What is the Summer Solstice?

 June 21st is also known as the summer solstice, the first day of summer, or the longest day of the year depending on who you talk to (this applies for the Northern Hemisphere, in the Southern Hemisphere substitute winter for summer and shortest for longest).  But why does the solstice occur and why is today the longest day of the year?

What is the Solstice?

As you are reading this it probably feels like you are sitting still but you are actually moving and moving quite fast at that.  You are on the Earth which is spinning on its axis and orbiting around the Sun.  All this motion is what accounts for the seasons and the length of the day, lets look at how this works.

Many people mistakenly think that the seasons are caused by the Earth getting closer or farther from the Sun and while the Earth does change it's distance from the Sun over the course of the year due having an orbit that isn't a perfect circle, this isn't the cause of the seasons.  Earth's elliptical shaped orbit causes it to change distance from the Sun but only very slightly, a 1.7% difference between the closest and farthest points in its orbit.  This change isn't enough to cause significant changes in temperature.  The fact that the Northern and Southern Hemispheres have opposite seasons helps show how distance from the Sun can't be the cause for the seasons; further evidence comes from the fact that Earth is actually farthest from the Sun in July and closest in January.

So what causes seasons then?  Well remember when we said the Earth is spinning on its axis, well that axis is tilted by 27.5 degrees compared to Earth's orbit around the Sun and this axis tilt is what causes the seasons by changing how direct the sunlight is and how long the Sun is in the sky for a particular spot on Earth.

As the Earth orbits around the Sun, there are times when a particular hemisphere is pointed towards the Sun and times when it is pointed away.  When a hemisphere is pointed towards the Sun it is summer in that hemisphere and when a hemisphere is pointed away from the Sun, it is winter in that hemisphere.  Whether a hemisphere is pointing towards or away from the Sun will change how high the Sun will get in the sky.

(Image courtesy of NASA)

When a hemisphere is pointed towards the Sun, the Sun will appear higher in the sky, when the hemisphere is pointed away from the Sun the Sun appears lower in the sky; we've probably all noticed this before in the summer and winter but never really thought of it.  How high the Sun is in the sky will have two effects.  The first is that the higher the Sun is in the sky, the longer it will stay above the horizon and the more hours of daylight we will have; this is easy to see in the image to the right.  The higher the Sun is in the sky, the more direct and intense the light from it will be whereas if the Sun is lower in the sky, it's light is spread out more and thus less intense.  In the summer the Sun is high in the sky so it stays above the horizon for longer and we get more direct light, leading to warmer temperatures.

The summer solstice is the point in the year when a particular hemisphere is pointing directly at the Sun, causing it to be at it's highest point in the sky; for the Northern Hemisphere this occurs on June 21st.  Since the Southern Hemisphere is pointed directly away from the Sun on this date, they will see the Sun at it's lowest point in the sky and will experience the winter solstice.

The Longest Day of the Year

When people say that the summer solstice in the longest day of the year you have to realize there are four different types of days that are referred to.  Many people refer to the day as the time when the Sun is above the horizon and it is light outside while others refer to a calendar day, the 24 hour period starting at midnight.  Astronomers of course have two other types of days that they refer to, the solar day and the sidereal day; the differences between these two types of days and their relationship with a calendar day, will be the subject of a future post here.

When people refer to the summer solstice as the longest day of the year, they are referring to how long the Sun is above the horizon or how long it is light outside.  As seen in the image above, on the summer solstice the Sun is at its highest point in the sky.  This means that the Sun has the longest path above the horizon which means it will stay above the horizon for longer and it will be light outside for longer.  Conversely on the winter solstice the Sun is at it's lowest point in the sky and spends the least amount of time above the horizon.

Why is it Hotter Later in the Summer?

The summer solstice is the day we get the most direct sunlight and the most hours of sunlight so why isn't today the hottest day of the year?  Or why is June not even the hottest month in the Northern Hemisphere?  Well the summer solstice is the day that the Northern Hemisphere receives the most energy from the Sun but the amount of energy that we receive from the Sun isn't the only thing that determines the temperature on that day.  Air, ground, and water do not heat up instantly when exposed to energy from the Sun, it takes time for them to absorb that energy and heat up.  This results in the warmest time of the year happening several weeks after the summer solstice.

Summer Solstices and Ancient Cultures

(Courtesy of Space.com)

Pretty much every ancient culture realized the importance of being able to tell what time of year it was as this helped with planting and harvesting crops, migrations, etc..  The best way to tell the time of year was to use astronomy and the positions of objects in the sky.  As a result, knowing where the Sun would be in the sky on the solstices was important knowledge for these cultures.  Legends and stories were created to help people remember where the Sun would be on these dates and many cultures even aligned key features of buildings with where the Sun would rise and set on the solstices; Stonehenge being a great example of this.

Lines on a Globe

(Courtesy of King5 Weather)

If you've ever looked closely at a globe you've probably noticed there are two other horizontal lines that run parallel to the equator, the Tropic of Cancer and the Tropic of Capricorn.   Most people who notice these don't really know what they mean or why they are there.  The tropics actually are directly related to the solstices and they mark where on Earth the Sun would be directly overhead on the solstices.  On the June solstice the Sun is directly over the Tropic of Cancer while on the December solstice the Sun is directly over the Tropic of Capricorn.