Message #135521

[Bianca]

How it works



Earth's multiple motions - spinning on its axis and orbiting the sun - are behind everything from day and night to the changing seasons


The sun comes up each day because Earth rotates once on its axis every 24 hours or so. Seasons are a result of Earth being tilted 23.5 degrees on its spin axis coupled with the planet's 365-day orbit around the sun.


(At the North Pole, the sun rises only once a year - at the start of spring. It gets higher in the sky each day until the summer solstice, then sinks but does not truly set until late September, at the autumn equinox.)


Imagine Earth as an apple sitting on one side of a table, with the stem being the North Pole. Tilt the apple 23.5 degrees so the stem points toward a candle (the sun) at the center of the table. That's summer for the top half of the apple.


Keep the stem pointing in the same direction but move the apple to the other side of the table: Now the stem points away from the candle, and it's winter on the top half of the fruit. The very top of the apple, representing the north polar region, is in total darkness 24 hours a day, during that season.


At winter solstice, the sun arcs low across the Northern Hemisphere sky for those of us below the Arctic Circle, and the stretch of daylight is at its shortest. By the time of the spring equinox, days have grown noticeably longer. At the summer solstice, the sun gets as high in our sky as it can go, yielding the longest day of the year in the Northern Hemisphere.


As long ago as the fourth century B.C., ancient peoples in the Americas understood enough of this that they could create giant calendars to interact with the cycle of sunlight. They built observatories of stone to mark the solstices and other times important for planting or harvesting crops. Shrines and even tombs were also designed with the sun in mind.