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What is “opposition”? April 10, 2014

Posted by aquillam in Astronomy, MichiganAstro.
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In common vernacular, the word opposition means “opposed to”. So when I say Mars is “at opposition”, I get a lot of funny looks. What would a planet be opposed to? So here is an explanation of the astronomical term ”opposition”.

In astronomy, the word opposition actually means opposite in the sky from something else, usually the Sun. For example, the full moon always rises at sunset and sets at sunrise: it is opposite the sun in the sky, so the full moon is always at opposition.

The entire sky is shown a few minutes before sunset, with the Sun on the horizon  in the ENE and the Moon just above the ESE horizon.

A few minutes before sunset on June 12. The Sun and Moon are on opposite sides of the sky. The full moon actually occurs a little after midnight local time, so it is already above the horizon at sunset.

Mars was at opposition on April 8, 2014, so it rose at sunset that night nearly due east (as the Sun was setting nearly due west), and set at sunrise the next morning.

Opposition is a good time to observe the outer planets. There’s the obvious reason that the planet is out all night, so you get plenty of time to observe. However, opposition also means the planet will be at its closest position to Earth, so it’ll be bigger and brighter than normal too. Lets take a look at why.

The Sun and the orbits of two imaginary planets with perfecty circular orbits.

Opposition for circular orbits. An observer on the inner planet would look in one direction to see the Sun, and in the opposite direction to see the outer planet. In this case, the two planets are also at their closest.

 

If the orbits of all the planets were perfectly circular, opposition and “closest approach” would be the same.

 

 

 

 

 

 

 

 

 

However, the orbits are actually elliptical, and the ellipses aren’t aligned with each other. The eccentricity and orientation of the ellipses is what determines when “closest approach” actually occurs. Here is an exaggerated diagram.

A diagram showing the Sun and the elliptical orbits of two planets. The position of the planets is shown for "closest approach"

A few days after opposition, the two planets are actually closer together (though the difference is very small!)

A diagram showing the Sun and the elliptical orbits of two planets. The position of the planets is shown for opposition, so they are in line with the Sun.

An observer on the inner planet would look in one direction to see the Sun, and in the opposite direction for the outer planet, so the outer planet is at opposition.

 

 

 

 

 

 

 

 

 

 

 

 

 

A diagram showing the Sun and the elliptical orbits of two planets. The position of the planets is shown for when opposition and "closest approach" occur at the closest point in their orbits.

The best possible opposition.

All the planets in our solar system have nearly circular orbits. This means that “closest approach” and opposition happen very close to each other, but not exactly at the same time. The very best configuration is of course when opposition occurs at the point when the orbits are closest.

 

 

 

 

This is of course very rare. It happened for Mars back in 2003. The opposition of 2035 will again be very close, but not as good as 2003.

Here is a diagram for this year:

An image showing the Sun and the orbits of Earth and Mars.

Mars at Opposition on April 8, 2014. Closest approach is a few days later, on April 14.

 

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