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.
Related Terms:
See this term in other languages
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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In Other Languages
- Arabic: المقابلة الفلكية
- German: Opposition
- French: Opposition
- Italian: Opposizione
- Japanese: 衝 (external link)
- Korean: 충
- Brazilian Portuguese: Oposição
- Simplified Chinese: 冲
- Traditional Chinese: 衝
Related Media
Retrograde Motion of Mars
Caption: Winner in the 2023 IAU OAE Astrophotography Contest, category Still images with smartphones-mobile devices.
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
Related Diagrams
Opposition and Conjunction
Caption: This diagram shows the Earth's orbit around the Sun and the orbits of two other planets, one orbiting the Sun outside the Earth's orbit and one orbiting the Sun inside the Earth's orbit. At different times in the orbits of both of these planets, their positions appear to line up with the Earth and the Sun. At these points each planet would appear close to the Sun when viewed from Earth. When this happens each planet is said to be in conjunction with the Sun.
For the planet that orbits the Sun within the Earth's orbit it can be in conjunction with the Sun twice in its orbit, once when it is between the Sun and the Earth and once when it is on the other side of the Sun from the Earth. When the planet is in conjunction between the the Sun and Earth it is said to be in inferior conjunction and when it is in conjunction on the far side of the Sun from the Earth it is said to be in superior conjunction.
For the planet orbiting the Sun outside the Earth's orbit, it can sometimes be on the other side of the sky. At this point the Earth lies between the planet and the Sun. The planet in this case is said to be in opposition.
Opposition and conjunction are not just limited to planets and other types of solar system objects such as dwarf planets, comets and asteroids can also be in opposition and conjunction. However only those objects that have an orbit that takes them outside the Earth's orbit can be in opposition.
Conjunction is also used to refer to alignments in the sky between planets in the Solar System. For example when Jupiter and Venus appear very close on the sky when viewed from Earth they are said to be in conjunction with each other.
Credit: Aneta Margraf/IAU OAE
License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
