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Saturn
Image of Saturn with its ring system captured by NASA's Cassini Orbiter (PIA 06193).
Saturn – The Lord of the Rings!
Saturn is the second largest planet in the solar system. Known for the gigantic ring system that surrounds it, it is perhaps the most spectacular planet when observed through a telescope. Despite its large size, it is considerably less massive than the planet Jupiter. For this reason, its density is so low that if there were a gigantic ocean of water in which we could submerge Saturn, it would float!
Like Jupiter, Saturn is also a gaseous planet, composed mainly of hydrogen and helium. The rings that surround it are composed mainly of ice and rocks, the diameter of the ring system is around 282,000 km and they have an average thickness of 10 meters. Saturn is, like Jupiter, a small solar system, with its 146 known moons. Of these moons, Titan is the largest and has an atmosphere rich in nitrogen and on its surface lakes of liquid methane flow and form.
Every year, given the movement of the planets around the Sun, there is a day when Saturn, Earth and the Sun are aligned. This day is called the Opposition of Saturn and marks the day when Earth and Saturn are at their closest point of approach, the distance separating them is just over 1,295 million kilometres. The day of the Opposition of Saturn is 8 September 2024. In 2025 it will be on 21 September.
Discovery and observations of Saturn
Unlike Jupiter, Saturn does not stand out much in the sky, being more diffuse than some of the brighter stars. Despite this, its movement across the celestial sphere is relatively easy to observe from month to month. As the planets orbit the Sun, their movement causes the planet to move across the sky from day to day. The closer the planet is to Earth, the faster it moves across the celestial sphere. Saturn is close enough and bright enough to observe this movement directly. Because of this, several ancient civilizations such as the Babylonians, Egyptians and Greeks have some record of Saturn either in their monuments or manuscripts. For the Greeks, the star in the sky represented Kronos, the father of the god Zeus. For the Romans, Saturn was the father of the god Jupiter.
Galileo Galilei and the arms of Saturn
On July 25, 1610, Galileo Galilei was the first person on Earth to observe Saturn through a telescope. In a letter written to the Grand Duke of Tuscany, Galileo describes his observation of Saturn.
In his first observation of Saturn, Galileo interpreted the image of the rings as three bodies “stuck” side by side with the largest in the center. This observation was contested by Father Cristovão Clávio, a figure for whom Galileo had great admiration and respect and who, curiously, was a student at the University of Coimbra. In a letter to Galileo dated December 17, 1610, he described that Saturn appeared oblong and that he had not observed what Galileo had described as three bodies side by side. Galileo and Clávio met to discuss these disparate observations.
Illustrations of Saturn by Galileo (1610 above and 1616 below).
Two years later, in 1612, Galileo observed Saturn again and, to his surprise, did not find the three bodies, only one, which confused him. This observation was due to the fact that at this time the rings were in profile and therefore were not visible through his small telescope (this phenomenon is explained below). In 1614 and 1616, Galileo observed Saturn again and, with the rings visible again, Galileo changed his interpretations again, which caused him great confusion and led him to conclude that Saturn had “arms”.
It was Christiaan Hyugens who solved the mystery of Saturn's "arms." Thanks to telescopes with more precise and refined optics, Hyugens deduced that the "arms" observed by Galileo were in fact a system of rings around Saturn.
A few years after Hyugens's discoveries, the Franco-Italian astronomer Jean-Dominique Cassini discovered a small gap in Saturn's rings that divided the structure into two distinct parts. This gap became known as the "Cassini Division".
Saturn's rings
The ring system has a diameter of approximately 180,000 km. The rings are composed mostly of ice, ranging from small grains to gigantic blocks. Data from the Cassini spacecraft indicate that the rings may have formed between 10 and 100 million years ago. The rings have been losing mass over time and it is possible that in 300 million years they will no longer exist (enjoy them while you can).
Panoramic image of Saturn's rings showing hundreds of divisions along the rings. Captured by NASA's Cassini Orbiter (PIA 07872).
Uma das hipóteses, elaborada por Édouard Roche, para explicar a formação dos anéis expõe que há muito tempo atrás uma das luas de Saturno aproximou-se demasiado do planeta e ultrapassou o que se designa como o Limite de Roche, ao passar esta fronteira a lua fragmentou-se e os seus detritos compoem hoje o material encontrado na região interna dos anéis. Um artigo publicado em setembro de 2022 analisou a dinâmica dos anéis e suporta a hipotese apresentada por Roche no século XIX.
What is the “Roche Limit”?
