By that, I am not referring to the latest escapades in American politics, but when planets reach opposition with Earth. During this time, the planet lies opposite of the Earth from the Sun. Why should we care about this? When opposition occurs, this is a planet’s closest approach to Earth and is the optimal time to observe it. On September 26th, Jupiter was in opposition as you can see below.
As Jupiter and the Sun are on opposite sides of the Earth, when the Sun set in the west on September 26th, Jupiter rose in the east. The following morning, as the Sun rose in the east, Jupiter set in the west. Hence, Jupiter was visible all night. Also, Jupiter’s day side was facing the Earth, allowing astronomers to capture the shadows of Jupiter’s moons eclipsing the surface of the giant planet.
While the optimal time to observe any planet that resides outside of Earth’s orbit is during opposition, historically, the opposition of Mars tends to draw the most anticipation. Today, the approaching opposition of Mars heralds the opening of launch windows to the red planet. During the late 1800’s, some astronomers observed Mars during opposition in the hope of discovering signs of intelligent life.
Before the age of space exploration, astronomers especially anticipated what is referred to as perihelic oppositions of Mars. During these oppositions, which occur every 15 or 17 years, Mars is close to perihelion. That is when Mars is closest to the Sun in its orbit (in Greek, peri means close and helion is the Sun). And if Mars is at its closest point to the Sun, it is also at its closest point to the Earth if this occurs during opposition. During the 19th century, the perihelic opposition of Mars in 1877 and 1892 would alter our perception of Earth’s closest neighbor for decades to come.
In 1877, at the Brera Observatory in Milan, Giovanni Schiaparelli was mapping the surface of Mars with an 8.6 inch refractor. In drawing his maps, Schiaparelli employed the same terminology used for the Moon. Dark areas were referred to as seas and lighter areas as land. This was not meant to be taken literally. For example, the Sea of Tranquility, where Apollo 11 landed, is not actually a sea but a dark area (or mare) of basaltic rock. More important than these distinctions, were the dark lines on Mars that Schiaparelli drew on his maps referred to as canali.
Schiaparalli intended the word canali to be interpreted as channel. However, businessman turned astronomer enthusiast Percival Lowell interpreted canali as canal. This preconception influenced Lowell as he made observations of Mars during the next perihelic opposition in 1892. In what is referred today as confirmation bias, Lowell proceeded to take his observations as evidence of an advanced civilization on Mars that built irrigation canals from the polar regions to transport water to the arid mid-latitudes. Lowell would continue to advance his case until he passed away in 1916. While this theory did not pan out, it did inspire quite a bit of science fiction (good and bad) during the following half century.
Lowell did go on to build the Lowell Observatory, where Pluto was discovered. It is still active with a recently built 4.3 meter telescope. Located in Flagstaff, the observatory offers tours for the public.
In all fairness to Lowell, he was not the only one to postulate that life might exist on Mars. The article below from August 7, 1892, presents a proposal from the Kew Observatory in London to attempt to communicate with beings on Mars via a light beam projected from Earth to Mars. This is a prototype for modern SETI projects.
With rovers on the surface and satellites in orbit around Mars, we no longer rely on opposition events for close up views of the red planet. However, we do rely on opposition to open up launch windows to Mars. As a Mars opposition occurs every 26 months, if a mission is unable to launch due to technical or budgetary issues, it has to wait another two years for the next launch window to open. The video below shows the trajectory of the Mars Science Laboratory which landed the rover Curiosity on the surface.
On August 27, 2003, another perihelic opposition put Mars closer to Earth than at anytime during the past 60,000 years. Actually, the differences in perihelic oppositions are difficult to discern with the naked eye. Nonetheless, the event launched an annual internet meme every August stating that Mars will appear as large as a full Moon. Even in 2003, this did not come close to happening. For Mars to appear as large as the full Moon, it would have to be located about a half million, rather than 34 million, miles from Earth. If Mars was to appear as large as a full Moon, something much more bizarre than opposition would have to be happening. So come August, if you see that meme on social media, it is safe to ignore it.
However, I do not recommend missing the opposition of Mars on December 8th this year. Just have realistic expectations of what to see. If the skies are cloudy on that date, worry not, Mars will be very bright in the skies for weeks to follow. As Mars approaches opposition, you will be able to discern its reddish hue with the naked eye. The next perihelic opposition of Mars will be on September 15, 2035.
During the 2020 launch window, the Perseverance mission began its voyage to Mars. The mission, just as in the 1892 opposition, will explore for signs of life on Mars. This time, rather than searching for an advanced civilization, the mission will seek out signs of microbial life during Mars ancient history when water flowed on the surface. While the search for intelligent life has moved beyond the Solar System, the missions launched today will afford us a view of Mars that Giovanni Schiaparelli and Percival Lowell could only dream about.
*Image on top of post is Percival Lowell at the observatory he founded in Flagstaff, Arizona in 1912. Credit: Mary Evans Picture Library.