Monday, September 23, 2013


Happy Fall!

Yesterday at 4:44 PM EST, Summer ended and Fall officially began.  The first day of Fall is also known as the Autumnal (Fall) Equinox.  The word Equinox means "equal night" and comes from the Latin words aequus (equal) and nox (night).  On this date the sun is striking directly on the equator, resulting in approximately equal amounts of daylight and darkness around the globe.

Have you ever looked at a globe and wondered "Why is it tilted?"  This tilt actually helps us explain the seasons.

This tilt represents the actual tilt of the earth on its axis.  This axis is tilted at 23.5 degrees from the vertical.   The Earth rotates around its axis approximately once every 24 hours.  It completes this rotation approximately 365 times for each trip around the sun - giving us years with approximately 365 equal twenty-four hour days. Each day is not exactly 24 hours and each year is not exactly 365 days - this is why we have a leap year every four years in which an extra day is added to make up for the slight difference between our calendar calendar year and the solar year.

The points on the globe that the axis revolves around are referred to as the North and South Poles.  The axis is always pointed toward the same location in the sky.  The North Pole points toward the "North Star" - Polaris.

As the earth revolves around the sun, sometimes the North Pole is closer to the sun, sometimes the South Pole is closer to the sun.  When the North Pole is at its closest, we experience Summer in Mid-Michigan and the Southern Hemisphere experiences Winter.  When the North Pole is at its furthest, we experience Winter and the Southern Hemisphere experiences Summer.

The hemisphere that is closer to the sun experiences has more hours of sunlight per day than the hemisphere that is further from the sun.  For example, when the Northern Hemisphere is closest to the sun (Summer Solstice), Mid-Michigan receives approximately 15 hours and 24 minutes of sunlight; when the Northern Hemisphere is at its furthest point from the sun (Winter Solstice), Mid-Michigan receives only 8 hours and 58 minutes of sunlight.

Twice a year, on the Vernal and Autumnal Equinoxes, the North Pole and South Pole are the same distance from the sun.   Yesterday, Mid-Michigan received approximately 12 hours of sunlight and 12 hours of darkness.  The days will continue to become shorter and nights longer until the Winter Solstice.

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