At 4:02 a.m. local time, the Sun passed into the northern hemisphere by crossing the Celestial Equator.  At that moment, spring officially began for the northern hemisphere and autumn for the southern hemisphere.  It is called the Spring Equinox and it is when the Earth’s tilt is neither towards nor away from the Sun.  The result is nearly equal periods of daylight and darkness.  For Phoenix, the Sun will rise at 6:32 a.m. and set at 6:40 p.m.

Over the next three months, the Sun will climb further north in our sky.  At 10:04 p.m. on June 20, the Sun will be as far north as it will get.  For the northern hemisphere, this is when summer officially begins.  For the northern hemisphere, the tilt of the Earth is towards the Sun.  The day is the longest (14 hours) of the year and the night the shortest (10 hours).  From that moment, the Sun will begin a six month retreat south.  At 1:44 p.m. on September 22, the Sun will pass the equator heading south.  This is the beginning of autumn in the north and spring in the south.  At 10:11 a.m. on December 21, the Sun will be at its southern most position.  This is called the Winter Solstice and marks the official start of winter in the northern hemisphere.  For the northern hemisphere, we are tilted away from the Sun.  It is when the days are the shortest (10 hours) and the nights are the longest (14 hours).  The Sun then begins the long trek north again.

The seasons are a result of the tilt of the Earth and have nothing to do with how close or far away the Earth is from the Sun.  On average, the Earth is ninety-three millions miles from the Sun.  In astronomy, this distance is referred to as one astronomical unit.  Because the Earth’s orbit is not a perfect circle but instead an ellipse, it is sometimes a little closer to the Sun and sometimes a little further away.  A planet’s closest point to the Sun is called perihelion.  For the Earth, it occurs in January and the Earth is about ninety-two million miles from the Sun.  It is also the point when the Earth is moving the fastest in its orbit.  Aphelion is when we are at the most distant point in the orbit and it occurs in July.  The Earth is 94 million miles from the Sun and moves the slowest in its orbit at aphelion.

Learn more about the seasons, the constellations, fascinating objects in the sky, and the latest astronomy new by joining us for Arizona Skies- shown daily at 1:30 PM in the Dorrance Planetarium.

This post was written by Mike George, our Senior Manager of Planetarium and Science Visualization.

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One Response to The First Day of Spring

  1. Alyssa Ast says:

    I had no idea the distance of the Earth had nothing to do with seasons. Great information!

    http://www.whattodoinarizona.info/phoenix/

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