In the field of geographical studies, the term 'solstice' refers to either of the two times of a given year wherein the Sun is farthest from the celestial Equator. While this definition is absolutely correct, there is a lot more to know about it. Did you, for instance, know that the December solstice (known as 'winter solstice' in the United States), which marks the beginning of winter in the Northern Hemisphere, actually marks the beginning of summer in the Southern Hemisphere? Not many people are aware of such facts about this phenomenon, and that has resulted in widespread myths about it.
The driving factors when it comes to solstice are Earth's revolution around the Sun and its rotation along its axis, which is tilted at 23.5°―both of which contribute to Sun's apparent position in the sky. Interestingly, these are also the driving factors when it comes to different seasons on the planet. As a result of revolution and rotation of Earth, the Sun is directly overhead at the tropic of Cancer and tropic of Capricorn. This journey of the Sun can be traced from the equator to the tropic of Cancer at 23.5°North, back to the Equator, and then down south to the tropic of Capricorn at 23.5°South over the year. When it reaches its northernmost or southernmost extreme, the Sun appears to stand still for sometime, before it resumes its journey. This very period when the Sun is still right overhead the tropic of Cancer or tropic of Capricorn, is known as solstice.
In fact, the term solstice is derived from the combination of two Latin words sol meaning the Sun and sistere meaning to stand still. Similarly, when the Sun is right at the Equator, it is referred to as equinox. As a result of the Earth's tilted axis, the areas near the south pole experience darkness for 24 hours when the Sun is at its extreme north. Similarly, when the Sun is at its extreme south, the areas near north pole are in the dark for 24 hours.
Other Names for Solstice
Even though solstice is known by different names in different parts of the world, its naming on the basis of month and hemisphere has got worldwide acceptance. Basically, this astronomical phenomenon occurs twice a year―once in June, owing to which it is named June solstice, and then in December, thus December solstice. While these names are given on the basis of month in which the phenomenon occurs, it can be also named on the basis of hemisphere in which the Sun is positioned. When the Sun is at its northern extreme, it is referred to as northern solstice, and when it is at its southern extreme, it is referred to as southern solstice.
Winter Solstice and Summer Solstice
Other than these names, this natural occurrence is also referred to as summer solstice and winter solstice. That, however, can be misleading as the two hemispheres have different seasons at any given point of time. While the Northern Hemisphere experiences mid-winter in December, the Southern Hemisphere experiences mid-summer in December. So the solstice which marks the beginning of winter in the Northern Hemisphere, will mark the beginning of summer down south. Generally, the term winter solstice is used to refer to December solstice owing to the fact that most of the land area on the Earth falls in the Northern Hemisphere.
In astronomy, the term solstice is used to refer to the exact moment when this phenomenon occurs. Colloquially, however, it is used to refer to the day on which it occurs. While June solstice generally occurs on June 20 or 21 (which is thus the longest day of the year), December solstice generally occurs on December 21 or 22. That being said, even though rare, there have been instances of June solstice occurring on June 19 or 22 and December solstice on December 20 or 23.