Here is a puzzle. Earth is closest to the Sun in early January, about 147 million km away, and farthest in early July, about 152 million km away. So why is Ontario freezing in January and hot in July? And why is it summer in Australia while you are shovelling snow? The answer is not distance at all. It is the tilt of Earth's axis. In this lesson you will see how that tilt makes the seasons, and how the Sun's energy also drives the weather, the water cycle, the northern lights and much of the electricity we use.
Earth's tilt causes the seasons
Earth spins on an imaginary line through the North and South Poles called its axis. The axis is tilted about 23.4° (often rounded to 23.5°) compared with a line straight up from the plane of Earth's orbit. The key fact is that the axis points the same direction in space all year, toward the North Star, Polaris. As Earth travels around the Sun, this means that in June the Northern Hemisphere leans toward the Sun, and in December it leans away from the Sun. The hemisphere leaning toward the Sun has summer, and the other has winter at the same time.
Leaning toward the Sun warms a hemisphere in two ways. First, the angle of sunlight: when the Sun is high in the sky, its rays hit the ground almost straight on, so the energy is packed into a small area. When the Sun is low, the same beam is spread over a much larger area, like a flashlight shining on the floor at a slant, so each square metre gets less energy. Second, day length: the hemisphere leaning toward the Sun has longer days and shorter nights, so the ground has more hours to warm up and fewer hours to cool down.
| Toronto (43.7° N) | Height of the Sun at noon | Hours of daylight |
|---|---|---|
| June solstice (about June 21) | about 70° above the horizon | about 15.5 h |
| March and September equinoxes | about 46° | about 12 h |
| December solstice (about December 21) | about 23° | about 9 h |
Solstices and equinoxes
| Approximate date | Name (Northern Hemisphere) | What happens |
|---|---|---|
| March 20 | Spring (vernal) equinox | Day and night are about 12 h each almost everywhere; the noon Sun is straight overhead at the equator. |
| June 21 | Summer solstice | Longest day of the year in the Northern Hemisphere; the noon Sun is straight overhead at the Tropic of Cancer (23.4° N). North of the Arctic Circle the Sun does not set. |
| September 22 | Fall (autumnal) equinox | Day and night are about 12 h each again; the noon Sun is overhead at the equator. |
| December 21 | Winter solstice | Shortest day in the Northern Hemisphere; the noon Sun is overhead at the Tropic of Capricorn (23.4° S). North of the Arctic Circle the Sun does not rise. |
The Sun drives the weather and the water cycle
The Sun heats Earth unevenly. Near the equator sunlight arrives almost straight down all year, while near the poles it arrives at a low angle. Warm air near the equator rises, and cooler air moves in to replace it; that moving air is wind. Huge patterns of wind and ocean currents carry heat from the tropics toward the poles. The Sun also evaporates water from oceans, lakes and plants. The water vapour rises, cools, condenses into clouds and falls as rain or snow. This is the water cycle, and it is powered by sunlight. When that rain fills a river and spins a turbine at a dam, the electricity is really solar energy that took a detour.
Auroras: the northern and southern lights
An aurora is a glowing curtain of light in the night sky near the polar regions. It is called the aurora borealis (northern lights) in the north and the aurora australis (southern lights) in the south. It starts with charged particles from the Sun, carried by the solar wind or a coronal mass ejection. Earth's magnetic field steers most of these particles away, but it guides some of them down toward the regions around the magnetic poles. About 100 to 300 km up, the particles crash into oxygen and nitrogen atoms in the thin upper atmosphere and give them extra energy, which the atoms release as light. Oxygen usually glows green (and red higher up), while nitrogen adds blue and purple. Much of northern Canada lies under the "auroral oval," so places like Yellowknife see auroras on many clear nights. During strong solar storms, auroras can be seen much farther south, even over southern Ontario. Many First Nations and Inuit communities have their own names and teachings about the northern lights.
Solar and wind energy
- Photovoltaic (PV) cells in solar panels turn sunlight directly into electricity.
- Solar thermal collectors use sunlight to heat water or air.
- Works best with a high Sun and clear skies; still produces some power on cloudy days.
- Produces no electricity at night, so it needs storage (batteries) or backup.
- Wind exists because the Sun heats Earth unevenly, so wind power is indirect solar energy.
- Moving air spins turbine blades connected to a generator.
- Works day and night when the wind blows; Ontario has large wind farms near the Great Lakes.
- Output drops on calm days, so it also needs storage or backup.