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Notes from the field ·

Why Is Sunlight Stronger in Summer? The Angle of Light, Measured

By Ahmed, founder

Longer days are only half the reason summer is warmer. The other half is that sunlight is stronger. You can show this with a flashlight and graph paper.

The experiment

You need: a flashlight, squared (centimetre) graph paper, a protractor or a card with marked angles, and a dim room.

  1. Fix the flashlight 20 cm from the paper, pointing straight at it (90°). Trace the bright patch and count the squares it covers.
  2. Keep the same distance, but tilt the flashlight to 60°, then 30°. Each time, trace the patch and count the squares.
  3. Record in a table.

What you find

The same flashlight beam covers more squares as the angle gets lower. The light spreads out over a bigger area, so each square gets less light and looks dimmer.

Connect to the Sun

When the Sun is high in the sky, sunlight hits the ground at a steep angle, so energy is concentrated. When it’s low, the same energy is spread over a larger area. At a latitude of 51°N, the noon Sun is about 62° above the horizon at the June solstice and about 15° at the December solstice, a huge difference in how concentrated the light is.

(Calculation: noon Sun height ≈ 90° − latitude + the Sun’s tilt position. At 51°N, 90 − 51 + 23.5 = 62.5° in June, and 90 − 51 − 23.5 = 15.5° in December.)

A low Sun also passes through more air, which scatters and absorbs more sunlight, another reason winter sunlight is weaker.

Put it together

Summer is warm because of:

  1. Longer days (more hours of sunlight) and
  2. A higher Sun (more concentrated sunlight).

Winter is the opposite. Neither depends on Earth being closer or farther from the Sun.

Arm angles

Have students stand outside at solar noon (or in class with printed Sun heights) and show the Sun’s height with their arms: 62° in summer, 15° in winter at 51°N, for example. It’s a good body-based way to feel the numbers.

The full lesson is Why is sunlight stronger in summer? from The Reason for the Seasons. It follows Is summer when Earth is closest to the Sun? Sign up for three lessons free.