The length of a day changes because the earth spins at a tilt. If the earth did not tilt, then the days would all be the same length and the sun would always be above the equator. The earth rotates around a line approximately 23.5 degrees to the poles, so the position on the earth where the sun is directly overhead moves. The southern most line where the sun is directly overhead is called the Tropic of Capricorn and the northernmost line is the Tropic of Cancer.

The length of a day depends on where you are on the earth; the extremes are the Poles. Due to the tilt of the earth, sometimes the North Pole points towards the sun, whilst the South Pole points away. This gives the North Pole 24 hours of daylight for about 6 months, whilst the South Pole is plunged into darkness. After this, the North Pole tilts away from the sun. On the equator, the sun is always nearly overhead, so the days are more constant with approximately 12 hours of daylight and 12 hours of darkness everyday.

In the UK, the day length depends on where you are. As it is north of the equator in the south there is less change, but most places still have a longest day of over sixteen hours, and a shortest of under eight. The longest day is called the summer solstice, and occurred this year on 21 June. On this day the sun was overhead at its nearest point to the UK, above the Tropic of Cancer. The shortest will be on 21 December, when the sun is furthest away, over the Tropic of Capricorn.

The tilt of the earth not only gives us the changing day lengths, but the seasons as well. When the earth is tilted with the Northern Hemisphere pointing towards the sun, the sun’s ray hit the UK more directly. A more intense sun gives days that are warmer, as well as longer. This is summer.

When the earth is tilted away from the sun, the sunlight has to pass through more of the earth’s atmosphere before it reaches the ground, so the sun is weaker. This gives the cold, short days of winter that are all too familiar in the UK, but at least we’re not in the Arctic Circle preparing for 6 months of darkness.

Other features in the Weather Basics series
Air Pressure
Clouds
Fog and Mist
Fronts
Frost
High Pressure
Humidity
Jet Streams
Lightning
Low Pressure
Mirages
Northern Lights
Rain or Showers?
Thunder
Watercycle

Other Features: Air Pollution Air Pressure Clouds Clouds, Nature Of Clouds, Types Of Day in the life of Helen Young Day & night Driving E-met: your weather Flooding Fog Fronts Frost Frost and Snow Frost Fairs Gardening Weather Global Warming - Overview Global Warming - What We Can Do Jet streams Hibernation High Pressure History of Weather Broadcasting Humidity Ice Age - part 1 Ice Age - part 2 Low Pressure Migration Mirages Motorbike Weather Napoleon Russia Northern Lights Our Graphics Our Indices Our Symbols Pollen Forecast Pressure Rain or Showers? Rain, Drizzle and Fog Royal Met. Society, The Scuba Diving Snowmen Sun Index Solar Energy Snowflakes Thunder Water cycle Winter on the Roads Worst Winters Water Energy Wind energy Year so far