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| Astronomy Essentials | Space on Jun 06, 2014

Why is Mars sometimes bright and sometimes faint?

The brightness of Mars in our sky depends on where our two planets are in orbit around the sun.

Here is the view from above the solar system in early June 2014.  Earth passed between Mars and the sun in April, and now we are flying ahead of it in the race of the planets.  That's why Mars is fainter now than it was in April 2014.

Here is the view from above the solar system in early June 2014. At the center of these diagrams is the sun (yellow dot). Earth (third orbit) passed between Mars (fourth orbit) and the sun in April 2014. By June, we are flying ahead of it in the race of the planets. That’s why Mars is fainter now than it was in April 2014. Image via Fourmilab.

In April 2014, Mars and Earth will be on the same side of the sun.  Earth will be passing between the sun and Mars.   Image via Fourmilab.

Earth passed between between the sun and Mars on April 8, 2014. Then, the distance between our two worlds was around its least for this two-year period. Image via Fourmilab.

Mars started its current cycle of visibility in Earth's sky in June, 2013.  At that time, Earth (third orbit from the sun) was just rounding the far side of the sun, as seen from Mars.  Image via Fourmilab.

Here’s a chart showing one year ago – June, 2013. At that time, Earth (third orbit from the sun) was just rounding the far side of the sun, as seen from Mars, giving us a distant line of sight to the planet. Image via Fourmilab.

Mars is the world orbiting the sun one step outward from Earth’s orbit. Earth takes one year to orbit the sun once. Mars takes about two years. The orbits of Earth and Mars are the reason Mars is one of the most fascinating planets to watch in our sky, and they are the reason Mars is sometimes bright and sometimes faint. On April 8, 2014, Earth went between the sun and Mars. That’s why Mars was so bright this April.

Now Mars is fainter, but it still lovely to behold, and its near the moon on the night of June 6.

Mars brightest in six years in April 2014

The brightness of Mars in our sky depends on where our two planets are in orbit around the sun. Sometimes Earth is close to Mars, and sometimes we are far away.

We are relatively close – and Mars appears at its brightest in our sky for that two-year period – every time Earth passes between the sun and Mars. At this time, Mars appears at opposition – opposite the sun in our sky – rising in the east when the sun sets in the west. Oppositions of Mars recur about every two years and 50 days.

So Mars alternates in appearing bright and faint in our sky. It’s bright in the years we pass between Mars and the sun. It’s faint in the years in between, when Mars lies far across the solar system from Earth – at times hidden from our view by the sun itself.

In 2013, Mars was in conjunction with the sun (behind the sun as seen from Earth) on April 18. It came back to visibility in Earth’s sky around the end of June, when Earth had rounded the sun as seen from Mars, giving us a line of sight to the planet.

On April 8, when Earth passed between the sun and Mars, the red planet is relatively close to us then, and it appeared as a bright red “star” in our night sky, rising in the east while the sun is setting in the west. Mars was excellent to view through April 2014. It was pretty good in May, too, and will be a fine sight throughout the summer of 2014, a charming companion to the beautiful skies of June, July and August. By September 2014, Mars will be noticeably very faint again – inconspicuous. It’ll be exceedingly faint and low in the southwest when 2014 ends.

Mars will also come close in May 2016 and July 2018. And so it will be close and bright every two years for billions of years to come!

Bottom line: Mars is bright when it and Earth are on the same side of the sun. It’s faint when it and Earth are on opposite sides of the sun. 2014 is a good year to see Mars. The best month for Mars viewing in 2014 is April. But the planet is still bright throughout the summer of 2014. Watch for it!