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Leap Year, explained

Professor Marshall Shepherd uses his Forbes column to embark on a perfectly understandable explanation of why we have seasons, the actual length of Earth's trips around the sun, et voilà, the need for a Leap Year:

Throughout the year, different parts of the Earth receive the Sun’s rays more directly. In Boreal Winter, the Northern Hemisphere is tilted away from the Sun. At the same time, Austral Summer is happening because the Southern Hemisphere is “tilted” more towards the Sun. By the way, many scientists believe that a protoplanet, Theia, smashed into Earth many years ago causing the axial tilt.

We typically refer to a “year” as the length of time required for Earth to orbit the Sun. Our modern Gregorian calendars tell us that this takes 365 days. Ah, but there is a subtle difference in these two representations of a “year.” The time length of Earth’s orbit around the Sun is called a “tropical” or “solar” year. This period is typically measured from Spring Equinox to Spring Equinox. This amount equals 365 calendar days, but there are roughly 5 to 6 hours of extra time because in reality the length of time that we consider a “day” (time it takes for Earth to revolve one time on its axis) is just short of 24 hours (approximately 23 hours, 56 minutes, and 4 seconds). To summarize, it takes the Earth around 365.242199 days to go around the Sun, but the calendar only has 365 days.

Public scholarship at work, people. Thanks, Dr. Shepherd, for your great work in the classroom, over the broadcast waves, at the White House and in the news media. Knowledge is power - sharing knowledge is leadership that empowers and inspires.

Image: Why we have seasons. Image courtesy of NASA Scilinks

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