Winter in the northern hemisphere happens because of how Earth is tilted as it orbits the Sun. This tilt changes how directly sunlight hits this part of the planet and how long days and nights are.
Main cause: Earth’s tilt
Earth’s axis is tilted about 23.5 degrees relative to the plane of its orbit around the Sun. In northern winter, the northern hemisphere is tilted slightly away from the Sun, so the Sun’s rays hit it at a lower angle and are spread over a larger area, providing less heating.
Shorter days and less energy
When the northern hemisphere is tilted away, days are shorter and nights are longer, so there is less time each day for the Sun to warm the surface. With weaker, more slanted sunlight plus longer nights, the ground and air lose more heat than they gain, leading to colder winter weather.
Not distance from the Sun
Winter is not caused by Earth being farther from the Sun; in fact, Earth is actually slightly closer to the Sun in January than in July. The key factor is the tilt changing sunlight angle and day length, which is why the two hemispheres have opposite seasons at the same time.
