An Extended Explanation of The Summer Solstice

 

The seasons in perspective for Earth's orbit

                                                         Focus on summer solstice

Today marks  the first day of summer, when the Earth reaches the summer solstice in its orbit.  So we are talking about not only a specific time (8:42 p.m, Mountain time today for Colorad0) but also a position and orientation in space. That position and orientation is what typically makes the days just before and after the summer solstice  warmer  in the Earth's northern hemisphere. (Our days here in Colorado have suddenly soared into the mid 90s.)

And note this has nothing to do with the distance from the Sun. In fact, on today's date Earth is actually approximately 1.5 million miles further away from the Sun, than it would be at the winter solstice (December 21). The diagram below clarifies this in terms of Earth's elliptical orbit:

Here, the generic (standard) date (June 21) for the summer solstice - as a position on the orbit  - is approaching the aphelion or greatest distance from the Sun. (Occurring July 3rd). Conversely, we see the winter solstice position in the orbit approaching the perihelion (nearest distance from the Sun) - which is on January 4th. The point is that distance from the Sun is not the determinative factor in the heating of the  northern hemisphere, but it's rather the inclination or tilt of the toward the Sun (seen in better relief in the 2nd diagram from the top.)

We can now go through the procession of one season to the next using the top illustration - which of course is not to scale.

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).

See Also:

The summer solstice is here. What to know about the longest day of the year

And

Brane Space: Tackling Intermediate Astronomy Problems (6)

And:

Brane Space: The Longest Day (Summer Solstice) - But Days Get Progressively Shorter Hereafter - How That Occurs