Seasons

A season is a period of the year with specific climatic conditions, influenced by Earth’s axial tilt and its revolution around the Sun. Seasons affect temperatures, precipitation, sunshine and natural phenomena such as plant growth and animal behavior.

Discover FizziQ

How to measure it in class

FizziQ measures the height of the Sun above the horizon with the theodolite, which uses the smartphone’s accelerometer as an inclinometer. By repeating the measurement at several dates in the year, the real cause of the seasons is brought to light.

Steps:

Choose a location with a clear view to the south and note the date. Always stand in the same place for all the sessions of the year.
Open FizziQ and select the Theodolite tool. Aim at the Sun without ever looking at it directly: orient the smartphone until its shadow is as short as possible, or aim at the tip of the shadow of a vertical stake.
Record the angular height of the Sun at the moment of its meridian passage (solar noon, around 1:40 pm in metropolitan France on summer time).
Also note the sunrise and sunset times of the day, and deduce the length of the day.
Repeat the measurement every three or four weeks, and at a minimum on the four key dates: the March and September equinoxes, the June and December solstices.
Plot the height of the Sun at noon and the length of the day as a function of the date, and compare the two curves.

Scientific activities on this topic

Day length and seasons - students compare sunrise and sunset times across the four seasons and relate the lengthening of the day to the tilt of the Earth’s axis.
Sun height and seasons

Learn more

Seasons do not come from the Earth-Sun distance

This is the most widespread misconception in all of school astronomy: people believe it is hot in summer because the Earth is supposedly closer to the Sun. This is false, and two arguments show it beyond appeal.

First, the Earth passes through perihelion - its position closest to the Sun - at the beginning of January, that is, in the middle of winter in the northern hemisphere. It is on the contrary at aphelion, farthest away, in early July, at the height of the northern summer. The distance therefore varies out of step with what the false explanation predicts.

Second, the Earth’s orbit is nearly circular: its eccentricity is e ≈ 0.0167. The Earth-Sun distance goes from 147.1 million km at perihelion to 152.1 million km at aphelion, a variation of only 3.4 %. The energy received varies as the inverse square of the distance, which represents only about a 7 % difference - trivial compared to the seasonal temperature differences.

The unanswerable argument remains this one: when it is summer in France, it is winter in Australia. Yet the two hemispheres are at the same distance from the Sun at the same instant. A cause linked to distance would produce the same season everywhere; that is not what is observed.

The real cause: the tilt of the rotation axis

The Earth’s rotation axis is tilted by 23.4° relative to the perpendicular to its orbital plane. This axis keeps a nearly fixed direction in space over the course of the year: it always points toward the pole star. As the revolution proceeds, it is therefore sometimes the northern hemisphere and sometimes the southern hemisphere that finds itself tilted toward the Sun. It is this apparent tipping, and not a change in distance, that creates the seasons.

Two effects that add up: angle of incidence and length of day

When a hemisphere is tilted toward the Sun, two things happen at the same time. The rays arrive more vertically: the same beam of light is spread over a smaller area of ground, so the power received per square meter increases. And the Sun stays above the horizon longer: the duration of sunshine lengthens. In Paris, the day lasts about 16 h 10 min at the June solstice compared with 8 h 15 min at the December solstice. The two effects go in the same direction and add up.

The seasonal lag

The longest day is June 21, but the warmest month is July or August. This delay of about a month comes from thermal inertia: the oceans and soils continue to accumulate heat as long as the energy balance remains positive. The maximum temperature is therefore not reached at the maximum of sunshine, but when gains and losses balance out.

Orders of magnitude

Tilt of the axis: 23.4°. Height of the Sun at noon in Paris (latitude 48.9°): about 64.5° at the June solstice, 41.1° at the equinoxes, 17.7° at the December solstice. Length of the day in Paris: 16 h 10 min in June, 12 h at the equinoxes, 8 h 15 min in December. Solar irradiance at the top of the atmosphere: about 1361 W/m² (solar constant). Variation of the Earth-Sun distance over the year: 3.4 %.

Formula

The height of the Sun above the horizon at solar noon is written:

h = 90° − φ + δ

where:

h: angular height of the Sun at solar noon (°)
φ: latitude of the location (°, positive northward)
δ: declination of the Sun, that is, its angular position relative to the celestial equator (°)

The declination varies over the year between −23.4° (December solstice) and +23.4° (June solstice), and equals 0° at the equinoxes. A common approximation gives:

δ ≈ 23.4° × sin(360° × (n − 81) / 365)

where n is the day number in the year (n = 1 on January 1).

The solar power received per unit of horizontal surface depends on this height:

P = P₀ × sin(h)

where:

P: surface power received on the ground (W/m²)
P₀: surface power of the incident beam (W/m²)
h: height of the Sun (°)

Application examples

In Paris (48.9° N) at the June solstice, the height of the Sun at noon is 90 − 48.9 + 23.4 = 64.5°. At the December solstice: 90 − 48.9 − 23.4 = 17.7°.
The sin(h) factor thus goes from 0.90 in June to 0.30 in December in Paris: a square meter of horizontal ground receives three times less energy per second in winter, for equal sunshine.
At the Arctic Circle (66.6° N), the height of the Sun at noon at the December solstice is 0°: the Sun grazes the horizon without rising, this is the polar night.
At the equator (0°), the height of the Sun at noon remains between 66.6° and 90° all year round, and the length of the day is about 12 h at all times: there are no marked thermal seasons, but an alternation of dry season and rainy season.
Fixed solar panels are tilted at an angle close to the latitude of the location, to compensate for the average height of the Sun and maximize annual output.
A horizontal sundial must have its gnomon parallel to the Earth’s rotation axis, hence tilted at an angle equal to the latitude: its design depends directly on the geometry that produces the seasons.

FAQ

Q: Why is it warmer in summer if the Earth is farther from the Sun? A: Because distance is not the determining parameter. The variation in distance over the year is only 3.4 %, whereas the height of the Sun at noon varies by nearly 47° at our latitudes. It is the angle of incidence of the rays and the length of the day that make the season.

Q: Why are the seasons reversed in the southern hemisphere? A: Because the Earth’s axis keeps a fixed direction: when the northern hemisphere is tilted toward the Sun, the southern hemisphere is tilted away from it. The two hemispheres are at the same distance from the Sun, which clearly shows that distance explains nothing.

Q: What would happen if the Earth’s axis were not tilted? A: There would be no more seasons. The Sun would keep the same height at noon all year round at a given location, and the day would last 12 h everywhere at all times. Only the small variation linked to the eccentricity of the orbit would remain.

Q: Why is the longest day not the warmest day? A: Because of the thermal inertia of the soils and above all of the oceans, which take weeks to warm up. The temperature keeps rising as long as the Earth receives more energy than it loses, hence a lag of about a month.

Q: How can the Sun be sighted safely to measure its height? A: You must never look at the Sun directly, neither with the naked eye nor through the screen while aiming. Its height is measured indirectly: plant a vertical stick of height H, measure the length L of its shadow, and the height of the Sun equals arctan(H/L).

Earth’s Axial Tilt - Theodolite - Inclinometer - Illuminance