June 20, 2021, was the summer solstice in the Northern Hemisphere and the longest period of sunlight of any day this year. Now that we have passed the summer solstice, the days are getting shorter. That raises the question of why the summer solstice is not the hottest day of the year since the Sun is up for the longest time. The answer to that is simple. It’s because every day until September’s fall equinox has more hours of sunlight than darkness, and there is a heat lag. It takes a long time for Earth to heat up after winter, so normally the hottest day is well after the summer solstice.
A more important point is why the summer solstice happens and how it is critical to life on planet Earth. The cause of the solstice and the equinox is that Earth is tilted on its axis by 23 ½ degrees. As it orbits the Sun, that tilt causes every point on Earth to experience different amounts of Sun each day and controls the angles at which the Sun’s rays hit Earth’s surface. On the equinox, the Sun is directly overhead at the equator. At noon, the Sun would shine down to the bottom of a well on the equator. Every other latitude on the planet would have the Sun’s rays hitting Earth’s surface at an angle that is not perpendicular.
As Earth revolves around the Sun, it is essentially a giant gyroscope with the poles always pointed in the same direction. It isn’t until the solstices that the Sun would shine right down a well at its northern or southern position, and that happens to be 23 ½ degrees north or south latitude. So why is that important?
If Earth did not have the axis tilt, we would not have the summer solstice in the Northern Hemisphere. The Sun would always be directly overhead at the equator all the time. Land near the equator would be so hot that life could not exist there. Areas to the north and south would be extremely cold. The temperature differences would create extreme winds, making life difficult. When scientists ran computer simulations, they found that 23 ½ degrees is ideal for minimizing overheating and cooling.
Earth also has a heat sink designed into it—the oceans. Most of the southern hemisphere is covered with water, while the northern hemisphere has more land. Since Earth’s orbit is an ellipse rather than a perfect circle, the southern oceans absorb much of the heat when Earth is closest to the Sun. It just happens that Earth is closest to the Sun when the Southern Hemisphere is in the summer season. It seems like it was planned that way.
We are thankful that we had the summer solstice in the Northern Hemisphere. The Earth’s tilt, spin, and distribution of land and water are not accidents. They are designed features of our planet that speak of God’s wisdom and planning in the creation.
— John N. Clayton © 2021
