How the Earth’s Terminator Changes With the Seasons

Caitlin Dempsey

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The terminator, also know as the twilight zone, is the moving line that divides the Earth or any other planetary globe between daylight and nighttime.  

The terminator passes through most locations on Earth twice a day. Once for sunrise and the second time for sunset.  

The exception to this are locations in high latitudes near the North and South Pole which experience long seasonal periods of total darkness during each hemisphere’s winter months and total sunlight during the summer months.

At any given point in time, about half of the Earth is covered in darkness and the other half is lit by the sun.  The path of this moving line varies depend on the rotation of the Earth along its axis which varies from season to season.


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Why does the Earth have a terminator line?

The terminator is a direct result of the Earth’s rotation and its position relative to the Sun. Our planet rotates on an imaginary line known as its axis, which runs through both the North and South Poles.

It takes approximately 24 hours for the Earth to complete a full rotation, during which time different parts of the globe experience daylight and nighttime.

A view of the Americas from a satellite image.
The Earth’s terminator is the moving boundary that separates the illuminated, daylit side of the planet from the dark, night side as a result of the Earth’s rotation and its position relative to the Sun. Image: NASA, Suomi, public domain.

How the Earth’s tilt creates seasons and changes in the terminator line

The Earth’s axis is tilted at an angle of approximately 23.5 degrees relative to its orbit around the Sun.

As the Earth revolves around the Sun over the course of a year, this tilt causes the Northern and Southern Hemispheres to receive varying amounts of sunlight, leading to the changing of the seasons.

At two specific points in the Earth’s orbit, known as the equinoxes, the Earth’s axis is neither tilted toward nor away from the Sun. During the equinoxes, the terminator runs straight from the North Pole to the South Pole, dividing the Earth into equal halves of day and night.

During the solstices, when the Earth’s tilt is at its maximum, the terminator’s path is more curved, resulting in the longest and shortest days of the year for the Northern and Southern Hemispheres, respectively.

The four seasons of the Earth’s orbit around the Sun

As the Earth orbits the Sun, the tilt of its axis results in four distinct seasons: spring, summer, autumn, and winter.

A small poster against a black background showing how the Earth's tilt affects the seasons.
The tile of the Earth’s axis and its rotation around the Sun is why our planet has seasons. Image: NASA, public domain.

Each hemisphere experiences these seasons in opposing order, with the Northern Hemisphere experiencing spring as the Southern Hemisphere experiences autumn, and vice versa. Similarly, when it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere.

The terminator’s position and shape change throughout the year, directly impacting the length of days and nights and the distribution of sunlight across the Earth’s surface. As the terminator moves, it alters the environment and influences the natural processes that govern our planet.

The March and September Equinoxes

During the March and September equinoxes, the Earth’s axis is perpendicular to its orbit around the Sun, and the terminator aligns perfectly with the Earth’s geographic poles. At this time, the terminator is at its straightest, dividing the Earth into equal halves of day and night.

Both hemispheres experience nearly equal amounts of sunlight, resulting in milder temperatures and moderate weather patterns.

Two images of the Earth from a composite satellite images, the top black and white image shows the split over the Earth between night and day on the March equinox and the bottom shows the same for the September equinox.
Both the March and September equinoxes are days when the Earth’s terminator is a north-south line. Images: NASA, public domain.

When does the March equinox occur?

The March equinox, also known as the vernal (spring) equinox in the Northern Hemisphere and the autumnal equinox in the Southern Hemisphere, typically occurs around March 20 or 21, depending on the year.

This is when the Sun is directly above the Earth’s equator, and day and night are approximately equal in length all around the globe. The specific date and time of the equinox can vary slightly each year due to the Earth’s elliptical orbit and the gravitational pull of other celestial bodies.

When does the September equinox occur?

The September equinox, also known as the autumnal equinox in the Northern Hemisphere and the vernal (spring) equinox in the Southern Hemisphere, usually occurs around September 22 or 23, depending on the year.

As with the March equinox, this is when the Sun is directly above the Earth’s equator, and day and night are approximately equal in length all around the globe.

Summer and Winter Solstices

The summer and winter solstices are the points in Earth’s orbit where the tilt of its axis is most pronounced. During these times, the terminator becomes more curved, with the illuminated side of the Earth extending farther north or south, depending on the hemisphere.

At the solstices, the Earth’s tilt reaches its maximum angle in relation to the plane of its orbit, resulting in one hemisphere receiving more daylight than the other.

Two satellite composite black and white images of the Earth.  The top is the June solstice with the dark line separating night from day tilted to the left and the bottom is the December solstice with the dark line representing the boundary between night and day tilted to the right.
The June and December solstices. Images: NASA, public domain.

Longest and shortest day of the year depending on the hemisphere

During the June solstice, the Sun is positioned directly over the Tropic of Cancer, resulting in increased sunlight for the Northern Hemisphere.

During the December solstice, the Sun is located directly over the Tropic of Capricorn, providing greater illumination to the Southern Hemisphere.

When does the June Solstice occur?

The June solstice usually occurs around June 20 or 21, depending on the year. In the Northern Hemisphere, it is the longest day of the year and is known as the summer solstice, when the Sun reaches its highest point in the sky and the amount of daylight is at its maximum.

At this point of the year, the Earth’s terminator moves northward in the Northern Hemisphere, resulting in longer daylight hours and shorter nights.

Conversely, in the Southern Hemisphere, this date marks the winter solstice, the shortest day of the year with the least amount of daylight as the terminator shifts southward, causing shorter days and longer nights.

When does the December Solstice occur?

The December solstice typically occurs around December 21 or 22, depending on the year. In the Northern Hemisphere, it is the shortest day of the year and is known as the winter solstice, when the Sun reaches its lowest point in the sky and the amount of daylight is at its minimum.

In the Southern Hemisphere, this date marks the summer solstice, the longest day of the year with the most amount of daylight. The specific date and time of the solstice can vary slightly each year due to the Earth’s elliptical orbit and the gravitational pull of other celestial bodies.

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.