Selected Questions-Answers On All Experts Astronomy Forum (The Summer Solstice - Why The Longest Day?)

Given today marks the summer solstice, or the longest day of the year, I decided one of the older All Experts questions would be apropos:

Question:  Can you explain for me why the summer solstice is the longest day of the year? In other words, what exactly makes it so?  - Brian, Miami, FL.

Answer:  A glance at the diagram below showing the Earth's axial tilt at different times in its orbit should make things clear.



Note first,  point A in the diagram (far left) , we have the commencement of summer in the northern hemisphere (summer solstice) because the Earth's axis is tilted toward the Sun hence the N. hemisphere receives more direct sunlight, radiation. Also at point A, there is the longest day.  This date is given the name "summer solstice". 

What makes it so?  Examining the diagram carefully you can see the greatest fraction of sunlit surface appears at this date.  (Adjacent dates, i.e. June 20, and June 22, approach this but don't equal the same large fraction.).  This is given the Earth's axis is now fully tilted (at 23.5 degrees) toward the Sun.  This greatest fraction of sunlight translates into the greatest duration of daylight for the date. Hence the longest day.  

Prior to the summer solstice and extending all the way from the date of the winter solstice (on or about Dec. 21st, at extreme right) the days are continually increasing in length as the fraction of sunlit surface continually increases. Note also at each of the equinoxes (March 21st - Spring or Vernal ;  Sept. 23rd- Autumnal) the lengths of the day are the same, at 12 hrs.   Hence the term "equinox" or "equal night". 

 In summary then, the tilt of the Earth modulates the extent or the amount of sunlight-radiation (including heat) a hemisphere receives at a particular time, or over a given interval. Onset of winter and summer are defined respectively by the extreme axial tilts of  either "fully away" from the Sun, and "fully toward", while Fall, Spring are defined by neutral tilts - i.e.  neither away nor directly toward.

Finally, it is important to point out that the "Longest day" on June 21st applies to the northern hemisphere only. For the southern hemisphere it is the shortest day while our winter solstice is the longest.  Again,  this can be grasped by examining the diagram and noting while the proportion of sunlit surface is large for us (in the N. Hemisphere) it is receding for the south.  The converse is true when one  looks to the image for Earth on the opposite side (Dec 21).