The Seasons, the Equinox, Solstices, Midnight Sun and Great Circles

In this topic, we will be discussing the following concepts;

1. Seasons.
2. Equinox.
3. Solstices.
4. The midnight sun.
5. Great circles.

1. Seasons:

A Season is a period during the year distinguished by special climate conditions. Each season has its amount of sunshine, temperature, and weather patterns that are repeated yearly, for example, in the Northern Hemisphere, winter generally begins on December 21st or 22nd.

Each Season has distinct climatic conditions as a result of the revolution of the Earth around the sun and the inclination of the Earth’s axis.

The Seasons in temperate regions are Summer, Spring, Autumn (or Fall) and Winter. In temperate regions, the characteristics and differences in the seasons are pronounced. Each season lasts about three months. The table below shows when the four seasons begin and end in the Northern and Southern HemispheresA hemisphere is half the Earth's surface. The four hemispheres are the Northern and Southern hemispheres, divided by the equator (0° latitude), and the Eastern and Western hemispheres, divided by the... More;

Season	Northern Hemisphere	Southern Hemisphere
Summer	June 1 – August 31	December 1 – February 28 (February 29th when it is a Leap Year)
Autumn	September 1 – November 30	March 1 – May 31
Winter	December 1 – February 28 (February 29th when it is a Leap Year)	June 1 – August 31
Spring	March 1 – May 31	September 1 – November 30

Summer is experienced in the temperate and polar regions. It is the warmest period of the year, therefore outdoor activities can take place. At this time the altitude of the Sun is at its highest in this area.

Autumn is the season experienced after Summer. During this season, the Sun is further away – so the altitude of the Sun is lower. The weather gets cold and leaves fall from trees. This is why Autumn is referred to as “The fall”.

Winter is the season experienced after Autumn. During this season, the Sun is very far away – so the Sun’s altitude is lower than it is in Autumn. Winter is experienced in the Temperate and Polar regions. It is a cold and bleak season. In the Polar regions, the Sun may not be seen on the horizon for the entire Season.

Spring is the season experienced after Winter. During this season the Sun’s altitude begins to rise again after Winter which causes an increase in temperature and the ice melts even in the Polar regions. This Season is favourable for the growth of plants and crops, this is the reason the season is known as “Spring”.

In the Equatorial Regions, the sun is never far away and the altitude of the midday sun is usually overhead throughout the year sometimes it is even vertical (90^o). The Climate shows little change with rainfall and temperature conditions almost constant throughout the year. Some equatorial regions show no significant variation at all in climate and are always hot and humid throughout the year.

2. Equinox:

Equinox is either of the two moments in the year (as about March 21 and September 23) when the Sun is exactly above the EquatorThe Equator is Zero degrees (0°) latitude. The Sun is directly overhead the equator at noon on the two equinoxes (March and Sept. 20 or 21). The equator divides the globe... More and day and night are of equal length.

The angle of Earth’s axis with the vertical is 23½° and its angle with the orbital plane or horizontal is 66½°, so most places all over the Earth experience a variation in the lengths of day and night.

These two dates of Equinoxes are known as the Spring Equinoxes (March 21st) and the Autumn Equinoxes (September 23rd) are the only times when the Earth experiences equal lengths of day and night because the Sun is vertically overhead at the Equator.

3. Solstice:

Solstice is either of the two times during the year when the Sun is farthest from the Equator. June 21st, when the Sun is farthest North of the Equator and about December 22nd when the Sun is farthest South of the Equator.

The Summer Solstice occurs on June 21st when the midday Sun is overhead the Tropic of CancerThis line of latitude is located 23°30' north of the equator. The Sun is directly overhead the Tropic of Cancer on the summer solstice in the Northern Hemisphere (June 20 or... More (latitude 23½°N) in the Northern Hemisphere and the Winter Solstice occurs on December 22nd when the midday Sun is overhead the Tropic of CapricornThis is a line of latitude located at 23°30' south. The Sun is directly overhead the Tropic of Capricorn on the summer solstice in the Southern Hemisphere (Dec. 20 or 21).... More (latitude 23½°S) in the Southern Hemisphere.

Note: The Tropic of Cancer and the Tropic of Capricorn mark the limit of the overhead sun. Outside these two points, the sun is never overhead at any time of the year. “The Tropics” refers to the area between the Tropic of Cancer and the Tropic of Capricorn.

4. Midnight Sun:

“Midnight Sun” refers to the natural phenomenon that occurs during the summer season when areas north of the Arctic Circle and south of the Antarctic Circle experience 24 consecutive hours of sunlight.

The Midnight Sun occurs in areas from the Arctic Circle (at latitude 66½°N) to the North Pole (at latitude 90°N) in the Northern Hemisphere. The Southern Hemisphere also experiences the Midnight Sun from the Antarctic Circle (at latitude 66½°S) and the South Pole (at latitude 90°S).

In these areas, in both the Northern and Southern Hemispheres the Sun remains in the sky and is visible on the horizon for 24 hours a day. The Sun can be seen in the sky even at midnight.

The number of days of the Midnight Sun increases from one day in the Arctic region to 6 months at the North Pole, during the Northern Summer. In the Arctic regions, they experience many days or months of consistent daylight in summer. In the Southern Hemisphere, the Midnight Sun is experienced around the 23rd of December.

Note: A part of the Midnight Sun is visible at the Arctic Circle from approximately 12 June until 1 July but there is 24 hours of daylight during the Summer Solstice on June 21st. The North Pole experiences the Midnight Sun for about 6 months, from approximately the 18th of March to the 24th of September. South Pole, Antarctica has midnight sun and experiences this from approximately 20 September to 23 March (about 6 months).

5. Great Circle:

A Great Circle is any circle that circumnavigates the Earth and passes through the centre of the Earth. A great circle always divides or bisects the Earth in half, thus the Equator is a Great Circle (but no other latitudes) and all lines of longitude are great circles.

We use imaginary lines to help us understand the Earth better some run horizontally and some run vertically. The lines that run horizontally on the Globe from East to West are called latitudes while the other lines that run vertically from North to south are known as longitudes.

Among the lines of latitude, only the equator is a great circle because it is at latitude 0°. It divides the Earth into two halves the Northern Hemisphere and the Southern Hemisphere but among the lines of longitude, there are many great circles. For example; the Meridians on Earth are longitudinal lines which are Great Circles.

Since the earth is a sphere, the shortest path between two points is expressed by the great circle distance, which corresponds to an arc linking two points on a sphere.