June 2005 Sunrises, Sunsets and the Longest Day

For starters, remember the Earth’s Equator is tilted 23 ½ degrees (more than ¼ of a right angle) away from the Sun’s apparent path through the sky.  In fact, the Earth’s axis tilts more towards the Sun for Arizona’s Spring and Summer half the year and tilts away from the Sun for the other half of the year.  During our Summer, because the Sun is more in the northern sky, we can see somewhat “over” the Northern edge of the Earth” and we observe the Sun staying above the horizon longer.  During the Winter, it’s just the opposite.

Also, the Earth’s orbit around the Sun is also not a perfect circle, but a slightly flattened ellipse.  Earth is closest to the Sun in December and farthest away in early Summer.  These effects result in our earliest Arizona sunrise occurring just before 5:20a.m. on the morning of June 10th.  Before and after that date, sunrises occur just a bit later.

Even though the Earth moves faster (when closer to the Sun) and slower (when farther from the Sun), the time it takes the Earth to spin around on its axis remains constant.  So, when Earth is moving more slowly around the Sun, it takes the Earth a bit LESS time to spin around and again face the Sun.  So, the time from one apparent Noon – or one sunset –  to the next is a bit shorter.  This means the length of the day is a bit shorter than it would otherwise have been in the Summer months – and a bit longer than it would otherwise have been in the Winter months.

In any event, the net effect of these two competing effects is to offset the dates of our earliest sunrise and our latest sunset.  June 10th is our earliest sunrise and June 30th our latest sunset.  (We’ll discuss shortest day and longest night as a story for our December Skies column).

Please note however there is NO offset effect on the total duration of “daytime” (time we see the Sun above the horizon).  The later sunrises after June 10th are more than compensated by the later sunset times until June 21st.  Likewise, between June 24th and July 2nd, essentially all the sunsets occur at the same time – and the later sunrises then control the length of the day. These effect also average out just fine, so the Sun stays above the horizon in June longer each day until the 1st day of Summer.  After June 21st, our days get progressively shorter.