Why is it hotter at the equator?

The equator is hotter because of the angle at which sunlight strikes it. Because the Earth is a sphere, the sun's rays hit the equatorial regions nearly head-on, straight down and concentrated onto a relatively small area of ground, delivering intense, focused heat. Toward the poles, the same sunlight arrives at a low, slanting angle and gets spread across a much larger area of the tilted surface, so the energy is diluted and each patch of ground receives far less heat. It is the same reason a flashlight beam is bright and hot when aimed straight at a wall but dim and spread out when aimed at a sharp angle. Sunlight reaching the poles also passes through more of the atmosphere on its slanted path, losing more energy along the way. So the equator gets concentrated, direct solar energy year-round, keeping it consistently warm, while the poles get thin, glancing sunlight, keeping them cold.

This same geometry, combined with the Earth's tilt, explains the seasons and why they are opposite in the two hemispheres. The Earth is tilted about 23.5 degrees on its axis, and as it orbits the sun, that tilt means different parts of the planet lean toward or away from the sun at different times of year. When the Northern Hemisphere tilts toward the sun, it receives more direct sunlight and has summer, while the Southern Hemisphere, tilted away, has winter, and six months later it reverses. The equator, sitting in the middle, stays relatively directly lit all year, which is why tropical regions near the equator do not have the strong summer-winter swings of higher latitudes; instead they tend to have consistent warmth with wet and dry seasons rather than hot and cold ones. So the equator's heat and the existence of seasons both trace back to the same root cause: the angle at which sunlight meets a tilted, spherical, orbiting Earth.