What is the opposition in astronomy?

What happens when a planet is in opposition? This is the case of Saturn on August 2, 2021. This configuration is conducive to its observation. When the Sun rises, the planet sets, and vice versa.

Saturn is in opposition on Monday August 2, 2021. It is a good time to try to observe the planet – with the naked eye, with binoculars, or even with a telescope if you have one. What exactly happens when a planet is in opposition?

In astronomy, we speak of opposition to describe a configuration between two stars, which have an angular deviation of 180 °, seen from a third star. Here we are talking about the Sun and a planet, as we observe them from Earth. Seen from Earth, the planet is opposite the Sun in the sky. Therefore, when the Sun rises, the planet sets, and vice versa. We can therefore see this planet almost all night long at this time.

During the opposition, a planet is at this point in its orbit closest to the Earth. This is what this animation produced by our motion-designer Nino Barbey shows: we can imagine an imaginary straight line linking Saturn, the Earth and the Sun.

This is what happens when a planet, here Saturn, is in opposition. // Source: Nino Barbey for Numerama

How far away is Saturn?

If we want to be more precise, we must say that Saturn went to the opposition at 8.14 a.m. on Monday, August 2. Its distance from Earth was then exactly 8,935 astronomical units (approximately 1.3 billion kilometers, because an astronomical unit is equivalent to 150 million kilometers). It also happens that Saturn reaches the perigee this August 2, at 1:08 p.m .: this name is given to the point in the orbit of a celestial object at which its distance from Earth is minimal (the opposite of the apogee). During the next few weeks, Saturn is therefore an interesting object to look for in the sky if you want to do some astronomy.

As the Lowell Observatory (Arizona, United States) points out on Twitter, the opposition of Saturn means that the planet appears to us a little bigger and brighter. It is possible to distinguish the planet with the naked eye, but if one wants to try to locate more details, binoculars are necessary.

Only the planets of the solar system which have an orbit outside that of the Earth can be opposed: Mars, Jupiter, Saturn, Uranus, Neptune (the latter two being quite difficult to see without an instrument). As for the Moon, it also goes into opposition, but this is more commonly called the full Moon.

Another planet will be in opposition in August 2021: on Friday 20, it will be the turn of Jupiter, the largest planet in the solar system, to be opposite the Sun in our sky.

Look at the world from space

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Source: Numerama by www.numerama.com.

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