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Terme du glossaire : Axe de la Terre

Redirigé depuis Pôle Nord

Description : L'axe de la Terre est une ligne droite imaginaire autour de laquelle la Terre tourne une fois par jour. Les deux endroits où l'axe et la surface de la Terre se croisent sont le pôle Nord géographique, à une latitude de 90°N, et le pôle Sud géographique, à une latitude de 90°S. L'équateur est également une ligne imaginaire qui correspond à l'intersection de la surface de la Terre avec un plan perpendiculaire à l'axe de la Terre. Ce plan est définit de telle manière à ce que les points sur l'équateur soient à la plus grande distance de l'axe de la Terre. La latitude de l'équateur est de 0°.

Les pôles géographiques ne sont pas situés aux mêmes endroits que les pôles magnétiques nord et sud de la Terre. Les pôles magnétiques sont définis comme les endroits de la surface où le champ magnétique de la Terre pointe respectivement vers le bas et vers le haut.

L'orbite terrestre possède également un axe : la direction perpendiculaire au plan orbital de la Terre. L'axe de la Terre est incliné d'un angle de 23,4° par rapport à cet axe orbital. Cette inclinaison est à l'origine des saisons terrestres. En particulier, au cours d'une orbite, il y a des phases où l'hémisphère nord ou l'hémisphère sud sont inclinés vers le Soleil, recevant ainsi plus de lumière.

Par rapport aux objets célestes les plus éloignés, la direction vers laquelle pointe l'axe de la Terre change lentement au fil du temps, dans ce que l'on appelle la précession axiale de la Terre. Actuellement, l'axe de la Terre croise la sphère céleste à proximité de Polaris, l'étoile polaire. Mais sur une période d'environ 26 000 ans, ce point d'intersection décrit un petit cercle dans le ciel. Les systèmes de coordonnées célestes qui définissent les coordonnées du ciel par rapport à la Terre doivent prendre en compte les infimes changements de coordonnées qui en résultent au fil du temps.

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Statut du terme et de sa définition : La définition initiale de ce terme en anglais a été aprouvée par un·e spécialiste de la recherche en astronomie et un·e spécialiste de l’éducation
La traduction de ce terme et de sa définition n'ont pas encore été aprouvées

Le glossaire multilangue de l'OAE est un projet du Bureau de l'IAU de l'Astronomie pour l'Education (OAE) en collaboration avec le Bureau de l'IAU de diffusion de l'Astronomie (OAO). Les termes et définitions ont été choisis, écrits et relues grâce à un effort collectif de l'OAE, les Centres et les Noeuds de l'OAE, les Coordinateurs Nationaux de l'Astronomie pour l'Education de l'OAE(NAECs) et d'autres volontaires. La liste complète des crédits pour ce projet est disponible ici . Tous les termes du glossaire et leur définition sont déposés sous licence Creative Commons CC BY-4.0 et doivent être créditées au nom de "IAU OAE".

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Diagrammes associés


Two views of the Earth with important lines of latitude and longitude marked

Latitude and Longitude

Légende : Two views of the Earth, one viewing the Northern Hemisphere, one viewing the Southern Hemisphere. The Earth rotates around its axis, an imaginary line that runs through the Earth from the North Pole to the South Pole. The Equator is an imaginary line that is the same distant from both the North Pole and the South Pole. The positions of two cities, Rome in the Northern Hemisphere and Sydney in the Southern Hemisphere are marked here with red dots. Arrows indicate the two spherical coordinates latitude and longitude. Latitude marks the position north or south of the equator. Here we can see Rome has the letter N in its latitude as it is in the Northern Hemisphere and Sydney has the letter S in its latitude as it is in the Southern Hemisphere. Latitude can vary from 90° N at the North Pole to 0° at the Equator to 90° S at the South Pole. Longitude measures the position around the equator. While the choice of the zero point for latitude as the equator seems obvious, the choice of the zero point for longitude is more subjective. By convention the zero point in longitude is the prime meridian which passes through the Royal Greenwich Observatory in the UK. This is marked here as a solid line originating at the North Pole. Longitude is measured in degrees east or west of the prime meridian. As both Rome and Sydney lie to the east of Greenwich, they have the letter E as part of their longitude. Moving west to east, longitude varies from 180° W on the other side of the world from the prime meridian to 0° on the prime meridian before reaching 180° E again on the other side of world from the prime meridian. This diagram shows the Earth at the December solstice. Two views are presented, one viewing the Northern Hemisphere and one viewing the Southern Hemisphere about nine hours later. The shaded region shows the night side of the Earth, with the day side being lighter. As it is the December solstice, the Sun appears overhead at the Tropic of Capricorn. This is a line of constant latitude at 23°26′09.3″ S. Six months later, at the June solstice, the Sun will appear to be overhead at the Tropic of Cancer at 23°26′09.3″ N. As the Sun appears over the Tropic of Capricorn more of the Southern Hemisphere is illuminated than the Northern Hemisphere. Indeed below the Antarctic Circle (the Polar Circle around the South Pole) the Sun does not set at this time of year leading to a Polar Day. Conversely, north of the Arctic Circle (the Polar Circle around the North Pole) the Sun does not rise at this time of year, leading to a Polar Night.
Crédit : Maria Cristina Fortuna/IAU OAE

