What if Earth’s tilt changed?

In summer, the Sun is high and days are long. In winter, the Sun is low and days are short.

That happens because Earth leans. What if Earth did not lean at all?

In summer, the noon Sun climbs high, the days are long and the sunshine feels strong. In winter, the Sun stays low, it gets dark early, and the light feels weak. Many people think this happens because Earth is closer to the Sun in summer, but that is not the reason.

The real cause is that Earth's axis leans. So what would happen if the lean changed, or disappeared? In this lab you can change Earth's tilt in a sunlight model and see.

Earth's spin axis is inclined 23.44° to the perpendicular of its orbital plane. As Earth orbits, that fixed lean alternately points each hemisphere toward and away from the Sun.

This lab asks how the pattern of sunlight would change for different obliquities: what happens to noon Sun height, day length and daily insolation at a chosen latitude if the tilt were zero, larger, or as large as 90°?

Earth leans the same way all year. In June, the top half leans toward the Sun.

In December, the top half leans away. That makes summer and winter.

Earth's axis always leans the same way in space, 23.4 degrees from upright. For half the year the northern half leans toward the Sun, and for the other half the southern half does.

In the model, at 40 degrees north (near New York, Madrid and Beijing), the noon Sun on June 21 climbs to about 73 degrees above the horizon and the day lasts nearly 15 hours. On December 21 the noon Sun reaches only about 27 degrees and the day lasts about 9 hours.

The solar declination δ = asin(sin ε · sin λ) swings between +23.44° and −23.44° over the year. At latitude 40° N the model gives a June-solstice noon altitude of 73.4°, 14.8 hours of daylight and a daily mean top-of-atmosphere insolation of about 499 W/m². At the December solstice: 26.6°, 9.2 hours and about 151 W/m².

That factor of more than three in daily sunlight, not the small change in Earth-Sun distance, drives the northern seasons.

In our pretend lab, Olly stands Earth up straight. No more lean!

Now the Sun is just as high every day. Every day and night lasts 12 hours.

Set the tilt to zero. Now at 40 degrees north the noon Sun climbs to 50 degrees every single day, and every day has exactly 12 hours of light. The year chart of the Sun's height goes completely flat: there is no summer and no winter.

Now try 45 degrees. In June the noon Sun is almost overhead and the day lasts nearly 20 hours; in December it barely rises and the day lasts under 5 hours. A bigger tilt makes much bigger seasons.

At ε = 0 the declination is always 0°, so at 40° N the noon altitude is 50° and the day length 12 h on every date, with a constant 332 W/m². At ε = 45° the June solstice gives 85.0°, 19.6 h and 633 W/m²; the December solstice 5.0°, 4.4 h and about 14 W/m². At ε = 90° the December Sun never rises at 40° N.

This is a thought experiment carried out with a model. Earth's tilt cannot simply be reset, and the model only calculates sunlight, not climate.

When the Sun is high, its light lands strong and bright on the ground. When it is low, the light is weak.

The lean changes how high the Sun gets.

When the Sun is high in the sky, its light hits the ground straight on and is packed into a small area, so it warms well. When the Sun is low, the same light is spread over a bigger area, like a flashlight shining at a slant, so it warms less. Long days add even more sunshine.

The tilt decides how high the Sun climbs and how long it stays up. No tilt means the same Sun height and day length all year, so no seasons.

Daily insolation depends on both the Sun's elevation (through the cosine of the zenith angle) and the length of the daylight arc, set by cos h₀ = −tan φ tan δ. Both are controlled by the declination, whose annual range equals ±ε.

The model's limits: a circular orbit at 1 AU, top-of-atmosphere sunlight with no clouds or air, and a flat horizon with no refraction. For tilts above about 54°, the model correctly shows the poles receiving more sunlight over a year than the equator, but what climate such a planet would have is an idea, not a result.

Mars leans almost as much as Earth. So Mars has seasons too!

Uranus leans so far that it is tipped right over.

Other planets have their own tilts. Mars leans about 25 degrees, close to Earth's 23.4, so Mars has seasons too. A Martian year lasts 687 Earth days, so its seasons last longer than ours. Uranus is tilted almost 98 degrees, so it is tipped right over on its side.

Earth's distance from the Sun does change a little, but Earth is actually closest to the Sun in January, during northern winter. That is a clear clue that distance is not what makes the seasons.

NASA's fact sheets list obliquities of 25.2° for Mars and 97.8° for Uranus. NASA notes that Mars, with its 25° tilt and 687-day year, has seasons of its own.

Earth reaches perihelion in early January, when the Northern Hemisphere has winter, and the two hemispheres have opposite seasons at the same moment: both facts rule out distance as the cause. The model ignores Earth's 3 percent change in distance, which shifts sunlight by only about ±3.4 percent.