Thursday, March 20, 2014

Vernal Equinox

Today at 12:57 PM EST, Winter officially ends and Spring begins in the Northern Hemisphere.  The day that this change occurs is known as the Vernal (Spring) Equinox.  The word equinox comes from the Latin words aequus (equal) and nox (night).  On the Equinox the sun strikes directly on the Equator resulting in approximately equal periods of day and night across the globe.

The Earth rotates around its axis approximately once every 24 hours.  However this axis is tilted at 23.5 degrees from the vertical.  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.

At any given time, fifty percent of the earth is in sunlight (Day) and the other fifty percent is in darkness (Night).  However, because the Earth is tilted on its axis sunlight does not always strike the Earth at the same angle.  This means during different seasons different parts of the Earth will receive varying amounts of sunlight and darkness.

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, the sun lights a larger portion of the Northern Hemisphere than it does the Southern Hemisphere.  When this happens, we experience Summer in Mid-Michigan and the Southern Hemisphere experiences Winter.  When the North Pole is at its furthest from the sun, we experience Winter and the Southern Hemisphere experiences Summer.  During our Northern Winter, the sun is striking a a larger portion of the Southern Hemisphere than it is the Northern Hemisphere.

Today the North Pole and South Pole are both at the same distance from the sun.  Direct sunlight is falling directly on the Equator, lighting the Northern and Southern Hemispheres equally.  Here in Mid-Michigan (and the rest of the Northern Hemisphere), we are passing into Spring and the Southern Hemisphere is passing into Fall.

My attempt at a diagram showing how the sun lights different parts of the globe on different days

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