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


The Earth with its tilted rotation axis at 4 points (December Solstice, March Equinox, June Solstice, September Equinox) in its orbit round the Sun

Solstices and Equinoxes

Légende : This diagram demonstrates the relative positions of the Earth and Sun at the two solstices and two equinoxes. The Earth’s rotation axis is tilted by 23.4° from the axis of its orbit. This means that for half the Earth’s annual orbit around the Sun, the Southern Hemisphere is tilted towards the Sun and the Northern Hemisphere is tilted away from the Sun. During this time the Sun appears to lie below the celestial equator. Due to this at any particular point in the Southern Hemisphere the Sun will appear to be higher above the horizon at a particular time of day and the days will be longer. Conversely during this time, the Sun appears lower in the sky at any particular point in the day in the Northern Hemisphere and the days are shorter. This effect is most pronounced in late December when the Sun appears at its most southerly point in the sky, corresponding to the longest day of the year in the Southern Hemisphere. This normally occurs on the 21st or 22nd of December (UTC), depending on when the most recent leap year was, but can very occasionally occur on the 20th or 23rd of December. This event is known as the December solstice. As the Sun’s rays hit the Southern Hemisphere at a more perpendicular angle during this time, the Southern Hemisphere is typically warmer in this time of year than at other times of year. This is often referred to as summer in temperate and antarctic regions of the Southern Hemisphere and thus the December solstice is known as the summer solstice. South of the Antarctic Circle at this time the Sun never sets and a Polar Day can last several months. Conversely in the Northern Hemisphere, the Sun’s rays hit the Earth at a more grazing angle than at other times of year so it is colder and this solstice is known as the winter solstice. North of the Arctic Circle the Sun never rises during this time and the Polar Night can last several months. As the year progresses the Earth moves in its orbit and the tilt of the Earth’s axis moves so it appears more side-on to the Sun. From the perspective of an observer on Earth this means that the Sun moves north in the sky, eventually reaching the celestial equator in late March, This usually occurs on the 20th of March (UTC) but can sometimes occur on the 19th or 21st of March. During this time all places on Earth will experience night and day that is approximately 12 hours long. The Sun’s rays now hit the Southern hemisphere at a more shallow angle than they did in December and hit the Northern Hemisphere at a more perpendicular angle. This means that the Northen Hemisphere will have warmed since December and the Southern Hemisphere cooled. This leads to the March equinox being referred to as the autumn equinox in the Southern Hemisphere and the spring or vernal equinox in the Northern Hemisphere. As the year progresses further the Sun moves further north on the sky, the Northern Hemisphere days lengthen and the Southern Hemisphere days shorten. By June the Earth’s orbit has progressed to the point where the Northern Hemisphere points towards the Sun and the Southern Hemisphere away from the Sun. At one point between the 20th and 22nd of June (UTC) the Sun reaches its most northerly point in the sky, this is known as June solstice. It is now summer in northern temperate and artic regions so this is known as the summer solstice in the Northern Hemisphere and the winter solstice in the Southern Hemisphere. This is the longest day of the year in the Northern Hemisphere and the shortest day of the year in the Southern Hemisphere. During this time regions north of the Arctic Circle experience a long Polar Day and regions south of the Antarctic Circle experience polar night. The year progresses further and the Earth moves in its orbit so that the tilt of the Earth’s axis again appears side-on for the Sun. The Sun moves south in the sky and again crosses the celestial equator between the 21st and 24th of September (UTC). At this time all places on the Earth experience equal lengths of day and night. This is known as the September equinox, also called the spring or vernal equinox in the Southern Hemisphere and the autumn equinox in the Northern Hemisphere. Note the sizes of the Earth, Sun and the Earth’s orbit around the Sun are not to scale in this diagram.
Crédit : Maria Cristina Fortuna/IAU OAE

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

Activités associées


Why Do We Have Day and Night?

Why Do We Have Day and Night?

astroEDU educational activity (links to astroEDU website)
Description: Explore day and night of Earth.

License: CC-BY-4.0 Creative Commons (CC) Attribution 4.0 International (CC BY 4.0) Icônes
Mots clés : Tilt
Tranches d'âge : 6-8 , 8-10 , 10-12
Niveau scolaire : Primaire , Secondaire
Domaines d'apprentissage : Modélisation , Axée sur la simulation , Apprentissage par enquête
Coûts : Faible
Durée : 1h 30m
Taille du groupe : Par groupe
Compétences : Poser des questions , Communiquer des informations


Le Bureau de l'astronomie pour l'éducation est hébergé par la Maison de l'astronomie sur le campus de l'Institut Max Planck d'astronomie à Heidelberg. Le Bureau de l'astronomie pour l'éducation est un bureau de l'Union Astronomique Internationale, avec un financement important des Fondations Klaus Tschira et Carl Zeiss. Les colloques Shaw-IAU sur l'astronomie pour l'éducation sont financés par la Fondation Shaw Prize.