Western Slope Skies - The Summer Solstice

Jun 11, 2021

Credit NASA
  Have you ever wondered why the summer solstice is the longest day in the year? This year the summer solstice will be on Sunday, June 20th, which will officially be our first day of summer.

During our summer solstice here in the northern hemisphere, the southern hemisphere is having its winter solstice at the same time, meaning it is their shortest day and their first day of Winter.  Solstice comes from the Latin words "sol," meaning "sun," and "sistere," meaning "standing still." The ancients noticed that as summer progressed, the sun stopped moving northward in the sky at its highest point, which happens at noon. It then began tracking southward again as summer turned to autumn. Neolithic humans may have initially observed the summer solstice as a marker to decide when to plant and harvest their crops. In Ancient Egypt, the summer solstice corresponded with the rise of the Nile River and may have helped to predict annual flooding.  The summer solstice may be the longest day of the year, but it will not coincide with the latest sunset or earliest sunrise. The earliest sunrise happens a few days before the summer solstice, and the latest sunset will be a few days afterward. This is due to the tilt of Earth's axis and the eccentric shape of Earth's orbit.  The summer solstice is not to be confused with daylight savings, since we are not changing our clocks. So, what exactly is the summer solstice? The Earth is spinning on an axis once per day, which is why we experience day and night.  If this axis were exactly perpendicular to our orbit around the sun, we would not have seasons and the length of the day would not change much throughout the year.  However, the axis of the Earth is tilted by about 23.5 degrees and this stays fixed as we orbit the sun.  This means that as we move around the sun throughout the year, different parts of the Earth will point slightly towards or away from the sun. For June solstice, the north pole tilts directly to the sun and the south pole directly away. Here on the Western Slope of Colorado, it will be rising north of east to its highest point in the sky at noon, and setting north of west, making the sun appear to move in its longest arc across the sky. That is what makes it the most interesting and longest day of the year. As someone who loves the sun, I personally love this day. Which do you prefer? More daylight or more nighttime?  Western Slope Skies is produced by the Black Canyon Astronomical Society. This feature was written and recorded by Katie Bent, an astronomy student of Dr. Catherine Whiting at Colorado Mesa University.