June Sunrises, Sunsets and the Longest Day

There are not one but two reasons why the longest day of the year is not also the day of the earliest sunrise (or the latest sunset) of the year. 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 slightly longer than if we were exactly on the Earth’s Equator.  (During the Winter, it’s just the opposite!)

Secondly, the Earth’s orbit around the Sun is also not a perfect circle, but a slightly flattened ellipse.  Our 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.  Both before and after June 10th, sunrises occur just a bit later in the morning.

Note that the Earth moves faster in its orbit when closer to the Sun in Winter and moves a bit slower when farther from the Sun in our Summer.  However, the time the Earth takes to spin around on its axis remains constant.  So, when Earth is moving more slowly around the Sun (Summer), 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 separate story in our December Skies column).

Please note however that to find the length of the “daytime” each day, we simply subtract the time of sunrise from the time of sunset.  This is equivalent to using a stop watch to calculate the length of the day. It will still show that the longest day is June 21st!

Why?  Because our later sunrises after June 10th are more than offset by later sunset times until June 21st.  Then, the later sunrises start to dominate the length of the day.  On June 22nd, the sun is above the horizon a bit less than on June 21st.  Between June 24th and July 2nd, all our sunsets essentially occur at the same time – and the later sunrises then completely control the length of the day. After July 2nd, our sunsets occur earlier each evening and the “daytime” rapidly shortens as we complete Summer and move into Autumn anticipating Winter.