Our amount of daylight hours depends on our latitude and how Earth orbits the sun. Earth’s axis of rotation is tilted from its orbital plane and always points in the same direction — toward the North Star. As a result, the orientation of Earth’s axis to the sun is always changing throughout the year as we revolve around the sun. Sometimes the axis points toward the sun and other times away from the sun.
As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude.
This tilting leads to a variation of solar energy that changes with latitude. This causes a seasonal variation in the intensity of sunlight reaching the surface and the number of hours of daylight.
The variation in intensity results because the angle at which the sun’s rays hit the Earth changes with time of year.
If you shine a flashlight at the ceiling, the region that is illuminated shrinks or grows depending on whether you point it directly at the ceiling or at an angle. Similarly, the sun’s energy spreads out over differing geographic areas when it reaches Earth’s surface. It is more concentrated during our summer months when the sun is higher in the sky.
This spinning of Earth like a top explains our daily cycle of night and day. The tilt of the Earth’s axis also defines the length of daylight. Daylight hours are shortest in each hemisphere’s winter. Between summer and winter solstice, the number of daylight hours decreases, and the rate of decrease is larger the higher the latitude. The fewer sunlight hours the colder the nights.
How fast Earth spins determines the number of hours in a given day. As Earth orbits the sun it spins about its axis approximately once every 24 hours. But this is slowly changing with time. About 650 million years ago there were only about 22 hours in a day.
