Explore what causes seasons on Earth in this interactive produced by WGBH and adapted from NASA materials. The interactive features four cities at different latitudes (New York City, Miami, Singapore, and Melbourne) and provides information about their seasonal conditions at eight points in Earth's orbit. Illustrations show how the Sun's path through the sky and the angle of sunlight hitting Earth's surface vary depending on latitude of a location and Earth's position in its orbit. Text boxes describe the shape of Earth's orbit, how the duration and angle of sunlight influence the energy received at Earth's surface, and seasonal lag.
Earth is in constant motion. Approximately every 365 days, Earth completes one revolution around the Sun, and approximately every 24 hours, Earth completes one rotation around its axis. Because Earth's axis of rotation is not perpendicular to the plane of Earth's orbit it is tilted about 23.5 degrees the way in which the Sun's light hits Earth's surface varies during the course of the orbit. The seasons predictable patterns of temperature and precipitation over the course of each year that locations on Earth experience are due to a combination of the tilt of Earth's axis and its place in its orbit.
The angle at which light strikes a surface greatly affects the amount of energy that the surface receives. For example, when a flashlight is positioned directly over a piece of paper, all the light is concentrated on a given circular area of the paper so that the paper receives direct, intense light from the source. However, if the paper is tilted and the same amount of light hits it at an angle, the light is spread out over a greater area and the intensity with which it hits that area is reduced. This idea also applies to the energy radiating from the Sun.