MERCURY
We have picked up general data of this planet in encyclopedias, magazines and Internet, and for indication of the teacher we have carried out deductions using the almanacs in relation to some topics that doesn't appear clear in these publications: the duration of the day of Mercury, as well as the movement of the stars in their sky and we deduced very curious circumstances. We have built a model with motors and engagements to visualize the movements of the planet and to leave clear all the above-mentioned, we have observed and photographed Mercury o the occasion of the great planetary conjunction of the month of May.
To compare it with a similar star and another different one we have chosen Venus and the Moon, although... which is it the similar one and the different one?
Mercury received this name of the Romans since for the messenger of winged feet of the gods, it seemed to move quicker than any other planet.
It is the nearest planet to the sun, and the second smaller the solar system. Its diameter is 40 %smaller than the Earth and 40 %bigger than the Moon. It is even smaller than the moon of Jupiter, Ganimedes, or the moon of Saturn, Titan. Their aspect is very similar to the Moon, like you can see at these images:
Anyway contrary to the Moon their surface is crossed by big fractures, maybe coming from the period of contraction that experienced in its first times, when the planet cooled down.
The general data are the following ones:
Diameter: 4880 km
Minimum distance to Sun: 45.9 million km
Maxim distance: 69.7 million km
Eccentricity of the orbit: 0.206
Duration of the translation: 87.97 terrestrial days
Duration of the rotation 58.64 " "
Duration of the day: 175.94 " " (in spite of erroneous data or not well interpreted in many places)
Inclination of the orbit: 7º
Inclination of the axis: 0º
Temperature: of -175 up to 415 centigrade grades
Satellites: None
Until beginnings of the 70´s were known very little of its structure, because the observations from Earth didn't show any detail of its surface. In 1974 a spacial probe interplanetary American, the Mariner 10, carried out three passages for its proximities, transmitting us the images of a world profoundly cratered and without atmosphere, that seems a bigger twin of the Moon. Thanks to the 7.000 images provided by the Mariner 10, today we know around 40% of the surface of Mercury. This, together with the informations that the sensors of the spacial probe have revealed on the geophysical characteristics of the planet, allows to have an idea, although it doesn't complete, enough approximate of this body, exposed more than the other ones to the rigors of the Sun.
In passed years the correct value of the day of Mercury was unknown. The observations carried out toward final of the XIX century by Schiaparelli equaled it to the period of revolution, reason why it seemed that the planet always directed toward the Sun the same hemisphere. But in 1962 the radio emissions coming from Mercury determined that the dark side was too hot as so that it has never received the solar rays, and later observations by radar have led to the determination of 58,65 days for the period of rotation around the own axis, fact confirmed fully by the observations of the spacial probe Mariner 10. A phenomenon of resonance taken place exactly by forces of tide of two translations each three rotations takes place.
The dimensions of Mercury are approximately 2/3 inferior to those of Mars, for what its volume would be of less than a third; but their mass of 3,30. 1023 kg. is almost half of that of the red planet, what indicates that its density is very high: the measures indicate a density totally similar to the terrestrial one, 5,42 g/cm3, making suppose a high percentage of iron in the interior composition of the planet. As consequence of the high mass of the planet the gravity in its surface is comparable with that of Mars: around the 50 per cent of the terrestrial one.
Its extreme proximity to the sun and the fact that, in practice, it hasn´t an atmosphere (it has been measured one near the surface and it is a billion of times less than the terrestrial one that contains sodium and potassium whose atoms, apparently, proceed of the skin of the planet) they make to experiment to this infernal world the highest existent thermal variations in a planet. Indeed, it passes from a maximum temperature in the exposed areas to the sun of 415 C (enough to melt the lead) to a minimum of -175 grades in the areas in the shade. Under these conditions the scientists exclude that the planet can harbor any form of life.
Mercury is a planet very difficult of observing from the Earth, although to the maximum of its splendor it reaches the magnitude -1. Their apparent angular distance of the Sun doesn't overcome the 28 grades, what means that the planet, for the terrestrial observer, is always inmerso in the reverberation of the solar light and it is only possible to see it on the bottom of the clear sky: or in the twilight that precedes to the daybreak or in the one after to the sunset in some few days a year in next dates to their maximum elongations. Of the five well-known planets from the antiquity and observables without help optics is the most difficult of seeing, and especially from the North hemisphere. As the other interior planet, Venus, Mercury also presents the phenomenon of the phases. When it is perfectly aligned between the Sun and the Earth, what happens rarely, is possible to see it pass as a black point on the disk of the Sun. The last passes of Mercury happened on the twelve of November of 1986 and on the fourteen of Novembre of 1999.
