Longest Day of the Year

Even though it is quite a common phenomenon, not many people are aware of the fact that there is something like the longest and the shortest day of the year. Technically put, a ‘day’ is the period of 24 hours, wherein the Earth completes a single rotation. Colloquially, however, the term is used to refer to the period between the sunrise and sunset, when it is bright outside. (In contrast, the term ‘night’ is used to refer to the period between the sunset and sunrise, when it is dark outside.)

On June 20th, 2016, the sunrise is scheduled for 05:47 and sunset for 19:59; which amounts to 14 hours and 12 minutes of daylight. The same was 9 hours and 18 minutes for January 1, 2016, (sunrise – 07:20 and sunset – 16:39), and will be 9 hours 28 minutes for December 1, 2016 (sunrise – 07:01 and sunset – 16:29). As you see, the length of a day increases and decreases over the course of a calendar year. Starting from January, it continues to increase till June 21st (at times June 20th or 22nd.) From this particular day, it starts decreasing, and continues to decrease till December 21st (at times December 20th or 22nd).

Summer Solstice and the Longest Day of the Year
Basically, days and nights are caused due to the rotation of the Earth, with the side facing the Sun experiencing day and other side experiencing night. As the Earth is tilted on its axis at an angle of 23° 26′, at one point it aligns in such a position that it is either inclined towards or away from the Sun. Such alignment, wherein the apparent position of the Sun is either perpendicular to the Tropic of Cancer or the Tropic of Capricorn, is referred to as solstice. It takes place twice in a year(;) once when the tilt of the Earth’s axis is inclined towards the Sun, and once when the tilt is inclined away from the Sun.

This phenomenon and the apparent position of the Sun in the sky enables the sunrays to reach the northern or southern extreme, which, in turn, determines the duration of a day. When the Sun is at the Tropic of Cancer, Northern Hemisphere receives more daylight, and therefore has longer days and shorter nights. When it is at the Tropic of Capricorn, things work the other way round, and the Southern Hemisphere experiences daylight for a longer period. Simply put, the hemisphere of the planet which is inclined towards the Sun experiences more daylight, and hence the days here are long and nights short.

Basically, the day on which the summer solstice occurs, with reference to a particular hemisphere, happens to be the longest day of the calendar year for that hemisphere. The length of a day will differ from one region to another depending on its latitudinal location. While the regions close to the Equator experience 12 hours of daylight, regions along the Tropics experience 14-15 hours daylight on this day. Interestingly, the places beyond 66.5°N and 66.5°S experience daylight for the entire 24 hours in summer, as the tilt of the Earth brings these areas directly under the Circle of Illumination for a part of the year. In fact, places like Canada, Greenland, Alaska and Sweden, witness a strange phenomenon referred to as the ‘midnight Sun’, wherein the Sun is visible even at midnight.
20th June or 21st June?

The alignment happens gradually over the course of time, and hence we witness the duration of the day increasing as we close in on June, and decreasing thereafter. On June 21st, the length of the day is at its peak, which again differs place to place. Though rarely, this phenomenon, which usually occurs on June 21st, can also occur on June 20th or June 22nd. (This change in the solstice dates can be attributed to the fact that we refer to the Gregorian calendar wherein every fourth year is a leap year.) In 2008, it occurred on 20th June; which was only the second time since 1975 when it didn’t occur on June 21st. In 1975, it occurred on 22nd June, and that will happen again only in 2203.

Though the actual astronomical event is restricted to a few seconds, the day on which this event occurs is informally referred to as the ‘summer solstice’. Being the longest day of the year, it also has a great significance in various cultures, where it is celebrated in the form of rituals, festivals and even gatherings, such as midsummer parties; all meant to welcome the Sun and the new season.

What is Solstice

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.

Solstice Explained

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