Solstices and Equinoxes
There are 2 solstices (June & December solstices / Summer & Winter solstices) and 2 equinoxes (March & September equinoxes) every year.
Solstices are the shortest and the longest days of the year. When it is the shortest day in the northern hemisphere, it is the longest day in the southern part of the world, and vice-versa. Solstices are possible when the north or south poles are completely tilted (tilted the most they can) towards and away from the sun.
Note: The Earth is farthest from the sun on the 3rd July (around 64, 555, 000 miles from the sun). This point is known as the Aphelion. And when it is closest to the sun on the 4th January (around 91, 445, 000 miles from the sun), it is called the Perihelion.
Let’s take a look at each solstice:
(i) The June Solstice (occurs on 20-21 June)
This solstice marks the start of summer in the northern hemisphere and that of the winter in the southern hemisphere. On this solstice, the northern hemisphere experiences the longest day of the year and the southern hemisphere experiences the shortest.
(ii) The December Solstice (occurs on 21-22 December)
This solstice marks the start of summer in the southern hemisphere and that of the winter in the northern hemisphere. Here, it is the southern hemisphere which experiences the longest day and the northern hemisphere experiences the shortest day.
Equinoxes, unlike solstices, equinoxes are either of the two occasions when day and night are of equal length. They are 6 months apart. During the equinoxes, there are 12 hours of day and 12 hours of darkness everywhere on the earth’s surface. The sun rises at 6 o’clock in the morning and sets at 6 o’clock in the evening in many parts of the planet.
Let’s take a look at each equinox:
(i) March Equinox (occurs on 20-21 March)
This equinox marks the start of spring in the northern hemisphere and the start of autumn in the southern hemisphere.
(ii) September Equinox (occurs on 22-23 September)
This equinox marks the start of autumn in the northern hemisphere and that of spring in the southern hemisphere.
An overview of what happens during solstices and equinoxes at the following locations:
(i) At the North Pole & Arctic Circle
During the March equinox, the sun rises at noon. The north pole receives daylight till the September equinox. That is, the north pole experiences no night-time during 6 months! Just after the September equinox, the sun sets and the north pole then experiences 6 months of darkness till the March equinox occurs again. At the Arctic Circle, the sun appears at the horizon for a very brief moment on the December solstice, and then disappears. The Arctic Circle experiences 12 hours of daylight and 12 hours of darkness during the March and September equinoxes.
(ii) At the South Pole & Antarctic Circle
During the March equinox, the sun sets at noon and the south pole remains in darkness till the September equinox. Just after that, the sun rises and the south pole receives daylight till the March equinox occurs again. At the Antarctic Circle, the sun briefly appears at the horizon on the June solstice, and then disappears, just like it happens during the December solstice at the Arctic Circle. The Antarctic Circle experiences 12 hours of daylight and 12 hours of darkness during the March and September equinoxes too.
(iii) At the Tropic of Cancer
During the June solstice, the sun shines directly over the Tropic of Cancer at noon. During the December solstice it is low in the sky (at noon itself). And during both equinoxes, the sun is off the zenith and the Tropic of Cancer experiences 12 hours of daylight and 12 hours of darkness, similar to what happens at the poles.
(iv) At the Tropic of Capricorn
The same thing occurs at the Tropic of Capricorn, the only difference being the sun is low in the sky during the June solstice instead of the December’s, and it is shines directly over the Tropic of Capricorn during the December solstice instead of the June’s. The Tropic of Capricorn receives 12 hours of daylight and 12 hours of darkness just like the poles and the Tropic of Cancer do.
(v) At the Equator
The equator always receives 12 hours of daylight and 12 hours of darkness, whether there is a solstice or an equinox. This is because it is in the perfect middle of the Earth. Although the solstices mark the peak of summer and winter, with respect to the amount of sunlight we receive, it doesn’t mean the warmest and coldest days occur on solstices! The warmest and coldest days depend entirely on the temperature of the surroundings, which is in turn dependent on the amount of heat the atmosphere gets from the sun and the amount of heat the atmosphere loses to the land and ocean. In general, we experience the warmest days of summer and the coldest days of winter when the temperature of the atmosphere, land, and ocean are in equilibrium, that is; when they are all equal!