Consider our Moon. The Earth exerts a greater force on the surface of the Moon that is facing Earth than on the opposite side. Why? Let us remember that gravitational force depends on distance, in fact, it is inversely proportional to the square of the distance. The further away you are from Earth, the smaller the Earth's force on you. Now, the side of the Moon that is facing us is closer than the side that is facing space. The distance that separates them is the radius of the Moon itself. If the Moon approaches the Earth and exceeds the Roche Limit, that is, the Earth-Moon distance decreases to less than 19,900 km, the gravitational forces of the Earth exerted on the Moon overcome the gravitational cohesion of the Moon and it disintegrates. The distance of 19,900 km is specific to the Earth-Moon system; other systems will have other values. The Roche Limit depends on the radius of the first body (in this case, the Earth) and the ratio of the densities of the bodies (Earth-Moon).
It is important to highlight that the origin and evolution of rings is still a subject of intense study and debate today.
The tilt of Saturn's rings
Saturn's rings are aligned with the planet's equatorial plane, but Saturn's axis of rotation is tilted at 27º. Because of this, every 13-15 years our planet intersects the orbital plane of the rings, which means that, at certain times, observations of the planet made from Earth can capture the rings edge-on. The next passage is scheduled for March 23, 2025. Consequently, throughout 2025, Saturn's rings will be edge-on, making them almost impossible to observe through small telescopes. In 2026 we will begin to see the rings as they should be, and in 2031, 2032 and 2033 we will once again have an excellent view of the ring system and the planet's South Pole.
It was this factor that caused great confusion in Galileo's observations. In 1610 Saturn's rings were clearly visible, but in 1612, when Galileo observed Saturn again, the rings were seen edge-on because the Earth's orbit was intersecting Saturn's equatorial plane. In 1614 and 1616 the rings returned and Galileo, as is normal given the knowledge of the time, had no explanation.
Saturn's atmosphere
Saturn's atmosphere is composed mainly of hydrogen (75%) and helium (25%). Winds in the equatorial region reach speeds of around 1800 km/h. Saturn's beige hue is due to ammonia clouds at a temperature of approximately -250º. Despite this, what makes Saturn's atmosphere so interesting is what happens at the planet's north pole!
Jupiter has a spot but Saturn has a hexagon!
Although it is not visible through a telescope, this is undoubtedly one of the most fascinating structures on Saturn.
Image of the large hexagon at Saturn's north pole, captured by NASA's Cassini Orbiter (PIA 21327).
This is a hexagonal cloud structure with a width of 30,000 km. At the center is a hurricane, centered at the pole, which, together with the hexagon, rotates in an anticlockwise direction. The hexagonal structure is formed due to the properties of the gas and the speeds of the jet streams in the region of the planet's north pole. Although we can replicate similar structures in the laboratory, one of the main mysteries is the mechanism that sustains this cloud formation over such a long period of time. Hurricanes on Earth tend to last for weeks, the hexagon has decades of recorded history and its origin may date back centuries.
Saturn at the Alqueva Lake Observatory
Image of Saturn captured at the Alqueva Lake Observatory in September 2023 through the telescopes used in the Astronomical Observation sessions.
What can I see at the Alqueva Lake Observatory?
Saturn, together with Jupiter, is undoubtedly one of the best objects in the sky to observe through a telescope. If atmospheric conditions allow, we can observe the surface of Saturn in good detail, while at the same time observing the ring system around the planet with excellent clarity. The better the conditions, the sharper the “Cassini Division”. In total, we can see:
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Some but not much detail in the clouds of Saturn's atmosphere
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Some of Saturn's largest moons, notably Titan.
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The wonderful ring system!!!!
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With “decent” atmospheric conditions, the Cassini division is easily observed.
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With fantastic atmospheric conditions, the Encke division can be observed.
Atmospheric stability, together with the planet's height in the sky, are the two crucial factors for obtaining a clear image of Saturn. The rings are always visible regardless of these factors; the detail observable within the rings depends more on the planet's position and the stability of the atmosphere.
When is the best time to see Saturn?
Saturn begins to appear at the end of July 2024 and will remain in the sky and visible in Astronomical Observation sessions until the end of January 2025.
To obtain a good image through a telescope, it is crucial to observe the planet when it is high in the sky. The higher the better and the colder the better.
The best time to observe Saturn at the Alqueva Lake Observatory during the Astronomical Observation sessions is during the months of October, November and December. In October we recommend the second session of the night while in November and December we recommend the first session.
In 2025, although it will still be visible in January during the first session of the night, Saturn will be missing from the night sky until it reappears at the beginning of August during the Astronomical Observation sessions.
You can consult our Astronomical Observation page for more information.