The surface of Mercury shows a power reflector or Albedo very little: the 7 per cent, that is very similar to the Moon´s one. Its fundamental characteristic is represented by the craters: there are of all the dimensions, from those that have a diameter from some meters to the big fissures, originated by the impact of big meteorites that reach diameters of hundreds of km. The biggest of these fissures is called Caloris Planitia, and it has a diameter of 1.300 km. Their structure reminds those of the Imbrium Sea or of the Oriental Sea on the Moon. Planitia is surrounded by a series of mountainous chains prepared concentrically, probably they arisen soon after the terrible shock wave that formed the own crater; in the center, the basin-crater is characterized by a flat surface due to the lava that covered the area after having produced the impact.
As the Moon, Mercury presents a morphological differentiation among the two hemispheres: one appears intensely covered with lands or highlands cratered; the other one for flat surfaces similars to the moon seas, with a percentage of craters very inferior. The craters of Mercury, however, present some structural differences with those of the Moon: they are less deep and in those of more size the area of the secondary products of the impact spread around, is much more reduced. Both peculiarities are due to the biggest force gravitational of Mercury. Other typical forms of the surface of Mercury, are the calls banks that extend for hundreds of kms. cutting the structures pre-existings: probably they are faults caused by phenomena of compression, as consequence of the cooling and the contraction of the surface to planetary scale. Also Mercury like the Moon, would present a dust layer that recovers the whole planet more or less homogeneously with an average thickness of 20 meters.
The evolutionary history of Mercury isn´t presented different from the Moon one. After the formation of the planet, the heat developed by the intense meteoric bombing melted the surface. Then was formed the region of the lands or highlands. Successive and imposing sporadic impacts gave place to the formation of the big basins or craters, characterized for covered with basaltic magma. The geophysical observations of the Mariner 10 suggest that the planet should possess a great nickel-iron nucleus, surrounded by a surface of silicates. The central nucleus of the planet could be in melting state. The Mariner 10 have also measured a weak magnetic field whose intensity is hardly the one per cent of the Earth. According to some specialists, it would be produced by the permanent magnetization of the rocks that they are on the planet.
All the above-mentioned is from the encyclopedia Encarta, several pages of Internet, and several articles and data that the professor has provided us in class, selecting, ordering and comparing the different sources. The following informations are our own with the guide, orientation and help of the teacher.
It is curious that in some books appear very strange data of the duration of the day of Mercury. It is clear that its year (a translation around the sun) lasts 88 days, but in more than a place we have found that the day lasts 58,7 days, which is not certain; and also it appears written in other places that it is the duration of the rotation, but that doesn't have any relation with the duration of the day. To leave doubts, following the indications of the teacher we have looked for in the Astronomical
Efemerides of San Fernando's Observatory the evolution of the point subsolar, that is to say the place of Mercury that has the Sun at the zenith, and we have found two important things: the first one is that one day in Mercury lasts 176 of our days, because it is the time that has to pass since the Sun is in the vertical of a certain point, until it is there again.
This says us that in this planet very curious circumstances take place: Duration of the translation 88 days; rotation 58,7 days and sun day 176 days. So that one day lasts exactly twice more that one year, and the triple of the rotation.
This so strange can be explained with a graph as this, where the positions of Mercury appear every 29,33 days = half rotation = 1/3 of their year, and following the position of the point where that the arrow arises.
1 - Noon in this point
2 - Final of the afternoon
3 - Beginning of the night
4 - Midnight. One year has passed, but only half-day.
5 - End of the night.
6 - Beginning of the day.
7 - Noon. It has been completed one day and two years.
Or better with a device with the help of engagements and a lamp that simulates the movements of Mercury in their fair scale and the positions and orientations in relation to the Sun, and it allows to see that they have to be completed two passages in order to complete one day at one point of Mercury.
Although at the beginning, we think that the right would be to place two engagements with relationship of teeth 2:3 (the first one fixed in the translation axis and gearing with the second that transmits with a rubber the rotation turn), to maintain the proportions of the rotation and the translation, we saw that the convenient thing was 2:1 because with each translation already takes place a rotation without putting engagements (like in the case of synchronous rotation of the Moon), and a turn more was needed each two translations.
Of course it is not on scale, and we have choosen an excessively big sphere for Mercury for better appreciating the rotation.
Here we can see the first astronaut that will arrive in Mercury, it will check with his shade and that of the flag (European, of course) that is midday, and that they will pass 176 terrestrial days (2 years in Mercury) for being midday again and the day it has been completed.
Using the pattern, we can see that they must pass two years to happen to complet one day.
The other curious fact that we have seen at the Efemerides is that the Sun is moving very slowly at the sky of Mercury because the subsolar point doesn´t changes too much, but in a certain moment it goes back and after continues going on. This means that looking from the surface of Mercury the Sun is moving slowly (like here from Easts to West), but in a certain moment it stops and goes backwards. The teacher tells us that this is consequence of the second law of Keppler, when Mercury is in the perihelion goes much more quickly, and the effect of the traslation movement is bigger than that of rotation, and that happens in Mercury because it has an eccentric orbit and a duration similar the translation and the rotation.
