Daylight duration
Radiation & sun day length astronomical day length
Daylight duration is the time between sunrise and sunset. Clouds can make a day gloomy, but they cannot shorten its astronomical length.
Why days differ in length
Earth’s axis is tilted relative to its orbit. In summer, the northern hemisphere leans more towards the sun, keeping it above our horizon for longer. Winter reverses the situation. Date and latitude determine most of the change.
Location matters
Sunrise and sunset depend on the date and location. They describe an astronomical interval for an unobstructed horizon, not a forecast of measured sunshine hours.
Why it is light before sunrise and after sunset
The atmosphere scatters sunlight even with the sun below the horizon. This extra light is twilight. Standard sunrise and sunset calculations also allow for atmospheric refraction and use the sun’s upper edge.
Daylight does not guarantee sunshine
A completely overcast day still counts as daytime. Sunshine duration counts only sufficiently strong direct sunlight. A long June day can therefore have few sunshine hours. It simply has more time to be gloomy, while still providing daylight.
Buildings, hills and trees change the visible horizon. The sun may therefore appear later or disappear earlier locally than the calculated times suggest.
In Hanover
The difference in Hannover
At Hannover’s latitude, daylight lasts nearly 17 hours around the summer solstice and about 7¾ hours around the winter solstice. These are approximate astronomical values for an unobstructed horizon, not averages of sunshine measurements.