There is something special about the first day of fall. The air begins to change, leaves slowly transition into new colors, and the days gradually become shorter. But what actually causes the seasons to change? The answer has less to do with how far Earth is from the Sun and much more to do with the way our planet is tilted as it travels through space.
Earth is tilted on its axis by about 23.5 degrees. As Earth orbits the Sun throughout the year, that tilt causes different parts of the planet to receive different amounts and angles of sunlight. When the Northern Hemisphere is tilted toward the Sun, we experience summer. As the Northern Hemisphere begins tilting away from the Sun, we transition toward fall and eventually winter. Interestingly, Earth is actually closest to the Sun during Northern Hemisphere winter, so our changing distance from the Sun is not what creates the seasons.
The first day of fall is marked by the autumnal equinox. An equinox occurs twice each year, in the spring and fall, when neither hemisphere is tilted more toward the Sun than the other. Around the equinox, sunlight is distributed more evenly between the Northern and Southern Hemispheres. The word “equinox” comes from Latin words meaning “equal night,” reflecting the near balance between daylight and nighttime around this point in the year.
After the autumnal equinox, the Northern Hemisphere continues moving toward shorter days and longer nights. As sunlight reaches us at a lower angle, the amount of solar energy received during the day changes, contributing to the cooler conditions we associate with fall. These gradual changes also influence plants and animals, from trees preparing for winter to animals changing their behaviors and food-gathering patterns.
The arrival of fall is a wonderful reminder that science is happening all around us. A changing leaf, an earlier sunset, or a cooler morning can become the starting point for a simple scientific investigation. At Challenge Island, we love encouraging kids to ask questions about the world around them, explore how things work, and turn curiosity into discovery. After all, sometimes the best science lessons begin simply by looking outside and asking, “Why?”
