Monday 24 June 2024

Wonder-filled Solar System – Part III


The Sun's Planets Also Exhibit Various Astonishing Phenomena - take Mercury, the closest planet to the Sun, for instance.

Wonders in Mercury

Mercury Experiences Two Sunrises in a Day

Mercury has an elliptical orbit, resembling a duck egg, meaning its distance from the Sun is not constant. Mercury revolves around the Sun at an incredibly high speed, the fastest among all the planets in the solar system. When Mercury gets very close to the Sun, its speed increases, and when it moves farther away, its speed decreases slightly. Additionally, the Sun's movement in Mercury's sky is somewhat influenced by Mercury's slow rotation on its axis. Mercury rotates very slowly on its axis, at a speed of just 10.9 kilometres per hour, a speed at which an ordinary person could run. Compared to any other planet, Mercury's rotation is so slow that it takes 59 Earth days to complete one full rotation on its axis. As a result, the Sun itself appears to slow down and even disappear for a while. When Mercury's orbital speed decreases, its axial rotation causes the Sun to rise again. Therefore, Mercury experiences a second sunrise on the same day.

In the image above, the time it takes for Mercury to complete one rotation on its axis is divided into sixteen equal parts. When Mercury is at position 1, at a distance far from the Sun, the observer sees the sunrise. As Mercury slowly rotates on its axis but moves very quickly in its orbit, by the time it reaches position 12, the Sun becomes temporarily hidden. However, because Mercury is very close to the Sun at that time and thus orbiting very quickly, the Sun reappears when Mercury reaches position 16. This phenomenon is why Mercury experiences two sunrises and two sunsets in one day.


One Day on Mercury Equals Two Years

The length of a year for planets in the solar system is based on the time it takes to complete one orbit around the Sun. Where Earth takes 365 days to orbit the Sun, Mercury takes only 88 days. Thus, one year on Mercury is equivalent to 88 Earth days.

Earth completes one rotation on its axis in 24 hours, making one day 24 hours long. We measure a day on Earth from one sunrise to the next at a specific location. Similarly, a day on Mercury would be the time it takes from one sunrise to the next at a specific point on the planet.

Mercury rotates very slowly on its axis, taking 59 days to complete one rotation. However, because it orbits the Sun quickly, Mercury covers nearly 2/3 of its orbit in those 59 days. Therefore, for an observer standing at a specific spot on Mercury, after seeing the sunrise, they would have to wait until Mercury completes two orbits around the Sun to see the next sunrise from the same location. This period equates to two Mercury years. Let’s explore how this happens.

In the above illustration, let's say an observer on Mercury at position 1 sees the sunrise. Then, at position 2, it is mid-morning; at position 3, it is noon; at position 4, it is afternoon. By the time it reaches position 5, Mercury has completed one orbit around the Sun, which means one Mercury year has passed. Yet, only then does the sunset occur. Moving on, at position 6, it is evening; at position 7, it is midnight; at position 8, it is dawn, and then back to position 1 for a new sunrise. By this time, Mercury has completed another orbit around the Sun, meaning it has taken 176 days, or two Mercury years, to return from position 1 to position 1 again. In the time it takes Mercury to orbit the Sun twice, it rotates three times on its axis. This way, one day on Mercury is equal to two years.


Mercury is Shrinking

Mercury, the closest planet to the Sun, receives the most sunlight. Despite this, the interior of Mercury is gradually cooling down, causing the planet to shrink. Scientists have found evidence that Mercury has been shrinking since its formation. Already the smallest planet in the solar system, it is becoming even smaller. But why? This mystery remains unsolved. Some researchers attempt to prove that Mercury is still geologically active. Despite continuously rotating for 4.5 billion years, there are still some geological faults and empty spaces inside Mercury, which are slowly being filled, causing the planet's interior to contract. However, there is no definitive proof of this explanation yet.

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