The Universe Seen From Mercury, Venus And Moon
Alien Skies: The Universe seen by Mercury, Venus and Moon
In this journey we will try to put together science and imagination, visiting skies completely alien to our daily earthly experiences.
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Our tour obviously starts from Mercury, the smallest and closest planet to the Sun. Although Mercury has a very thin atmosphere, from its surface a hypothetical Earth explorer would see the stars even during the day, in a black sky dominated by a solar disk up to three times the size observable from Earth.
But these are details that our hypothetical traveler could appreciate only having a really resistant equipment: first of all an adequate protection for eyes and skin, since the proximity of the Sun and the lack of the atmosphere would make the bombardment of ultraviolet rays simply lethal; and then a bombproof thermal insulation, since on the small planet there are temperature changes of more than 600 degrees centigrade (with absolute minimums of -170° and maximums of +430°.
the only chance to escape the heat even during the daytime period is to reach the poles, where the sun rays arrive oblique and create twilight regions at more “human” temperatures.
Solved these “small” practical problems, our visitor could enjoy a very unusual spectacle, even if very slow in its development … that is, the Sun rises in the east and advances very slowly in the sky (about three times slower than the sidereal motion of the stars) increasing more and more the apparent diameter; near the meridian (South direction) the motion slows down until it stops completely; then the Sun produces a double inversion and resumes the path to dive towards the horizon, which will reach 88 days later, returning to its original size!
But what is special about Mercury to make such a strangeness possible? Only two things: a very elliptical orbit and the Sun practically two steps away. In practice, the strong gravitational attraction exerted by the Sun has blocked the rotation of Mercury on its axis in a resonance ratio of 3:2 with the revolution motion: in time, that is, in which the planet makes three complete turns on the same, it also travels two whole orbits around the Sun. This ratio between rotation and orbit period based on small integers, called resonance, affects the duration of the Mercurian day, making it very different from Earth day.
Here on Earth, the sidereal day, that is, the time our planet takes to make an entire turn on itself to the fixed stars, is 23 hours 56 minutes and 4 seconds; and is almost equal to the solar day, which is the time of rotation to the Sun, of about 24 hours. Both are much shorter than the Earth’s year, which lasts 365 days and a few hours.
On Mercury, however, because of the rotation in resonance with the orbit, the sidereal day lasts just under 59 Earth days, equal to two-thirds of the year, which is about 88 days. And the solar day, that is the time that the Sun takes to go back on the same meridian of Mercury, lasts 176 Earth days (two Mercurian years): it is this long interval that to an inhabitant of the planet would appear as the real day.
The consequence of this is that the apparent motion of the Sun in the sky of Mercury is affected much more than in the terrestrial skies by the competition between the rotation motion of the planet and that of revolution. The first makes the Sun move, as on Earth, from east to west; the second, on the contrary, makes it move from west to east. In general, even on Mercury, it is the motion of rotation that prevails, so for most of the Mercurian day the Sun proceeds normally in the sky from east to west. However, when it approaches the perihelion, due to the proximity to the Sun and the high orbital eccentricity, the planet obeying Kepler’s second law (that of “equal areas at equal times”) gradually increases its speed, reaching 56.6 km/s: a real sprint, compared to 38.7 km/s of the orbital speed at the aphelion.
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