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Mars – The red planet!
Image of Mars captured by the NASA/ESA Hubble Space Telescope .
Mars
Mars is the fourth planet in the solar system and the last of the rocky planets. With about half the diameter of our planet, it is the planet most visited by space probes. The scientific community's great interest in Mars is due to the possibility that this planet may have had conditions in the past to support water in a liquid state and perhaps even life.
Mars is rocky and has an atmosphere rich in carbon dioxide. Its surface has some craters that geological processes have not yet erased, the largest canyon in the solar system, Valles Marineris, which would be large enough to cross the United States of America, and the largest volcano in the solar system, Olympus Mons, which has a base that is 500 km long and 27 km high, which is three times the height of Mount Everest.
Every year, given the movement of the planets around the Sun, there is a day when Mars, Earth and the Sun are aligned. This day is called the Opposition of Mars and marks the day when Earth surpasses Mars in the orbital plane. Because of this, it is the day of the year when the planets are closest, the distance separating them is just over 96 million kilometres. The Opposition of Mars will be on 16 January 2025, and the planets will only meet again on 19 February 2027.
Discovery and observations of Mars
Mars stands out very well in the sky, not because it has a red glow or because it is quite bright from time to time, but because from night to night the planet moves just under 1º. It seems like little, but over the course of 30 days, Mars almost travels across an entire constellation.
Mars also exhibits a characteristic that can be observed over two years of continuous observation. The ancients began by observing a pale pink object that was only visible early in the morning, just before dawn. This object moved relative to the stars, becoming brighter over the following year and rising earlier and earlier. Then it would abruptly stop and reverse its direction. At its brightest, it was an intense red color, visible all night, and was the third-brightest object in the night sky. After moving in the "wrong" direction for about 70 days, it would stop and reverse direction again. Gradually, it became darker, barely visible in the night sky, and set earlier and earlier. After another year, it would return to being a pale pink object, this time visible only just after sunset. Shortly after this, it was no longer visible in the sky. It remained hidden for about a hundred days until the cycle repeated itself again. Each cycle lasted a little over two years.
The oldest records of Mars come from the Egyptians, from 1570 BC to 1293 BC, where the star is referred to as "Horus of the Horizon." The Greeks named the star Ares, their god of war. In 356 or 357 BC, Aristotle observed Mars passing behind the Moon, leading him to conclude that Mars was farther away than the Moon. The Romans changed the name of the star to Mars, in honor of their god of war.
Galileo Galilei
In 1609 Galileo Galilei was the first to observe Mars through a telescope. Although he could not make out any detail on the planet's surface, Galileo noticed that Mars was not perfectly round. In a letter to a colleague Galileo described his observation of the planet.
Galileo's conclusion is due to the fact that the observation was made with Mars in phase. Just as the Moon has phases (first quarter, last quarter, etc.), the other planets in the solar system also exhibit the same behavior. In the case of the Moon, the surface that is illuminated by the Sun does not always correspond to the side that is observed from Earth. Mars is “full” on two occasions: The first is when it is on the opposite side of the Sun (Earth – Sun – Mars), however, in this situation Mars is in the sky at the same time as the Sun, which makes it impossible to observe it. The second is when it is on the opposite side of the Earth (Mars – Earth – Sun), which ends up corresponding to the day of Mars' opposition. Therefore, Mars fills as it approaches opposition and wanes as it moves away from opposition.
Christiaan Huygens, using a telescope with more refined optics, was able to observe some details of the planet's surface in 1659, the drawings of which still survive today.
Christiaan Huygens
Three drawings made by Huygens in 1659. The first (A) was made on November 28th at 7:00 p.m., the second (B) was made on the same day but at 9:30 p.m. The last (C) was made three days later on December 1st at 6:30 p.m.
Huygens called the lighter regions “deserts” and the darker regions “seas”. This designation is still used today.
Phobos is the largest moon of Mars with a diameter of just over 22km that orbits the planet 3 times a day. Unlike our Moon, Phobos is not spherical, it almost looks like a potato.
Phobos
Image of the moon Phobos captured by the NASA orbiter – Mars Reconnaissance Orbiter (PIA 10368).
Although it currently orbits Mars, Phobos approaches Mars by about 1.8 meters every 100 years, which could lead to a collision with the planet, or to fragmenting into a ring system around the planet.
Deimos is the smallest moon, approximately 12 km in diameter, which orbits the planet every 30 hours.
Like Phobos, Deimos is also not a sphere, sharing the appearance of a potato.
Deimos
Image of the moon Deimos captured by the NASA orbital probe – Mars Reconnaissance Orbiter (PIA 11826).
Perihelical Opposition is the day when opposition occurs near perihelion, the point in the orbit where the planet is closest to the Sun. With Mars, this happens every 15 to 17 years. The next Perihelical Opposition will be in September 2035, not long from now.
The Perihelical Opposition
Comparison of the telescope image of Mars at two different times. Image on the left (A) is during opposition in January 2025. Image on the right (B) is during perihelical opposition in September 2035.
Mars at the Alqueva Lake Observatory
Mars is a fascinating object to observe through a telescope, but the planet's proximity to Earth creates expectations that are difficult to manage, because, despite being close to Earth, Mars is just over half the size of Earth.
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It is possible to distinguish the regions of the polar ice caps.
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It is possible to distinguish deserts from seas.
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The moons Phobos and Deimos are visible when Mars is at or near opposition.
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Atmospheric stability together with the planet's height in the sky are the two crucial factors in obtaining a clear image of Mars through a telescope.
When is the best time to see Mars?
Mars begins to appear in early December 2024 and will remain visible in the
Astronomical Observation until the end of July 2025.
To get a good image through a telescope, it is crucial to observe the planet when it is high in the sky. The higher the better. The colder the better!
The best time to observe Mars at the Alqueva Lake Observatory during the Astronomical Observation sessions is throughout the months of January and February, with special emphasis on the month of January. The Opposition of Mars will be on January 16, the day when Mars and Earth are just over 96 million km apart.
Considering the size of the planet, to get a good image of Mars, every bit of help is important, making January a particularly good month to observe the planet and all the details of its surface.
After January and throughout the rest of 2025, as Earth continues its usual journey around the Sun, Mars lags behind, its brightness in the sky and size in the telescope diminishing.
Unlike the giant planets that are always clearly visible through the telescope, Mars has a specific period that guarantees the best experience. January 2025!!!
Mars will only reappear in Astronomical Observation in January 2027.
You can consult our Astronomical Observation page for more information.