Glossary term: Star Trail

Description: In astrophotography, star trails are striking images of the night sky made using very long exposures. This is the effect of the camera rotating with the Earth and the resulting motion of the stars across the night sky causing trails in the images. Star trails show the diurnal motion of the Earth, and if the camera is pointed at the celestial North or South Pole, the trails form semi-circles centered on the pole location in the sky. In addition, the pattern of star trails can be used to determine the observer's latitude. Star trail images can also be created by taking a series of short exposures over a period of time, and then stacking (overlaying) them on top of each other in photo editing software.

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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

Star trails form short, diagonal lines in the background of a volcanic eruption

Star Trails And Lava Plume on Mount Etna February 2021, by Dario Giannobile, Italy

Caption: Third place in the IAU OAE Astrophotography Contest, category Star trails. This spectacular image captures some of the most dramatic expressions of Nature, the eruption of a volcano and the night sky. The volcano in the foreground is Mount Etna, located in Sicily/Italy, during an eruption in February 2021. In the background, we can see the star trails resulting from the Earth's motion around its imaginary axis. As Mount Etna is located about 37° N, we can notice that the arcs are quite open, as this is far from the North Pole, from where the view from the sky would provide traces drawing circles. As we move to the Equator, either from the Southern or Northern Hemisphere, the star traces become more and more parallel, instead of circular, as the viewer moves away from the zenith pointing to one of the Earth's rotational axes. Therefore, the sky presents us with valuable information about location and time and was an important practice for ancient civilizations to follow the movement of the celestial bodies. The image also captures the varying colours of stars which result from differece in the temperature of stars: higher temperature stars are bluer, compared to the lower temperature white and red stars.
Credit: Dario Giannobile/IAU OAE

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

An all-sky image. Above a ring of stones shaped like door-frames, the bright curved paths of star trails seem to form circles

Stone Star Circles, Startrails above Stonehenge, by Till Credner, Germany

Caption: Second place in the IAU OAE Astrophotography Contest, category Star trails. Astronomy is one of the oldest (if not the oldest) of the sciences, and as such has connection to various cultures over millennia. This image in a way conveys this relationship by being contextualised in Stonehenge. There is much research into what astronomers call archeoastronomy sites, and how they connect to the sky (for example, seasons, phases of the moon and much more). Civilizations across time and from all over the world have their own views and interpretations of what they see in the sky, and this has been tied not only to culture but also to the people’s day-to-day and seasonal activities. The “concentric circles” which are often referred to as “star trails”, are the result of the apparent motion of the sky, which is in reality due to the rotation of the Earth on its axis. The small dot appearing towards the top center of the image is Polaris – The North or Pole Star. Polaris is only visible to observers in the Northern latitudes. The height of the Pole Star can be used to infer the observer’s actual latitude. Stonehenge is located at around 51° North. This image is taken from one of the most notable ancient sites in the world, brings us back in time, and makes us wonder about the stories told by the people that lived in that place many millennia ago.
Credit: Till Credner/IAU OAE

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

Bright star trails for arcs around the center of the image behind the silhouette of a tree

Half day exposure to the north star, by Fabrizio Melandri, Italy

Caption: First place in the IAU OAE Astrophotography Contest, category Star trails. This star trail image shows the apparent movement of the stars in the night sky, which results from the Earth’s rotation around its polar axis. The trees in the foreground serve as an anchor as we turn with the Earth. The bright white semi-circle in the center of the image is the North (Pole) star – Polaris, which is located very close to the North Celestial Pole. One way to imagine that the Earth is at center of hollow crystal sphere, and the stars are embedded on this sphere, as the Earth spins on its axis, an observer on Earth sees the stars rising and setting, following an arc, because the horizon prevents the observer from the full arc for stars that are further away from the Celestial Pole. The North Celestial Pole (and its opposite the South Celestial Pole) is essentially formed by extending the line of the Earth’s axis. Capturing this image requires the photographer to take a long exposure with the camera on a tripod and pointing towards the Pole Star (North Hemisphere). In the Southern Hemisphere there is no star as close as Polaris is to the Celestial Pole, so the position of the South Celestial Pole is found using the South Cross and Pointer Stars (Alpha and Beta Centauri). Although overall the image has a slight blue tint, it does capture the varying colours of stars, it is easy to distinguish between the higher temperature blue stars and the lower temperature of the whitish stars, and even lower temperature reddish stars. The reason for this variation in colour is because higher temperature stars emit more in shorter wavelengths (bluer), compared to lower temperature stars which emit in longer wavelengths (redder).
Credit: Fabrizio Melandri/IAU OAE

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