Lesson 4: Earth’s Tilt, the Angle of Sunlight, and Seasons

What causes winter in the United States to occur in December– February and summer to occur in the United States in June–August? What is happening in Brazil, and why?

Earth’s curved surface causes differential heating from the Sun’s light. Additionally, Earth’s consistent tilt toward the North Star throughout its orbit causes the Northern and Southern Hemispheres to point toward or away from the Sun at different times of year, thus experiencing winter and summer in opposite times of year. Earth is tilted on its axis at an angle of 23.5 degrees. This tilt causes Earth’s surface to experience variations in temperature partly due to the changing angle at which the Sun’s light strikes Earth’s surface in relation to latitude. Because of Earth’s tilt, the change in the angle of the Sun’s light striking Earth means that an entire hemisphere receives more-direct sunlight at certain times of the year, specifically during the summer months. When the North Pole tilts toward the Sun, the Sun’s energy is more concentrated in the Northern Hemisphere causing temperatures to be higher, thus it is summer. During this same time, the Southern Hemisphere is tilted away from the Sun, so the Sun’s energy is less concentrated, and they experience winter. Earth’s consistent tilt causes the Sun to not be directly overhead at the equator all year long. When the Northern Hemisphere points toward the Sun, the sunlight is more concentrated and temperatures increase (become warmer); conversely, when the Northern Hemisphere points away from the Sun, the sunlight is more spread out and temperatures decrease (become cooler). The same happens in the Southern Hemisphere when the South Pole points either toward or away from the Sun. Thus, the angle of sunlight related to Earth’s tilt is one critical factor in determining temperatures around the globe. Because the Earth’s tilt remains consistent as it orbits, the angle at which sunlight strikes Earth at different times of year causes the Northern and Southern Hemispheres to experience more and less intense sunlight and thus opposite periods of higher average (warmer) and lower average (cooler) temperatures (seasons). In June–August, the Northern Hemisphere is tilted toward the Sun, thus it is summer there but winter in the Southern Hemisphere. In December February, it is the opposite because the Southern Hemisphere is tilted toward the Sun.

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