Glossary term: Sky

Description: The sky is what we see when we are outside, our view unencumbered by any structure on Earth, looking up or at least higher than the horizon that marks the boundary of what we can see of Earth and earthbound structures. When looking at a clear sky at night, we can see distant planets, stars, and even a few galaxies (the Andromeda galaxy in the northern and the Magellanic Clouds in the southern hemisphere). The twinkling of the stars is evidence that we are still looking through the gases of Earth's atmosphere. During the day, sunlight scattered by air molecules makes the sky shine with a blue color, blocking our view into the cosmos. Clouds or fog covering the sky will also keep us from observing any astronomical objects.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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".

Related Media

The arc of the Milky Way over some trees. Two small galaxies can be seen in the lower middle of the image.

Milky Way Over Quiver Tree

Caption: This breathtaking photograph was captured on 17 June 2023, near Keetmanshoop, Namibia, with a smartphone. Dominating the night sky, the majestic arc of the Milky Way creates a celestial bridge across the heavens. The image captures a range of notable astronomical objects: the Large and Small Magellanic Clouds, seen towards the bottom of the image and appearing as fuzzy clouds; Antares, seen towards the top left of the image; and the coalsack nebula (referred to by various names by Indigenous cultures around the world), seen vertically above the Large Magellanic Cloud. Silhouetted against this astral backdrop, the trees — which are actually succulent aloe plants native to southern Africa — add a touch of Earth's unique beauty. Historically, these plants were known as ‘quiver trees’ because groups of local Indigenous people would use their hollowed branches to hold darts. The serene Namibian landscape, combined with the brilliance of the southern hemisphere's stars, offers a glimpse into the majesty of our Universe.
Credit: Jianfeng Dai/IAU OAE (CC BY 4.0)

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

Four dish-shaped telescopes point at the sky with the arc of the Milky Way overhead.

Milky Way Over H.E.S.S Observatory

Caption: The darkness of the skies at H.E.S.S Observatory reveal the seemingly innumerable stars that make up the Milky Way galaxy, making it challenging to discern the constellations as seen taken with a smartphone from Namibia in June 2023. The stars Alpha and Beta Centauri are visible bottom left of the image just about the tower. The orange-hued star visible just above and to the right of the left H.E.S.S telescope is Antares, a red-giant star that is part of the constellation Scorpius.
Credit: Jianfeng Dai/IAU OAE (CC BY 4.0)

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

The Big Dipper drifts lower towards the horizon on the left, on the right a comet rises in the sky.

Big Dipper and Comet Neowise C2020 F3

Caption: This time-lapse documents the trajectory of the iconic Big Dipper across three frames taken in July 2020. Captured from three locations in Italy, Tre Cime di Lavaredo Auronzo di Cadore, Monte Rite, Cibiana di Cadore, and Casera Razzo, Vigo di Cadore, this visual odyssey showcases the captivating journey of the Big Dipper with the addition of trails of stars painting a celestial canvas. It not only traces the path of this renowned asterism but also features the rare appearance of comet Neowise C/2020 F3, an extraordinary event that graced our skies during July 2020.
Credit: Giorgia Hofer/IAU OAE (CC BY 4.0)

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

The Milky Way appears as a diagonal stripe bottom left to top right. It becomes more horizontal as the video progresses.

Southern Sky Over La Silla

Caption: This video shows the Milky Way’s sprawling brilliance across the celestial expanse. Among the stars’ grand theatre, the Southern Cross constellation claims its place in the scene, distinguished by its distinctive cross shape as seen in the top of this video and slightly to the right just over the Milky Way, slowly disappearing as the video progresses. This celestial marker, a prominent feature in the southern hemisphere, holds cultural and navigational significance, having served as a navigational guidepost for centuries. Accompanying the Milky Way’s grandeur are the two Magellanic clouds, celestial companions seen dancing in the distant sky. The Carina Nebula adds its ethereal glow to the cosmic panorama, painting a radiant hue against the darkness. In the midst of this celestial ballet, a gentle green hue envelops the atmosphere, a phenomenon known as airglow, adding a touch of subtle luminescence to the night sky. The terrestrial also makes an appearance, with planes flying overhead and vehicles driving between observatory buildings. This time-lapse, taken from the La Silla Observatory in Chile, is a window to the captivating dance of stars, offering a glimpse into the awe-inspiring beauty of our galaxy and the celestial landmarks that grace the southern sky.
Credit: José Rodrigues/IAU OAE (CC BY 4.0)

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