Daytime

Daytime is the period when the Sun is above the local horizon for a specific location.

During this time, a region is illuminated by daylight. This light is a combination of direct sunlight and indirect light coming from the sky. At any single moment, about half of the Earth faces the Sun. This means roughly half of the planet is experiencing daytime. However, because the atmosphere bends light, the actual area covered by illumination is slightly more than half the surface.

The mechanism of daytime is driven by the Earth's rotation on its axis. As the planet spins, the part of the surface facing the Sun changes continuously. A surface observer defines daytime as the period between sunrise and sunset. Sunrise occurs when the Earth's rotation toward the east causes the Sun's disc to appear above the horizon. Sunset happens when the continuing rotation causes the Sun to disappear below the horizon to the west. Because the Sun is a luminous disc rather than a single point of light, these events are not instantaneous. Furthermore, the atmosphere bends and diffuses light, which lengthens the periods of sunrise and sunset. This creates a period called twilight, where indirect light keeps the sky bright before sunrise or after sunset.

The length of daytime varies based on latitude and the time of year. This variation occurs because the Earth's axis is tilted 23.44° relative to the ecliptic. The ecliptic is the plane of Earth's orbit around the Sun. As the Earth orbits the Sun, this axis remains relatively fixed in relation to the stars. This tilt causes seasonal variations in how much daylight different parts of the planet receive.

At the Equator, the daytime period is consistently about 12 hours long regardless of the season. In contrast, middle latitudes experience moderate changes. In higher middle latitudes, like Paris or Montreal, the sky might stay light until 10 pm in summer but turn dark by 5 pm in winter. In lower middle latitudes, the difference between summer and winter daylight is about 4 hours.

Different zones on Earth experience unique solar patterns. The tropics are located between 23.44° north and 23.44° south of the Equator. In this zone, the Sun will pass almost directly overhead, or culminate, at least once a year. The boundary at 23.44° north is the Tropic of Cancer. The boundary at 23.44° south is the Tropic of Capricorn. On the Tropics of Cancer and Capricorn, the Sun is directly overhead only once per year, during the solstice. Closer to the Equator, the Sun can be overhead twice per year. This phenomenon is sometimes called Lahaina Noon or a zero shadow day.

Near the poles, the seasonal variations in daylight are extreme. The Arctic Circle marks the boundary at 66.56° north, while the Antarctic Circle marks 66.56° south. Within 23.44° of the poles, there are days when the Sun never sets. This 24-hour summer daylight is known as the midnight sun. Conversely, there are also days when the Sun never rises. Even during the summer solstice, the Sun only reaches a maximum height of 23.44° above the horizon at the poles. This low angle, combined with the reflection of solar radiation from snow and ice, helps keep polar regions cold.

The Sun's position also changes throughout the year in a way called the equation of time. This term describes how the exact instant of solar noon varies. Solar noon is the moment when the Sun reaches its highest point in the sky. Over the course of a full year, this variation is approximately 30 minutes.

While the length of day changes, the Equator remains warm because the Sun stays close to the zenith, which is the point directly overhead. At the Equator, the Sun's path is nearly perpendicular to the horizon. This results in the shortest sunrise and sunset durations, sometimes taking only two minutes to cross the horizon on the equinoxes.

Understanding daytime is essential for studying how energy is distributed across our planet. The amount of directness in solar rays affects temperature more than the length of daylight itself. For example, the poles remain cold even with 24 hours of daylight because the Sun's rays hit at such low angles. The Equator stays warm with only 12 hours of daylight because the Sun is so much more direct. These patterns of light and heat drive the complex systems of our global climate and seasons.