As opposition ( astronomical symbols : ☍) is called in astronomy the constellation , in which two celestial bodies from the Earth viewed at an angular distance ( elongation ) of 180 degrees are to each other. As a rule, only the opposition of one celestial body to the sun (as a second celestial body) is of interest , because only then can it be seen in the sky all night and is highest above the horizon at midnight .
If the moon is in opposition to the sun, we have a full moon . Celestial bodies that orbit the sun like the lower planets Mercury and Venus can never come into opposition to the sun and can therefore only be seen in the evening or morning sky, but not during the whole night.
In close proximity to the opposition of a celestial body in relation to the sun, the smallest distance from the earth is regularly reached, which is of course an advantage for earth-based observations. Due to the eccentricity of the planetary orbits, however, this smallest distance from the earth is often not reached exactly at the time of opposition, but the difference of a few days that occurs is generally of no practical importance.
If two celestial bodies are in opposition to the sun at almost the same time, a so-called triple conjunction can occur because of the apparent loop paths ( opposition loop ) of the celestial bodies involved . Here, the two celestial bodies meet three times at a distance of only a few months.
In the case of (sufficiently) exact opposition, i.e. when the sun, earth and the third celestial body - in this order - are on one line, the third body will be shadowed if the size and spacing is right. In the case of the small and near moon, a sufficiently perfect opposition leads to a total or ring-shaped lunar eclipse, while a less precise opposition leads to an asymmetrical partial eclipse.
Special forms of opposition