<h2>As 21 June approaches, people around the world are getting ready to mark the longest - or shortest - days of the year. But why do the days change in length? And who is most affected?</h2>

A: It is the longest day of the year, when the sun rises earliest and sets latest. In the Northern Hemisphere, this takes place on 21 June. In the Southern Hemisphere, the longest day is six months earlier on 21 December. This year, the UK will enjoy 16 hours and 38 minutes of daylight on Monday. Meanwhile, in Australia on the shortest day of the year, the sunlight will only last 9 hours and 32 minutes.

For both Hemispheres, 21 June officially marks the beginning of an astronomical season: in the North, the summer; in the South, winter.

Q: What happens during the summer solstice?

A: There are two solstices a year, in June and December. In June, the solstice occurs when the tilt of Earth's axis is most inclined towards the sun so that it shines directly above the Tropic of Cancer. This means the Northern Hemisphere is tilted more towards the sun, so gets more light. For everywhere north of the equator, this is a summer solstice.

In December, the opposite takes place. The Earth has moved around the sun and so the Southern Hemisphere is tilted inwards. The Tropic of Capricorn is directly under the sun, and people south of the equator enjoy their summer solstice.

Q: So, what's a winter solstice?

A: There are two every year as well - and they take place at the same time. While the Northern Hemisphere enjoys a summer solstice in June, the Southern Hemisphere experiences its shortest day. The same happens in reverse come December.

While lots of people see summer solstices as days to celebrate, they actually signal the moment the sun's path stops moving northward in the sky - and the start of days becoming shorter. Many people celebrate the winter solstice as the birth of the sun - that is, the moment the days begin to lengthen once more.

Q: What about an equinox - is that the same thing?

A: Actually, they are the opposite. An equinox happens when the sun shines directly on the equator. This means that day and night are approximately the same length. The word equinox even comes from the Latin meaning equal night.

There are two equinoxes each year, one in September and one in March. Together, the solstices and equinoxes mark the points at which the world's seasons change.

Q: Do we all experience the same changes?

A: Not at all. Regions of the Earth can be divided into different zones based on how far from the equator they are: tropical, temperate and polar. The temperate zone lies between the tropics and the polar regions. It includes much of Europe, North America, Central Asia and southern Australia. These areas tend to experience four seasons as the Earth moves around the sun.

The tropical zones experience little change in the length of days as they are close to the Equator and experience less of the tilt. Meanwhile, polar regions see extreme changes to light. In Tromso, Norway, the sun never rises between November and January. In the high summer, it never sets.

Q: Do people celebrate?

A: Yes! Over the centuries, the June solstice has inspired many festivals and celebrations. In the Alps, locals climb the peaks to light fires that burn throughout the longest night. Many still dance around Maypoles.

Stonehenge remains one of the most popular destinations on 21 June. Built between 3,000BC and 1,600BC, Stonehenge aligns with the solstice. Once a year, on 21 June, the sun shines directly through the stones onto the central altar.

While the real purpose of the standing stones remains a mystery, they continue to inspire generations of people to celebrate the sun.