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Glossary term: Trajectoire du soleil (Arc diurne)

Description: La trajectoire du Soleil ou arc diurne retrace le mouvement apparent du Soleil dans le ciel, tel qu'il est perçu par un observateur situé à une position fixe sur la Terre. Chaque jour, la trajectoire trace un arc, commençant par le lever du soleil dans la partie orientale du ciel et se terminant par le coucher du soleil dans la partie occidentale. Ce n'est qu'aux équinoxes de printemps et d'automne que le soleil se lève exactement à l'est et se couche exactement à l'ouest. Au solstice d'hiver, l'arc est le plus court et le plus bas dans le ciel. À partir de ce moment, la longueur de cet arc, et avec elle la durée du jour, augmente, et chaque arc successif est plus haut dans le ciel que le jour précédent, atteignant sa hauteur maximale, et la durée maximale du jour, au solstice d'été. Ce processus se répète ensuite dans l'ordre inverse, pour atteindre à nouveau l'arc le plus court et le plus bas au solstice d'hiver.

Au nord du cercle polaire arctique et au sud du cercle polaire antarctique, le soleil ne se lève pas pendant de longues périodes en hiver et ne se couche pas pendant de longues périodes en été. Il peut donc y avoir des nuits qui durent plusieurs mois en hiver et des jours qui durent plusieurs mois en été. Au cours d'un de ces jours polaires, le soleil parcourt le ciel en cercle, passant d'un point haut à midi à un point bas à minuit sans jamais traverser l'horizon.

L'ensemble des arcs solaires peut être capturé dans une image d'exposition à long terme appelée solargraphe.

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Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher
The translation of this term and its definition is still awaiting approval

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

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Le Soleil descend de la partie supérieure gauche de l'image vers le milieu. Sa trajectoire se reflète sur la surface du lac.

La dernière heure de la journée

Caption: Mention honorable au concours d'astrophotographie 2023 de l'UAI OAE, catégorie Images fixes de l'arc diurne du Soleil et solargraphes : La dernière heure du jour, par Fabrizio Melandri. Cette image, prise le 2 mai 1997 au lac Scaffaiolo en Italie, capture magistralement la descente du Soleil dans une séquence de reflets en miroir sur les eaux calmes. Cette représentation époustouflante du temps met en valeur la beauté froide de l'environnement et constitue un témoignage intemporel des instants fugaces de la dernière heure de la journée. Cette image reçoit une mention honorable dans la catégorie des images fixes des arcs diurnes du soleil et des solargraphes.
Credit: Fabrizio Melandri/UAI OAE (CC BY 4.0)

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons


12 images de coucher de Soleil. De janvier à décembre (de haut en bas), le Soleil va du SO au NO, puis reviens vers le SO.

Où le Soleil se couche chaque mois

Caption: Troisième prix du concours d'astrophotographie 2023 de l'UAI OAE, catégorie Images fixes des lieux de lever et de coucher du Soleil au cours de l'année : où le Soleil se couche chaque mois, par John Paul Pile. Prise avec un smartphone depuis San Rafael, Bulacan, Philippines, cette série d'images a remporté le troisième prix dans la catégorie Images fixes des lieux de lever et de coucher du Soleil au cours de l'année. Elles montrent l'évolution de la position du coucher du Soleil au cours de l'année en raison de l'inclinaison de l'axe de rotation de la Terre sur le plan de l'écliptique (plan de l'orbite de la Terre autour du Soleil). De janvier à décembre 2022, chaque image reflète l'évolution de la position du Soleil au crépuscule par rapport au même horizon local. Observez le changement de feuillage et l'apparition d'un poteau électrique en octobre. Cette série d'images est un témoignage visuel de la danse céleste entre la Terre et le Soleil, et des transformations de la nature au fil des saisons.
Credit: John Paul Pile/UAI OAE (CC BY 4.0)

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24 images of the Sun around the edge of a circular image. In the lowest image, a black circle blocks the centre of the Sun.

