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Mars at Opposition

Written by: Lucia Brimer

Several evenings ago, I walked outside after dark, and low in the eastern sky was a very bright object glowing reddish-orange. This is Mars! Have you ever noticed that sometimes Mars is very bright and other times it is much dimmer? This is due to the orbits of Earth and Mars. When Mars gets close to the Earth, it is much brighter.

The montage of images at left shows how Mars has changed in apparent size (and brightness!) from March – Sept. 2020. So, why does Mars change so much in brightness and in how close it is to Earth? Check out the illustration below.

Image Credit: Jonathan T. Grayson

A top-down image of the orbits of Earth

and Mars. Credit: NASA

The Earth orbits the Sun at an average distance of 93 million miles (150 million km). It takes 365.25 days to complete one orbit. However, Mars, at an average distance from the Sun of 142 million miles (228 million km) – about 1.5 times the distance of the Earth from the Sun – orbits more slowly. It takes about 687 Earth days, or nearly 2 Earth years, for Mars to orbit the Sun. This means that every 2 years the Earth and Mars will “pass by” each other fairly closely on the same side of the Sun, as in the above illustration. On the year between, Earth and Mars will be on opposite sides of the Sun and will be very far apart. The distance between Earth and Mars makes a very big difference in how bright the planet looks in our night sky. When the Earth is directly between the Sun and Mars, then Mars is in opposition, and it can be seen all night long. Mars is in opposition today, Oct. 13, 2020.

Also, notice that both orbits are elliptical, not circular. Earth’s orbit is nearly circular, but Mars’ orbit is more elliptical. So, if Earth and Mars pass each other when Mars is close to perihelion as illustrated above, they will be much closer to each other than if they pass each other on the opposite side, when Mars is near aphelion. [Perihelion is an orbit’s closest point to the Sun, and aphelion is an orbit’s farthest point from the Sun.] This is why some oppositions occur when the Earth-Mars distance is much closer than other times. This year, Mars’ perihelion was on Aug. 3. Due to the complex geometry, Mars’ closest approach to Earth is not necessarily on the date of opposition. This year, Mars was closest to Earth on Oct. 6, when it was 38.6 million miles (62.1 million km) away. The closest approach this Century was on Aug. 27, 2003, when Mars was only 34.6 million miles (55.8 million km) from Earth. On the other hand, when opposition occurs close to Mars’ aphelion, the 2 planets are a lot farther away. For example, on March 5, 2012, the distance between the 2 planets was 62.6 million miles (100.8 million km). This means that at opposition in 2003, Mars appeared to be nearly twice as big as it did in 2012! The next time that Mars will be closer than this year will be on Sept. 11, 2035.

Mars is known as the Red Planet due to its reddish-orange color. Mars is named after the Roman god of war, known as Ares in Greek. Perhaps this is because the ancient Greeks and Romans thought the planet was covered in blood? We now know that the color is due to a large amount of iron oxide in the rocks and dust on Mars. Mars is not a bloody planet, but a rusty planet!

Since Mars is 1.5 times farther from the Sun than the Earth, it is much colder there. The temperature on Mars can range from a high of about 70 degrees F (21 degrees C) on the equator in summer to a low of about -225 deg F (-153 deg C) at the poles in winter. The average temperatures are below freezing. However, its rotation rate, or length of its day, is very similar to the Earth. A Martian day, or sol, is 24 hours 39 minutes. Its axial tilt is also similar to Earth. Mars’ tilt is about 25 degrees, compared to Earth’s tilt of 23.5 degrees. Therefore, Mars has seasons like the Earth does. However, seasons on Mars are not marked by the growth of plants, but by annual dust storms, which occur during the southern hemisphere’s summer. About every 3 Martian years, the dust storms grow into massive, planet-circling dust storms. These global dust storms can hide the Red Planet’s surface features.

Comparison images of Mars taken by Hubble, before (left) and during a global dust storm that engulfed it (right). Credit: NASA

Mars will be very easy to see in the eastern sky all night throughout October. Jupiter and Saturn will be in the southern sky and Mars in the eastern sky. Normally Jupiter is much brighter than Mars, but for this month, Mars is the brightest planet.

You can find Mars in the constellation of Pisces. There are not many bright stars in the area, so it will be the brightest object by far in the eastern sky. The image at left, from the open source program Stellarium, shows Mars in the constellation Pisces the Fish, with the constellation Cetus the Whale below it. The orange color of Mars is obvious. If you have a small telescope, you may be able to see dark features on the planet’s surface. Astronomers are not expecting any dust storms to hide the features. So, check out the best view of Mars until 2035!

Find out more about Mars, the spacecraft that have visited there, and what they found, in my next blog.