Winter Comes When Earth Is Closest to the Sun
Many people picture summer as the time when Earth swings close to the sun and winter as the time when it moves far away. That sounds sensible, but it is not true. For people in the Northern Hemisphere, Earth is actually closest to the sun in early January, in the middle of winter.
Earth's orbit is only a little bit oval, so the change in distance matters very little. The real cause of the seasons is the tilt of Earth's axis. Earth spins like a top that leans to one side. It keeps leaning the same way as it travels around the sun.
For part of the year, the Northern Hemisphere leans toward the sun. Sunlight strikes the ground at a steep angle, and the light is concentrated on a small area. Days are long, so there is more time for the ground to warm. That is summer.
Half a year later, the Northern Hemisphere leans away. Sunlight arrives at a low slant and spreads over a larger area, so each patch of ground gets less heat. Days are short too. That is winter.
You can see the effect with a flashlight in a dark room. Shine the beam straight down on a table, and you will see a bright, small circle. Tilt the flashlight, and the same light stretches into a larger, dimmer oval.
The seasons are opposite in the two halves of the world. When it is summer in Canada, it is winter in Australia.