Why do we have summer? On June 21 the Sun rose in the northeast and was within 10 degrees of straight overhead at its highest. (At 1 PM daylight savings time.) When the Sun went down in the northwest on June 21 it had been in the sky for almost 14 ½ hours. The longest day of the year. This left only about 9 ½ hours for the air to cool off before the Sun rose again. Having five more hours of sunshine than dark is one of the two main reasons for summer’s higher temperatures. (Rainwater Observatory in French Camp exhibit shows 3 parallel arcs of the Sun’s path in the sky in summer (longest), fall/spring (middle), and winter (shortest on the right)
Every day after June 21 the Sun will rise a bit later and farther south in the mornings. It will also set a little earlier and farther south each evening. These changes will begin slowly so you may not notice it for a few weeks. Even though the hours of sunshine are getting shorter, there will still be more hours of daylight than dark, so the heat continues to build for another six to eight weeks to reach the hottest days of the year in August.
By mid-August, the southerly motion of the sunrise and sunset quickens and you’ll begin to notice that it’s getting dark earlier. On September 23 the Sun will finally rise due east and set due west. We will have equal lengths of day light and dark. We call this day the “autumnal equinox”, the first day of fall.
You may have guessed what happens next. The sunrise and sunset locations will continue their southernly motion. The hours of daylight will continue to shorten. Finally, the Sun will stop rising and setting further south and we will have the fewest hours of daylight of the year on December 21, the winter solstice. (Solstice means “the Sun stops”) After this first day of winter, the Sun will begin the rise and set further north again and the cycle of seasons will repeat.
By the way, how fast the Earth travels in its orbit around the Sun depends of the force of the Sun’s gravity. If the Earth’s orbit were a circle, then the distance to the Sun would stay the same and all the seasons would be the same length. They are not. Winter is the shortest season at 89 days. It may seem odd, but the Earth is closest to the Sun on January 3 during the winter. The Earth moves faster and finishes its winter orbit segment quicker. Summer is the longest season at 94 days. The Earth is three million miles farther away from the Sun on July 4. Weaker gravity and a slower orbital speed mean we get five more summer days than winter days. The spring and fall seasons both have 91 days.
Next time we’ll look to the sky to see how to find the three “summer triangle” constellations and discover the “milky way”. Get a head start at https://skymaps.com/skymaps/tesmn2306.pdf
Notice: July 10 is the deadline to register to vote in the August 8 primary elections.
For questions: James Hill, Mississippi NASA/JPL Solar System Ambassador. jhill6333@gmail.com
