If you don’t know what an opposition means in astronomy, here is a short explanation.
When an object is in opposition in astronomy, it means it is directly opposite our Sun from Earth. A good example is the Full Moon, where the Moon is close to being directly in opposition, hence why it shows a fully lit face back to Earth.
A planetary opposition is when our Sun, Earth and any given outer planet are directly in line with each other.
This applies for every Superior Planet, a planet that has an orbit outside of Earth’s. Therefore, Mars, Jupiter, Saturn, Uranus, and Neptune all have days where they are directly opposite the Sun from Earth.
On nights such as these, these are the characteristics:
- The planet is in retrograde motion, where it appears to go backwards for a brief period of time before going back on its normal path through the ecliptic.
- The planet rises directly at sunset, reaches the meridian (highest point in the sky) at midnight and sets directly at sunrise. Therefore the planet is visible all night.
- The planet’s disc is fully illuminated, showing a full phase. When Mars is at opposition, it looks circular, but when it isn’t, it looks almost like the shape of a gibbous.
- It’s at or near a point closest to Earth, making it appear brighter and slightly bigger through a telescope, thus it’s usually the best time to view and take pictures of that planet in question.
Mars’ apparent brightness and angular size as seen from Earth fluctuates the most due to its own size and different distances – as close as 40 million miles and as far away as 200 million! When at its furthest from Earth, it can appear no brighter than mag. +2 stars like Polaris, which are not the easiest to spot from a city sky. But when Mars is closest, not only will it appear as bright, if not brighter than Jupiter, but through a telescope can appear larger than Saturn’s apparent disc without its rings. So yes, when Mars is a lot closer to us, we notice it!
Planetary oppositions will not always be the same date every year, as over time their positions change along the ecliptic due to their own orbits around the Sun.
So when you hear about a planet that is in opposition, you now know what it means, and know it’s the best window of time to observe it!
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