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by Irene Pease on
Every few years Earth passes Mars in their path around the Sun. During this year’s pass, Earth and Mars will be closer than they’ve been since 2003, and as close as they’ll be until 2035. Why are the planets especially close this year, and what can we expect to see in the sky or through a telescope?
[Mars in the night sky, briefly highlighted with a circle, is brighter and redder than the stars. It moves leftward against the background of stars]
Mars, like the other planets, appears to move against the backdrop of stars.
[Outlines of Sagittarius and Capricornus appear. Mars stops and moves rightward for a time.]
This month you can find it between Capricornus and Sagittarius, moving backward, then forward again.
[Mars stops again and resumes moving leftward.]
Why does it seem to change direction?
[Earth and Mars’s orbits around the Sun, planets moving counter-clockwise. Mars’s orbit larger than Earth’s, both mostly-circular with the Sun slightly off-center. Year is labeled in upper left corner: 2009. Gap between the planets’ paths is narrower on the left side of the Sun than on the right.]
The closer a planet is to the Sun, the faster it travels.
Since Earth is closer, it passes Mars, on average, every two years and fifty days.
Whenever Earth passes a planet that is farther from the Sun, we observe retrograde motion.
[Split screen. Left side: Earth and Mars continue their orbits. Right side: Mars moves leftward in Earth’s night sky.
Earth approaches and passes Mars in orbits; Mars in sky stops and moves rightward. Turing point in sky, and positions of planets in orbits are marked by dots. Mars in sky moves rightward and Earth in orbit passes Mars. Line connects orbits at closest approach. Dot marks Mars’s position in sky. Mars in sky stops and moves leftward again. Turing point in sky, and positions of planets in orbits are marked by dots. Marker in sky for closest approach is centered between the turning point markers.
Split-screen ends, orbit view re-centers, Earth and Mars still moving around Sun. Line for closest approach remains, at lower right side of the Sun.]
A close approach occurs during retrograde motion.
How does our view of Mars during a close approach compare to our view when Mars is farther away?
[Images of Mars as viewed from Earth appear outside of Mars’s orbit as the planets continue moving.
As Earth passes Mars in its orbit the view is much larger. A line between the orbits marks the close approach. Label: March 5, 2012, 62.6 million miles. Motion continues. Other views of Mars are relatively small in comparison.
After a full Martian year of Mars views, all but the close approach view fade.]
During close approaches, we get exceptional views of Mars.
Earth-view of Mars appears next to 2010 closest approach line. Label: January 27, 2010, 61.7 million miles.
Earth passes Mars in orbit, line marks closest approach and Earth-view of Mars appears. Label: April 14, 2014, 57.4 million miles. The approaches shown are on the right side of the Sun, where separation between Earth and Mars’s paths is wider. 2014 view is slightly larger than the others.]
Due to the elliptical shape of orbits, some approaches are much closer than others.
Line marks closest approach, upper right of Sun. Larger Earth-view of Mars than previous years. Label: May 30, 2016, 46.8 million miles.
Line marks closest approach, upper left of Sun, where gap between planet’s orbits is small. Earth-view of Mars is nearly twice as large as the 2012 view. Label: July 31, 2018, 35.8 million miles.
This month will be Earth’s closest approach to Mars since 2003.
Earth and Mars won’t be this close for another 17 years.
[Line marks closest approach, lower left of Sun. Earth-view of Mars is slightly smaller than 2018 view. Label: October 6, 2020, 38.6 million miles.
[A tiny red-orange dot on left half of screen. Label: Telescope View]
In telescopes, Mars is usually a very small, reddish circle.
For reference, this is the Moon at the same magnification.
[Image of north east quadrant of Moon dwarfs Mars, extends beyond screen.]
This month, the small circle of Mars is much larger than usual — large enough to reveal markings of Mars’s surface features.
[Mars-dot enlarges 7 times into a circle with some dark features. Moon image fades. Mars globe on right side with features clearly visible. ]
Which features can you identify in the telescope view?
[Telescope view and globe turn simultaneously through one rotation. Dark spots on telescope view correspond to clear dark features on globe.
Labels for dark regions as globe rotates: Sirenum, Cimmeria, Tyrrhenum, Hellas (light region), Syrtis Major, Meridiani, Erythraeum.
Night sky view, Mars moving in retrograde.]
Seize the night… see the red planet during this extra-close approach.