Glossary term: Opposition

Description: When two astronomical objects appear to be lined up or nearly lined up with an observer, in opposite directions in the sky, they are said to be in opposition. It is not necessary for both objects to be actually visible for the observer. For instance, at a full moon, the Sun, observer on Earth, and Moon are lined up, so the visible part of the Moon's surface is fully lit up by the Sun – unless the alignment is perfect, in which case there is a lunar eclipse. When a planet, comet, or asteroid is said to be "in opposition", this commonly refers to the Sun and observers on Earth. When a planet is in opposition, it looks particularly bright, appears to move in a direction opposite than usual ("retrograde motion" as Earth moves faster on its inside track), and is particularly close to Earth.

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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".

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A montage of images of Mars, seen here as a series of red dots in a squashed z-shaped pattern.

Retrograde Motion of Mars

Caption: This image captures the celestial waltz of Mars as it demonstrates its intriguing retrograde motion against the background of fixed stars. This event, when Mars appears to backtrack in its orbit, arises from the different speeds at which Earth and Mars orbit the Sun. Earth’s faster movement occasionally positions it ahead of Mars, creating the illusion of the Red Planet moving in reverse from our perspective. This retrograde motion occurs when Mars is on the other side of the sky from the Sun, when it is said to be in opposition. Following Mars from 14 August 2022 to 5 April 2023, this smartphone image stands as a testament to perseverance and precision in the tranquil setting of Bataan, Philippines. Enduring unpredictable weather and ever-shifting celestial alignments, the photographer meticulously captured each shot at regular intervals of five to eight days. The process involved aligning 35 distinct images of Mars, taken without any external lens or telescope, alongside a stacked background image composed of 54 frames lasting 15 seconds each, portraying the starry expanse. Fusing these images involved precisely aligning them and cropping Mars in order to centre its position, revealing its retrograde movement against the backdrop of stars. This intricate process, blending the images seamlessly into the background by masking, highlights the planet’s unique motion. In the lower right corner, the Pleiades star cluster is visible.
Credit: Rob Kerby Guevarra/IAU OAE (CC BY 4.0)

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