The primary reason why it is bright during the day is due to sunlight, which is the emission of visible light from the sun. This light travels through the Earth's atmosphere and illuminates the surface, making it bright. The intensity of daylight varies depending on the time of day, season, and geographic location.

Sunlight is composed of a spectrum of colors, which together create the white light that we see. The Earth's atmosphere scatters these colors, a phenomenon known as Rayleigh scattering, which is more effective for shorter wavelengths (blue and violet light) than longer wavelengths (red and orange light). This is why the sky often appears blue during the day and red during sunrise and sunset.

According to the National Aeronautics and Space Administration (NASA), the sun emits approximately 3.8 x 10^26 joules of energy per second, which is about 10^17 times more energy than all the energy consumed by humans on Earth. This immense energy output is what makes the sun the primary source of light and heat for the Earth.

The brightness of daylight also depends on the angle of the sun relative to the Earth's surface. During the summer months, the sun is higher in the sky, resulting in longer daylight hours and more intense sunlight. Conversely, during the winter months, the sun is lower in the sky, leading to shorter daylight hours and less intense sunlight.

Data from the U.S. National Oceanic and Atmospheric Administration (NOAA) shows that the average daylight duration in the United States varies from about 9 hours in the winter to over 15 hours in the summer. This variation is due to the tilt of the Earth's axis, which causes the sun to be higher in the sky during the summer months and lower in the sky during the winter months.

In conclusion, the brightness of daylight is primarily due to sunlight, which is emitted by the sun and scattered by the Earth's atmosphere. The intensity and duration of daylight vary depending on the time of day, season, and geographic location.