What is the Summer Solstice? - Definition & Facts

Instructor: Dominic Corsini
Why does the amount of daylight change throughout the year? This lesson helps answer that question through an investigation into the summer solstice. An illustration, lesson summary, and brief quiz are included.

Summer Solstice Defined

Many people throughout the northern hemisphere have likely noticed that it gets dark earlier in the winter than it does during the summer. In fact, late December marks some of the shortest days of the year with respect to daylight hours. Yet inevitably, as we move into spring the days start getting longer and continue doing so until we reach the summer solstice. The summer solstice is the longest day of the year and signifies the first official day of summer, often occurring around June 21st.

How the Summer Solstice Works

To explain the details behind the summer solstice, it is best we refer to an illustration.


There are two items worth noticing in this image. The first is the fact that Earth is tipped on its axis, and the second is that Earth is revolving around the sun in a counterclockwise manner. To begin, focus your attention on the Earth found on the far right of our image. This Earth has the northern hemisphere (top half) tilted away from the sun and the southern hemisphere (bottom half) tilted towards the sun. This means that the northern hemisphere is receiving less direct solar radiation than the southern hemisphere does. Translation: the northern hemisphere is experiencing winter, while the southern hemisphere is experiencing summer.

Now, in addition to the cold brought on by less direct solar radiation, the Earth's tilt also causes the northern hemisphere to experience shorter days during late December. As the Earth continues on its path around the Sun, the amount of solar radiation (and length of day) begins to increase. For example, the Earth shown in the back of our image has a northern hemisphere experiencing spring. As we continue along, the Earth positioned on the far left now has its northern hemisphere tilted towards the Sun. This is the position of the Earth when the northern hemisphere experiences summer and the southern hemisphere experiences winter.

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