The Earth doesn't sit straight up and down; it tilts on its axis at an angle of about 23.5º. During the summer solstice, the planet reaches the point in its orbit where one of its poles is tilted at its maximum angle directly toward the Sun.
In June (June 20–22): The Northern Hemisphere is tilted toward the Sun.
In December (December 21–22): The Southern Hemisphere is tilted toward the Sun.
2. The Sun Hits Its Highest Point
Because of this tilt, the Sun appears to travel its highest and longest path across the sky during the day. On the solstice, the Sun is directly overhead at noon at the Tropic of Cancer (in the north) or the Tropic of Capricorn (in the south).
In fact, the word solstice comes from the Latin words sol (sun) and sistere (to stand still). For a few days before and after the solstice, the Sun's peak height in the sky appears to "stand still" before gradually lower day by day as the planet moves toward autumn.
3. We Experience the Longest Day of the Year
Because the Sun stays in the sky longer, the hemisphere experiencing the solstice gets the most hours of daylight of any day in the year.
The further away you are from the equator, the longer your day will be.
At the Arctic Circle (or Antarctic Circle in December), the planet tilts so much toward the Sun that the Sun never actually sets at all. This phenomenon is known as the "Midnight Sun."
4. Solar Radiation Intensifies
The hemisphere tilted toward the Sun receives solar rays at a much more direct, steep angle. Instead of the sunlight spreading out over a large area (like it does in winter), the energy is concentrated on a smaller area, causing the atmosphere to heat up.
Note for your students: Even though the solstice is the day we receive the most sunlight, it is not usually the hottest day of the year! That usually happens a month or two later (in July or August) because it takes time for the Earth's massive oceans and landmasses to heat up—a process called "seasonal lag."
Quick B1 Vocabulary Bonus!
If you ever want to bring this topic into your English classes, here are some great high-frequency verbs and nouns related to the solstice:
To tilt: The Earth tilts on its axis.
To peak: The Sun peaks at midday.
To shorten / To lengthen: After the solstice, days begin to shorten.
Hemisphere: The Northern Hemisphere experiences summer in June.