Summer days longer than winter days the same reason that Earth has seasons: because Earth’s axis is tilted in relation to the Sun.
If the axis were straight up and down, different parts of the world would still have different lengths of daylight and darkness, but they would remain constant throughout the year.
When the Northern Hemisphere, or half, of Earth is tilted toward the Sun, the Sun’s rays hit it more directly.
The more direct—and therefore warmer—sunshine creates summer in the Northern Hemisphere. The tilt of Earth also makes the Northern Hemisphere experience longer days during this time, and the Sun appears to pass higher in the sky.
Six months later, Earth has moved halfway through its orbit, and the Southern Hemisphere is tilted toward the Sun. This means it is summer in the Southern Hemisphere and winter in the Northern Hemisphere.
You can see the change in the angle of the Sun very gradually over the course of several months. In the Northern Hemisphere, the Sun climbs higher in the sky each day as summer approaches, until one day—on or about June 21—it seems to stop getting higher.
After that, it drops lower each day as winter comes closer. Then it stops—on about December 21—and begins to climb higher again. June 21 is called the summer solstice, or “summer sun standing still”; December 21 is the winter solstice, or “winter sun standing still.” In the Southern Hemisphere, summer and winter are reversed.
Earth is divided into two sets of hemispheres. The equator divides the Northern and Southern Hemispheres.
The Eastern and Western Hemispheres are divided by the 0° meridian, or the prime meridian, on one side of the globe and the 180° meridian on the other (both not shown).
The two dotted lines show the tropics of Cancer (north of the equator) and Capricorn (south of the equator). Within these two imaginary lines the Sun’s rays are strongest.
You should never look directly at the Sun. The light is so powerful that it could damage your eyes.