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Celestial Lineup Makes For A Very Bright Mars

When Mars is "in opposition," it, Earth and the sun are all in alignment. <em>(Note: This is a diagram meant to illustrate the concept of opposition. It is not drawn to scale.)</em>
Alyson Hurt
When Mars is "in opposition," it, Earth and the sun are all in alignment. (Note: This is a diagram meant to illustrate the concept of opposition. It is not drawn to scale.)

Updated at 7:37 a.m. ET Friday

Every two years or so, the sun, Earth and Mars line up — and that's what is happening now. It's a celestial orientation known as Mars opposition. Leaving aside any significance this might have for astrologers, from an astronomical point of view there's one thing you can say for sure about this Mars opposition: Mars will be brighter in the night sky than it's been for 15 years.

Mars is always brighter than usual during opposition. That's because from our vantage point here on Earth, the sun is shining directly at Mars. It's like pointing a flashlight directly at what you want to see. In this case, the sun is the flashlight.

But this opposition is special because of where Mars is in its orbit around the sun. That orbit is not a perfect circle. It's elliptical, slightly egg-shaped. As a consequence, there's a point in every Martian year where Mars is closest to both the Earth and the sun.

This year that happens on July 31, just a few days past opposition. On that day Mars will be only 35.8 million miles from Earth, and the closer an object is, the brighter it appears.

And even more significantly, Mars is about to reach the point in its orbit where it's closest to the sun — that's important because there is more sunlight that bounces off it, making the planet appear brighter.

There another strange thing that happens with Mars around the time of opposition. Most of the time, if you plotted the position of Mars in the night sky, it would appear to be moving eastward on successive nights. But for a few months around the time of opposition, it appears to change direction.

This strange behavior flummoxed early astronomers who thought the planets revolved around the Earth, not the sun. But it makes sense if you realize that the sun is at the center of the solar system.

This is a case where a video is worth a thousand words, or maybe 10,000 words, so if you want to understand this so-called retrograde motion, I strongly encourage you to watch the video.

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Joe Palca is a science correspondent for NPR. Since joining NPR in 1992, Palca has covered a range of science topics — everything from biomedical research to astronomy. He is currently focused on the eponymous series, "Joe's Big Idea." Stories in the series explore the minds and motivations of scientists and inventors. Palca is also the founder of NPR Scicommers – A science communication collective.
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