A Straight Line in Space
Sometimes, the Sun, the Earth, and a third celestial body form a straight line in space, with our planet in the middle of the configuration. When that happens, astronomers classify the celestial body as being at opposition.
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Among our cosmic neighbors, the superior planets (those with orbits further from the Sun than Earth) of Neptune, Uranus, Saturn, Jupiter, and Mars come into this position, while the two inferior planets (those with orbits closer to the Sun) of Mercury and Venus do not.
The Moon arrives at opposition every month during the Full Moon phase. At that moment, the Sun and the Moon are at opposite sides of Earth, so the side of the Moon that is visible to us is fully lit up.
But opposition isn’t limited to planets and the Moon. Other celestial bodies, like asteroids, can move into this position as well.
What are inferior conjunctions?
Close, Large, and Bright
Opposition is a good time to observe a celestial body—when it is closest to the Earth, reflects the most sunlight, and makes its brightest appearance in our skies.
Mars can make a particularly dramatic impression at opposition. Once every 26 months or so, the red planet aligns with Earth and the Sun to become the second-brightest planet in the night sky at times, outshone only by Venus. Stargazers are looking forward to the next time Mars is at opposition: February 19, 2027.
In general, opposition provides a long and excellent window for spotting our cosmic neighbors. Bodies at opposition are visible above the horizon from sunset to sunrise, and can be a little easier to locate since they can be found at their highest point around midnight.