20th June, 2012, the longest day of the year, marks the beginning of summer for various countries in the Northern Hemisphere. Interestingly, the same also happens to be the shortest day of the year, and marks the beginning of winter for the Southern Hemisphere. In other words, the United States will experience the longest day of the year come 20th June, while the same will be the shortest day for Australia and other countries of the Southern Hemisphere. For these countries, the longest day will be 21st December, 2012, which incidentally will be the shortest day for the countries in the Northern Hemisphere. Basically, this phenomenon which causes the longest and shortest day on the planet can be attributed to the Sun's apparent movement between the Tropic of Cancer (northern extreme) and the Tropic of Capricorn (southern extreme). For those residing in the Northern Hemisphere, Christmas is usually associated with snowfall; for them, the fact that the Australians celebrate it in summer is bound to come as a surprise. In astronomy, the term 'solstice' refers to an astronomical event which occurs when the apparent position of the Sun is at its northernmost or southernmost extreme in the sky. Other than the Sun, the tilt of the Earth at 23.5°, its rotation and its revolution also have a crucial role to play in this phenomenon. In the course of its journey, the Sun reaches the Tropic of Cancer in the month of June (June solstice), and the Tropic of Capricorn in the month of December (December solstice). Even though the use of the term 'Summer Solstice' for June solstice is restricted to the Northern Hemisphere, it is widely popular as this hemisphere is home to around 90 percent of the world's population. In the Southern Hemisphere, June solstice is referred to as the 'winter solstice' as it marks the beginning of winter. Simply put, June solstice happens to be the summer solstice for people living in the Northern Hemisphere, and December solstice happens to be the summer solstice for people in the Southern Hemisphere. When the Sun reaches the Tropic of Cancer, it is summer solstice for the Northern Hemisphere. This usually occurs on the 20th or 21st of June, and in rare cases, on 19th or 22nd of June. For the Southern Hemisphere, summer solstice usually occurs on 21st or 22nd December, and at times on 20th or 23rd December. This can be attributed to the fact that we refer to the Gregorian calendar which has 365 days in a normal year, and 366 in a leap year. A study carried out at the Swinburne University of Technology revealed that the orbital and daily rotational motion of the Earth also has a role to play in this occurrence. The Sun's journey can be traced from the Equator to the Tropic of Cancer, back to the Equator, then to the Tropic of Capricorn and once again to the Equator. When the Sun is at the Equator, it is referred to as the 'equinox'. As in case of solstice, even an equinox occurs twice a year; first in March and then in September. The journey of the Sun continues year after year, and is also responsible for the occurrence of seasons on the planet. ❍ Going by the technically sound definition of a solstice, it is possible to determine the exact time of the event, correct up to the minute. On 20th June, 2012, for instance, it will occur at 23:09 UT. In colloquial sense though, the term is used to refer to the day on which the solstice occurs, i.e., 20/06/2012, 21/06/2013, 21/06/2014, etc. ❍ The term 'solstice' is derived from the combination of Latin words 'sol' meaning 'the Sun' and 'stitium' meaning to 'stand still'. While the people living in the Northern Hemisphere celebrate summer solstice in June, people in the Southern Hemisphere celebrate it in December. (Though it is known to mark the beginning of summer in most of the cultures, in some it marks mid-summer.) ❍ Irrespective of whether it is the Northern Hemisphere or the Southern Hemisphere, the day this astronomical event occurs marks the longest day of the year for that particular hemisphere. The subsequent days become shorter until we reach the winter solstice in December, which happens to be the shortest day of the year. ❍ In case of June solstice, the Sun remains above the horizon for the entire 24 hours in regions beyond the north of the Arctic Circle; this phenomenon is referred to as the 'midnight-sun' and the region which experiences it is referred to as the 'Land of Midnight Sun'. Those places which experience the midnight-sun include northern Alaska, northernmost regions of Canada, Norway, Sweden and Finland, and much of Greenland. ❍ Summer solstice celebrations differ from one part of the world to another, with various cultures celebrating the day in a peculiar manner. Some light bonfires and dance around them to welcome the Sun, while some light bonfires to keep spirits out of the town. Similarly, some people worship the god of light, and some celebrate it as the wedding of heaven and the Earth. ❍ Bonfires seem to have a special place in solstice festivities; people light these bonfires and stay awake the entire night to welcome the Sun. In some cultures it is believed that these bonfires keep evil spirits away, and in some it is believed that whispering your wish to a pebble and throwing it in a solstice bonfire makes it come true. As one can see, the relevancy of summer solstice is not just restricted to the field of Astronomy, but goes well beyond that into mythology, folklore and culture. Even those who are not familiar with the science of solstice, are well aware of the Stonehenge, an ancient monument which is believed to have been built in accordance to the solstice alignment, wherein thousands of individuals assemble to welcome the Sun.