearly equal
Explanation
At the spring equinox (around March 20) and autumn equinox (around September 22), the Sun passes directly over the equator, making day and night nearly equal in length almost everywhere in the world.
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Reversed Seasons Between Hemispheres
The Northern and Southern Hemispheres have opposite seasons
Explanation
Because of the direction of Earth’s axial tilt, the Southern Hemisphere has winter while the Northern Hemisphere has summer, and vice versa. That’s why Christmas falls in the middle of summer in Australia.