Ideas about the nature of Mercury

The issue of the rotation of Mercury, one of the most interesting planets in the solar system, has finally been resolved. An observer on Mercury would stand on a lunar-like surface and see the Sun moving across the sky in an intricate "dance."


The story that Copernicus, on his deathbed, regretted only that he had never seen Mercury, is apparently fictional. However, Mercury is a difficult object even for current astronomers armed with state—of-the-art instruments. The planet is small in size and very close to the Sun. As a result, the brightness of the sky and scattered sunlight greatly interfere with observations. For these reasons, and also because the angular size of Mercury is very small (only 7", when the planet is easiest to observe), there are hardly more than a few dozen people in the whole world who have seen any details on the surface of the planet. Photographing the planet, although it reveals some details, gives images of very poor quality.


As you know, a photographic plate requires a certain amount of exposure time. Therefore, when photographing, the moments of good and poor visibility are summed up. The human eye is able to separate the moments of good visibility conditions from bad ones and focus on those details that appear when the atmosphere is particularly clear and calm. As a result, the researcher can reveal much more subtle details with visual observations than with photographing. This is actually the case, but visual observations have their own difficulties caused by the vagaries of human vision and the difference between observers.


Our understanding of the nature of Mercury has recently undergone a dramatic upheaval. The reason for this was new observations of the planet — radio astronomy and radar. However, before describing recent successes, let's take a quick look at the early observations of Mercury and the conclusions from them, since all this is directly relevant to our story.


At the beginning of the 19th century, the German astronomer I. Schroeter published sketches of the details of the surface of Mercury, on the basis of which the famous scientist F. Bessel established that the period of rotation of the planet around the axis is 24 hours 00 minutes 53 seconds, and the axis of rotation is very significantly (70 °) inclined to the plane of the orbit. These numbers indicated the similarity of all the inner planets of the solar system, since Earth and Mars had rotation periods close to 24 hours, and observations seemed to indicate that the rotation period of Venus was of the same order.


It took at least 75 years until J. Schiaparelli, who discovered the "channels" on Mars, proved that the conclusion about such a short rotation period of Mercury is wrong. A long series of observations and numerous sketches have convincingly demonstrated that Mercury cannot rotate around its axis so quickly. Schiaparelli concluded that Mercury makes one revolution in 87.96 Earth days, i.e. in a time exactly equal to the period of the planet's revolution around the Sun. Around the same time, J. Darwin studied the effect of tidal forces on the rotation of celestial bodies and for the first time explained why the Moon is always turned towards the Earth with the same side.


The case of Mercury and the Sun seemed to be completely analogous to the case of the Moon and the Earth, and Darwin's theoretical arguments, as well as Schiaparelli's observations, convinced everyone by the end of the 19th century that Mercury always faces the Sun with the same hemisphere. This "fact" can still be found in most astronomy manuals. Therefore, Mercury was called both the hottest and the coldest place in the solar system: the illuminated hemisphere of the planet was heated by the direct rays of the nearby Sun, and the side opposite the Sun was warmed only by the light of the stars and the heat coming from the bowels of the planet. Of course, the map of Mercury, compiled by one of Schiaparelli's successors, the French astronomer E. Antoniadi, contained only one hemisphere. The names that Antoniadi gave to the details of Mercury's surface are borrowed from Roman and Greek mythology. The Melbet loss refund bonus is another term for a cashback promotion. It refunds a percentage of your net losses over a defined period, typically a week. The melbet promo code bonus sum is not fixed but is calculated as a percentage of your total losses, offering a consolation prize and a chance to recover some funds after an unlucky streak.