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.
The United States will experience the longest day of the year between the 20th and 22nd 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 20th to 22nd December, 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.
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.
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'.
❍ 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. (In some cultures it marks the beginning while 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.
The places that 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. Some people worship the god of light, while others celebrate it as the wedding of heaven and the Earth.
❍ Bonfires have a special place in solstice festivities; people light them and stay awake the entire night to welcome the Sun. In some cultures it is believed that these bonfires keep evil spirits away, in some it is believed that whispering your wish to a pebble and throwing it in a solstice bonfire makes it come true.
The relevancy of summer solstice is not restricted to the field of Astronomy, but goes well beyond that into mythology, folklore and culture. Those unknown to the science of solstice, are still aware of the Stonehenge which is believed to have been built in accordance to the solstice alignment, wherein thousands of individuals assemble to welcome the Sun.