Mars at Perihelic Opposition
September & October 2020
Mars will look larger than normal. Your telescope may show surface features!
There’s about two months in which Mars will look larger than normal in a telescope centered on October 6, 2020. Why? Mars is the fourth planet from the Sun. Earth is the third. There are times, about every two years, in which the two planets are on the same side of the Sun. This is called opposition. Mars is opposite the Sun in the sky. No, Mars will never look unusually large and NEVER as large as our Moon. You’ll still need a telescope to see Mars as a tiny disk and with a good amount of magnification. If you have a telescope, this is a good time to try to eke out some dark and light markings on the surface of Mars. The first thing you’ll see is the pale, orangish color of the planet. That is the sand covering this desert world. There is iron oxide (rust) there, providing the color. Then you might see the southern ice cap comprised of frozen water. The southern hemisphere is facing the Sun, so it has warmed up enough to evaporate the frozen carbon dioxide at the pole (CO2) also known as dry ice. Remember, its freezing point is at a much colder temperature than water’s. Then, if you are really lucky with good optics and steady viewing, you’ll see dark markings on the surface. That will be exposed lava rock called basalt. Guess what, the Moon and Earth both have basalt from geologic activity too! If you see any features at all, you are doing really well.
So what’s with the perihelic label? All orbits of all celestial objects are elliptical, or oval, in shape. Some orbits are almost all round, some are very elliptical, like a comet’s. None are circular as there is always something affecting them gravitationally. The point in an orbit in which the object is closest to what it is orbiting, like a planet orbiting the Sun, is called perihelion, or closest approach to the Sun. When you combine opposition with its perihelion, then Mars, in this case, is even closer to Earth. Again, it will never be very close. It will just allow us to see Mars a tiny bit larger than a regular opposition.
This event has one more aspect that is to our favor. The planet will be seen high in the sky in the Earth’s northern hemisphere. This allows us to see it higher above the Earth’s murky and turbulent atmosphere. Maybe even smaller features can be spied on this “red” planet. Look at the accompanying illustration and see the small change in the apparent size of Mars and compared to the Moon, all to scale and through a telescope. If you desire a more complete explanation of the orbits of Mars and why it looks larger, please see the October 2020 issue of the Bays Mountain Astronomy Club’s newsletter.