That history of the Sun going backwards agrees with something that we have found a long time ago at a magazine, it said that they had been located in Mercury two points hotter than the rest (one in Caloris planitia) they will be those that have the sun at the zenith during more time, stopping, going back and going forward. As every year it goes by the perihelion and in one day it makes it exactly twice, they have to be two points of the opposed equator. We have thought that in the points of the equator that are at 90º of the previous ones it will happen when the Sun is in the horizon, and therefore from there every day there will be two exits and two settings of Sun. To sum up more this thing so strange, we have seen in the efemerides that this retrogradation of the sun it is of a grade in the sky of Mercury, because it is what the point subsolar goes back. We have also calculated for trigonometry the size that you see the Sun from Mercury when it is in the perihelion, and it is of 1.7º (this fact is wrong in some of the pages of INTERNET). This means that the history is this way: We sit down in that concrete point to see the setting of Sun (by the way, it lasts enough), and as soon as it has just set, it begins to show again, backwards, almost in their third two parts, it stops a little, and it leaves again. How strange thing, isn´t it?. Then the teacher tells us that if we have looked properly, it will be this way, and the same thing will happen from there every morning in the daybreak.
Besides we wanted to deduce with all the data that we had, how you can see the sky from Mercury. You can see the Sun bigger than from here because they have it nearer; although it changes its size for the eccentricity of its orbit, from 1.7º in the perihelion, to 1.38º in the afelio. As every day it lasts exactly two years, from each observation place in Mercury the size of the Sun will depend on the place in which is seen in the sky. Also as the axis it is not inclined, the journey regarding the horizon is always same, on the contrary than in the Earth. When it passes above a determinated crater it will have always the minimum size, it always will go backwards at other one, it always will sunset at one determinated, etc.
We have read that in Mercury there is a very slim atmosphere, but it doesn't say if it will be enough to disperse the solar rays during the daylight and to obstruct to see the stars. However we have found an imaginary drawing of a view from Mercury in that it is seen that it is during the daylight, defined shadows appear, but as in the Moon the sky is black and you can see the stars. Two brilliant stars that would be Venus and the Earth appear. According to this, the stars would be seen during the daylight (at least the most brilliants), they would move in an uniform way giving three turns every day, but our Polar star would not be fixed, it would be at a place near the star of the Dragoon Grumiun, 7º from the pole of the ecliptic (because the turn axis is not inclined, but the plane of the orbit is inclined 7º regarding to the ecliptic). The Sun would move on the stars but unpredictably, with variable speed but always more slowly than the stars, spending twice for each zodiacal constellation every day (once it would not be seen to be at night, and it would happen under the horizon). As in one night there is a rotation and a half, from the equator of Mercury in a single night all the stars of the sky will be seen; and even all visible star at the beginning of the night will put on and it leaves before dawn again.
To compare Mercury we have chosen to the Moon like similar star and Venus like different star. Although officially it would be seemed Venus for being a planet and because they are neighbouring, besides they are the uniques that don't have satellites. The similar star is really our Moon, its aspect, surface full with craters, similar size, almost absence of atmosphere what causes in both cases extreme temperatures, black sky and to be able to see the stars by daylight.
On the contrary Venus has a dense atmosphere, you can´t see any star from its surface, uniform but high temperatures, and terrible pressure. It is also bigger, and it rotates the other way around. Although seen from the Earth both they are similar, they have phases and you can see they only in the twilights, if you want to visit it the differences they are total; better to go to Mercury.
To end up, we will mention the observations that we have made of Mercury. They say that it is very difficult to be able to see Mercury; but we have done it without any problem during a heap of evenings of April and May with the other 4 visible planets at first sight in the famous conjunction; concretely from April 19 up to May 13. We have found out that seeing so many days to Mercury has been possible because it has coincided with the maximum elongation. And although in the Efemerides we have seen that there are two elongations bigger this year (in February and September 6 degrees more than that of May) our teacher has explained to us that due to the inclination of the ecliptic in spring this of May was the best in the whole year. We give some pictures that we have obtained of Mercury with Mars, Saturn, Venus and Jupiter, besides another they made some partners in 1999 from Bilbao where appears Mercury, Venus, Saturn and Jupiter.
Obtained pictures the 4-5-2002
Obtained picture the 20-4-02 obtained Picture the 1-3-99
Finally, we believe that the exercise that we have carried out of the calculation of the duration of the day in Mercury starting from the data of the anniversaries, can also be used in other stars, mainly with Venus. Then to compare it with the duration of the rotation (fact that can be found easily), and to think why they are different.
Starting from these and other data, to study the movement of the stars in the skies of other planets. This is our work proposal for other groups.