Atlas of Light
Atlas of Light
Imagine that our Earth occupies the center of the Universe,
that every celestial body moves around it:
the Sun, the Moon, the planets, the stars, the galaxies.
Let's now observe the variations of luminosity on its surface...
The Earth's surface is bathed in two types of light: :
- natural lights from celestial objects located relatively close to the Earth: the Sun, the Moon, the polar auroras resulting from the multiple interactions of the solar wind with particles in the upper atmosphere. The Earth's rotation around its axis results in the alternating of day and night, whose respective durations vary according to the seasons of the year, as a direct consequence of the Earth's revolution around the Sun.
- artificial lights generated by human activities. Added together, they amount to a form of pollution: light pollution, which masks faint celestial objects such as the Milky Way, as well as many stars, and damages the healthy functioning of terrestrial ecosystems.
The succession of day and night on Earth
In order to get the most global view possible, let's place ourselves in a geostationary orbit some 30 kilometers above the equator.
From this angle, our planet looks like a perfect sphere, richly colored, surrounded by a thin layer of atmosphere. It seems to float in space, in the middle of more or less brilliant stars. Among these is the Sun, the closest star, whose apparent rotation is evidenced by the progressive illumination of the Earth's surface, as the hours of the day and the seasons of the year pass.
At intermediate latitudes (between -60° and +60°), day regularly follows night, which our artificial lighting now illuminates, as shown by this satellite imagery (Black Marble / Earth at Night) recently published by NASA. Consequently, from the Earth's surface, the starry sky now appears to be populated no more by a myriad of luminous points, but by a few of the brightest celestial bodies such as the Moon, the planets Mercury, Venus, Mars, Jupiter and Saturn, and stars such as Sirius, Betelgeuse and Antares.
The succession of days and nights on Earth at intermediate latitudes (between -60° and +60°)
The artificial lighting offers a night vision of the Earth's surface - a shot called Black Marble / Earth at Night (*).
At polar latitudes however (above +60° and below -60°), there is no trace of man-made light pollution. Only a few (northern or southern) auroras are visible in the perfectly dark night sky, filled with thousands of luminous points whose apparent movements are parallel to the horizon circle. At these latitudes, there is no appearance or disappearance of stars in the night sky. Simple rotations, eternal rotations, at variable heights above the circle of the horizon according to the seasons.
Thus, the winter Sun makes its apparent rotation below the circle of the northern horizon. Its height below the horizon is minimal on the winter solstice. At the spring and autumn equinoxes, on the other hand, its apparent path is parallel to the horizon circle. On the summer solstice, it is several tens of degrees above the horizon. Naturally, the situation is reversed at southern latitudes. For this reason, the duration of the polar day ranges from 0h - during the northern or southern winter - to 24h - during the northern or southern summer. Observe the variations of the Earth's insolation throughout the seasons with the help of the animation at your disposition.
The movement of the stars above the Arctic Circle is parallel to the horizon. Because, during the boreal winter, the Sun remains constantly below the horizon, the night lasts 24 hours (*).
The movement of the stars above the Antarctic Circle is parallel to the horizon. Because, during the austral summer, the Sun remains constantly above the horizon, the day lasts 24 hours (*).
The succession of seasons on Earth
The interactive map below shows the terrestrial zones illuminated by the Sun at the current time (date of the day and Greenwich Mean Time). Change the hour of the day, the day and the month of the current year and observe the effects of these variations on the insolation of the different terrestrial zones: continents, oceans, equator, polar latitudes, etc. The transition from one season to the next will become evident.
Simulation of the variations of insolation of the various terrestrial zones throughout the hours of the day and the seasons of the year (**).
By default, the display is for the present time. Modify the date and GMT time by positioning your cursors on the areas of the following form:
On spring and autumn equinoxes, the lengths of day and night equal 12h anywhere on Earth. At the summer solstice, daylight hours are at their longest - between 12h and 24h, depending on the latitude. At the winter solstice, on the other hand, daylight hours are at their shortest - between 0h and 12h, depending on the latitude. Easily determine the dates on which these celestial events occur using the Culture Diff' software designed for this purpose.
Variation of sunshine on the Earth's surface over the year (at 13:00 GMT),
from the winter solstice (left) to the summer solstice (right) through the spring equinox (center) (**).
Anthropogenic light pollution is changing our planet. At an altitude of 50,000 km, artificial lighting is shaping the contours of the Earth's continents, even though they are plunged into darkness.
This 3D view of the Earth illuminated by the Sun, surrounded by the constellation of Orion and the star Sirius, is the result of numerical simulations combining satellite images of the Earth and algorithms of positioning of the objects (Earth, Sun, stars) on the celestial vault. It simulates a view acquired on March 21, 2022 at a distance of 50 000 km (*). Although plunged into darkness, the artificial lighting draws the contours of the American continent.
Anthropogenic light pollution is changing our skies. From the Earth's surface, only the brightest celestial objects are now visible to the naked eye. Because artificial light fills the sky, creating an opaque veil.
Simulation of the heliacal rising of the star Sirius in the sky of the Occitanie region, southern France:
shortly after appearing in the southeastern corner of the sky it will disappear, fading into the glow of the rising Sun.
Move the cursor laterally to materialize the effects of light pollution on the landscape contours (*).
Below, a photograph of the Pic de Bugarach in the Upper Valley of the Aude, Occitanie region, France, illuminated only by the headlights of a car.
(*) This simulation was performed using the WebWorldWind interface developed by the NASA/ESA consortium and distributed under the Apache 2.0 license.
(**) This simulation was performed using the OpenLayers geographic mapping tool. The script to determine the day and night zones was borrowed from Jean-Marc Viglino. It is protected by the CeCILL-B compatible BSD OpenSource license.