What is opposition in astronomy?

Jupiter is at opposition on Monday, September 26, 2022. This is a good time to attempt observing the distant planet, even with the naked eye. You can also hope to distinguish it with binoculars. What exactly happens when a planet is at opposition?

What is opposition in astronomy?

In astronomy, we speak of opposition to describe a configuration between two stars, which have an angular difference 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 throughout the night at this time.

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

How far away is Jupiter?

If we want to be more precise, we must say that Jupiter goes into opposition at 9:33 p.m. on Monday, September 26, 2022. Its distance from Earth is then 3.953 astronomical units (or approximately 591 million kilometers, because an astronomical unit equals 150 million kilometres). It also happens that Jupiter reaches perigee on the same day, at 4:16 am: this name is given to the point in the orbit of a celestial object at which its distance from the Earth is minimum (the opposite of the apogee). At the moment, Jupiter is therefore an interesting object to look for in the sky if you want to do a little astronomy.

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