Today’s Wonder of the Day was inspired by Audrey from Wheeling, WV. Audrey Wonders, “Why does the sun set earlier every night?” Thanks for WONDERing with us, Audrey!
Have you ever given much thought to sunrises and sunsets? Wouldn't it be great if the Sun rose every morning at the same time? Wouldn't you enjoy the convenience of knowing exactly when the Sun would set at the same time each night?
Unfortunately, the world doesn't work that way. If you live in North America, you probably look forward to those long summer days when you can play outside for hours on end because the Sun doesn't set until late in the evening.
Things would be different if: (1) Earth orbited the Sun in a perfect circle; and (2) Earth's axis was perpendicular to the plane of its orbit (straight up and down). If that were the case, the Sun would rise and set at the same times every day. It would also take the same path across the sky every day of the year.
However, neither of those conditions is true for Earth. Instead of a perfectly-circular orbit, Earth's orbit around the Sun is slightly elliptical. This means that the Sun travels across the sky at slightly different speeds from day to day depending upon where Earth is in its orbit.
When the North Pole is tilted toward the Sun, the Northern Hemisphere experiences summer, and the Sun is high in the sky at noon. During the winter, the North Pole is tilted away from the Sun, and at noon the Sun doesn't get nearly as high in the sky.
Earth's tilt also explains why the longest day of the year occurs on the summer solstice (usually around June 21). Likewise, the shortest day of the year occurs on the winter solstice (usually around December 21).
The combination of Earth's elliptical orbit and the tilt of its axis results in the Sun taking different paths across the sky at slightly different speeds each day. This gives us different sunrise and sunset times each day.
Once the summer solstice passes, you'll notice the days begin to get shorter each day. This trend continues until the winter solstice, the shortest day of the year. After the winter solstice, days get slightly longer each day up until the summer solstice, and the process repeats year after year.