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).

So Wait....The Days Grow SHORTER In The Summer? Yes!

It is astounding and kind of ironic that too many (who likely never had exposure to basic astronomy ) fail to realize the days actually get progressively shorter during the summer (which begins at the summer solstice, or about June 21st.)   Reference to the above diagram can help to clarify this, including that (for the northern hemisphere):

1) The shortest day of the year is at the winter solstice (Dec. 22-23)

2) The longest day of the year occurs at the summer solstice (June 21-22)

Let's examine this in a bit more detail, paying attention to the proportion of n. hemisphere solar exposure between points A and C. At point A in the diagram, 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, we have the longest day.

Summer lasts up to point B which marks the autumnal equinox,  but during this interval the length of day is continually growing shorter.  Again, the proportion of N. hemisphere area exposed to the Sun continually decreases which means from point A onwards the days continually grow shorter. The N. hemisphere observer notes the Sun's altitude in the sky is becoming lower and its path across the sky (diurnal circle) less.

This extends to point C, or the winter solstice, which marks both the shortest day (again for the N. hemisphere) and the beginning of winter. Note the tilt of the Earth's axis is away from the Sun, indicating less direct solar heat, radiation - while the S. hemisphere now receives more - hence it is now summer in that hemisphere.

The winter season extends from point C to point D, with the day lengths now increasing all the while, until the vernal equinox is reached around or about March 21, the first day of Spring. Spring then extends from March 21 to the summer solstice, with the length of days increasing further as the observer notes the  Sun now gains greater altitude in the sky, and thus longer diurnal circles (as it approaches the summer solstice (A).)

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.

The main takeaway? Days grow shorter during the summer - they grow longer beginning with the winter solstice (Dec. 22-23) and extending to the summer solstice.