The Eclipse Clock-Eclipse on a Polar Day

Caption: Constructed by combining multiple images over the course of a 24-hour period, the image was captured in Union Glacier, Antarctica, during the total solar eclipse of 4 December 2021, and showcases the day arc of the Sun. It illustrates the unique phenomenon of a polar day, during which the Sun travels around the sky without setting. During polar days, areas within the polar circles experience 24 hours of continuous daylight, and the Sun doesn’t set for an extended period. The image also offers a rare perspective of a solar eclipse, where the Moon passes between the Sun and Earth, and as viewed from Earth. This can be seen in the lower image of the Sun, where the Moon covers the solar disc.
Credit: Stephanie Ziyi Ye/IAU OAE (CC BY 4.0)

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons


Seven images of the Sun. From the left it sinks, reaching its lowest in the centre image, before rising higher to the right.

When the Sun Bounces

Caption: This image was taken at Norway's North Cape. Owing to the latitude of this location, the Sun never truly sets during the summer months, from around April to August. This results in the unique spectacle of continuous daylight, known as a polar day. This captivating series of images, taken in 2022, showcases the Sun's relentless journey across the horizon every 30 minutes. At its lowest point in the sky, the Sun does not set, but appears to bounce off the horizon. The globe on the far right in the photo stands as a sentinel, marking the northernmost point of the European continental mainland.
Credit: Milos Obert/IAU OAE (CC BY 4.0)

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Curved bright lines rise from the left, peaking on the right. The further to the left a line rises, the higher it peaks.

The Path of the Sun

Caption: This enchanting image, captured in Germany between 21 June 2018 and 21 December 2018, is a poetic blend of art and science, showcasing nature's rhythm and the passage of time. It combines images taken at different times of day over the course of six months. Each curve tracks the Sun’s path (or arc) across the sky on a particular day. The sweeping arc of the Sun is distorted here by the projection, making it appear like a curve. The position of the daily arc across the sky changes with the Sun’s seasonal movement in the sky. Captured using a simple pinhole camera fashioned from a coffee can, it shows the Sun's journey over the course of six months, and the gradual shift of the daily arc from the summer, when it is higher in the sky, to its lowest arc at the winter solstice. Gaps in the Sun’s path represent days or times of day when the Sun was obscured by clouds. This image is a vivid testament to the changing seasons, revealing the Sun's dance in the sky as days grow shorter or longer.
Credit: Frank Niessen/IAU OAE (CC BY 4.0)

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Three paths of the Sun across the sky. The highest almost crosses the middle of the image, the lowest is one sixth of the way

Solstices and Equinox

Caption: This captivating image showcases the journey of the Sun through the sky during the winter solstice (bottom), equinox (middle), and summer solstice (top) in Bursa, Turkey, between 21 December 2007 and 20 June 2008. Using a fisheye lens, the photographer captured the Sun's positions at regular intervals from sunrise to sunset on the three different days, illustrating the changing lengths of day and night throughout the year. The variations in the Sun’s daily path across the sky, its height at midday, and the positions of sunrise and sunset, as well as the length of a day, depend on Earth’s axial tilt relative to its orbit around the Sun. The series was taken from a rooftop in Bursa, where the local terrain adds its own unique touch by occasionally obstructing the horizon. Oriented with north at the top and south at the bottom, this visual diary serves as a stunning representation of the Sun’s path for those residing in mid-northern latitudes.
Credit: Tunç Tezel/IAU OAE (CC BY 4.0)

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Many images of the sunset in a montage. The Sun’s setting position changes from the left in winter to the right in summer.

Occasive Amplitude

Caption: Captured from Gatto Corvino village in Sicily, this composite showcases the Sun setting at various points over the sea from the winter to the summer solstice (2016–2017). The centre frame marks due west. As Earth orbits the Sun, the position of the setting (and rising) Sun changes from being northward of due west (east) in summer to southward of due west (east) in winter. During the equinoxes — when day and night are of equal length — the Sun rises directly in the east and sets directly in the west. The changing location of the Sun on the horizon is due to Earth’s axial tilt as it orbits the Sun. In temperate regions of Earth, the Sun’s height above the horizon at noon is at its highest and lowest points in the sky on the summer and winter solstices respectively.
Credit: Marcella Giulia Pace/IAU OAE (CC BY 4.0)

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