Why do we have summer and winter, and why is it not simply because Earth is closer to the Sun?
Topic 4.7 Solar Radiation and Earth's Seasons: explain how the tilt of Earth's axis and its orbit produce variations in insolation that cause the seasons.
A focused answer to APES Topic 4.7, covering solar radiation (insolation), the 23.5 degree axial tilt, the solstices and equinoxes, the angle of incidence, why the tilt and not distance causes the seasons, and latitude effects, with a worked insolation question.
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What this topic is asking
The College Board (Topic 4.7) wants you to explain how Earth's axial tilt and its orbit change the amount of solar radiation (insolation) reaching different places at different times, producing the seasons. The key is the angle of incidence, not distance.
Solar radiation
Why the tilt causes seasons
Solstices and equinoxes
- Solstices: one pole is tilted most toward or away from the Sun, giving the longest or shortest day. At the Northern Hemisphere summer solstice the North Pole leans toward the Sun (most insolation, longest day in the north).
- Equinoxes: neither pole is tilted toward the Sun, so day and night are roughly equal everywhere.
Latitude and insolation
Near the equator, sunlight strikes nearly vertically all year, so insolation is high and steady (little seasonal change). Near the poles, sunlight strikes at a low angle, spreading energy over a larger area and passing through more atmosphere, so insolation is low and highly seasonal (including months of light and dark). This uneven heating is the same difference that drives the global wind patterns of Topic 4.5.
Why this matters
The pattern of insolation set by the tilt and orbit is the root cause of climate. It drives the atmospheric circulation (Topic 4.5), sets the latitudinal arrangement of terrestrial biomes (Topic 1.2), and controls the primary productivity (Topic 1.8) that feeds ecosystems. Understanding that the tilt, not distance, causes seasons is one of the most frequently tested ideas in Unit 4.
Try this
Q1. Identify the approximate tilt of Earth's axis. [1 point]
- Cue. About 23.5 degrees.
Q2. Explain why the tropics receive more insolation per unit area than the poles. [2 points]
- Cue. At the tropics sunlight strikes nearly vertically, concentrating energy on a small area, while at the poles it strikes at a low angle and spreads the same energy over a larger area, so each unit receives less.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Define solar radiation (insolation). (b) Explain why the tilt of Earth's axis, not its distance from the Sun, causes the seasons. (c) Describe what happens in the Northern Hemisphere during its summer solstice. (d) Explain why polar regions receive less solar energy per unit area than the tropics.Show worked answer →
A 4-point FRQ on solar radiation and seasons.
(a) Define (1 point): solar radiation (insolation) is the solar energy received by a given area of Earth's surface.
(b) Explain (1 point): Earth's axis is tilted about 23.5 degrees, so as Earth orbits, each hemisphere is tilted toward the Sun for part of the year (more direct sunlight, summer) and away for part (less direct, winter); distance barely changes and is not the cause.
(c) Describe (1 point): at the Northern Hemisphere summer solstice the North Pole is tilted toward the Sun, giving the longest day, the most direct sunlight, and the most insolation.
(d) Explain (1 point): near the poles sunlight strikes at a low angle, spreading energy over a larger area (and passing through more atmosphere), so each unit of surface receives less energy than the tropics, where sunlight is more direct.
Markers reward the energy-per-area definition, the tilt (not distance) reason for seasons, the pole-toward-Sun summer solstice, and the low-angle reason for low polar insolation.
AP 2018 (style)1 marksSection I (multiple choice). The seasons on Earth are caused primarily by: (A) Earth's changing distance from the Sun (B) the 23.5 degree tilt of Earth's axis (C) variations in the Sun's output (D) the rotation of Earth on its axis. Justify your choice.Show worked answer →
A 1-point MCQ on the seasons. The answer is (B).
The seasons result from Earth's roughly 23.5 degree axial tilt: as Earth orbits the Sun, each hemisphere alternately tilts toward and away from the Sun, changing the directness and day length of sunlight. (A) is a common misconception (distance changes only slightly and Earth is actually nearest the Sun during Northern Hemisphere winter); (C) and (D) do not cause seasons (rotation causes day and night). The trap is the distance misconception.
Related dot points
- Topic 4.5 Global Wind Patterns: explain how uneven solar heating and the Coriolis effect drive atmospheric circulation cells and global wind belts.
A focused answer to APES Topic 4.5, covering uneven solar heating, convection and the Hadley, Ferrel and polar cells, the Coriolis effect, the trade winds and westerlies, and why deserts and rainforests sit where they do, with a worked latitude-climate question.
- Topic 4.4 Earth's Atmosphere: describe the composition of the atmosphere and the four main layers, and explain how temperature changes with altitude.
A focused answer to APES Topic 4.4, covering atmospheric composition, the four layers (troposphere, stratosphere, mesosphere, thermosphere), the temperature profile, the ozone layer, and the role of the atmosphere in weather and protection, with a worked composition calculation.
- Topic 4.8 Earth's Geography and Climate: explain how geographic features such as mountains and proximity to water shape regional climate, including rain shadows and El Nino and La Nina.
A focused answer to APES Topic 4.8, covering how mountains, latitude, ocean currents and proximity to water shape regional climate, the rain shadow effect, and the El Nino and La Nina (ENSO) cycle, with a worked rain-shadow question.
- Topic 1.2 Terrestrial Biomes: describe the global distribution of the major terrestrial biomes and explain how temperature and precipitation determine the type of biome found in a region.
A focused answer to APES Topic 1.2, covering the major terrestrial biomes, how temperature and precipitation define them, latitude and altitude patterns, and biome shifts under a changing climate, with a worked climograph question.
- Topic 1.8 Primary Productivity: define gross and net primary productivity, explain the factors that control them, and calculate net primary productivity from data.
A focused answer to APES Topic 1.8, covering gross and net primary productivity, respiration, the GPP-NPP relationship, limiting factors, productivity across biomes, and ecological efficiency, with a full worked NPP calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)