Did you know # 16: The Sun who wanted to go back to bed

Because of its eccentricity and a period of revolution in resonance 3: 2 with its period of rotation, Mercury sometimes presents singular days: if the sun rises to the East as it is accustomed, it decides some days of stopping in the middle of the sky and finally going back to bed where he came from.

Seeing the Sun travel through our skies is one of those earthly phenomena we are so used to, that we have a little trouble imagining spontaneously that they could be different on other planets. On Earth, the Sun always rises in the East and always goes to the West with Lucky Luke , exceptions made on days too cold during which it takes a long time to get out of bed, or even, prefer to stay there (we speak well obviously here polar days and nights ).

Skies and planets

However, the movement of the Sun in the sky is not an astronomical constant, but depends on our point of view - in particular, the speed of rotation of the planet, and its orbital speed . Until then, everyone knows and already recognizes this fact. That said, we can have a little difficulty to imagine the movement in the skies of the other stars, since we imagine instinctively that it is relatively continuous everywhere - this is not necessarily the case for all the stars of all the skies. Thus, for example, since the Moon always offers us the same face to contemplate, it means that an observer posted on the Moon, would see constantly the Earth at the same place , in the Lunar sky.

So the skies are ultimately as changeable as the planetary features. On Venus, whose rotation is anti-clockwise, the Sun has the habit of rising to the West and lying to the East . Lucky Luke the pistolero probably looks more like the Samurai who draws faster than his shadow . But it is true that a solar day on Venus still lasts 116 days and 18 hours, so the Sun gets up much less often, but stays up much longer. Whimsical

characterof the Sun is still more noticeable in the Mercurian sky. Mercury, the closest planet to the Sun, undoubtedly has a much more intense view of our star, since at 58 million km (on average), the solar disk appears more than three times bigger in his sky, than in our Earth sky. The sun makes its show more than anywhere else.

When the sun dances

Except that some days, while he is pointing out for a day that promises to be among the hottest and most sunny in our solar system ( Mercury has only a fine exosphere - not really an atmosphere - lacking clouds, and the temperature can reach 427 ° C in full sun ), he scowls and after a few short turns in the sky, eventually go back to bed where he had risen.

It happens that, for a hypothetical observer looking at the sky on the surface of Mercury, the solar star seems to start the day by getting up, then slows down and stops in the sky, and then go back to where it came from. As if the sun were suddenly taken from Acute Monday-phobia , and decided like others that a fat morning could finally be very seductive.

This effect occurs thanks to the particular configuration of Mercury, whose solar day is worth 176 days, and the year, 88 days. The day and the night each equate to a Mercurian year. The rotation period of the planet is 58.7 days. Mercury also has a particularly fast orbital speed due to its orbital eccentricity. From these particular data results the equally special aspect of the sky.

And the astonishing solar ballet can surprise even the most experienced astronomer:at the cold equatorial poles ) and at a specific time of the year ( 4 terrestrial days before the mercurial perihelion ), the Sun begins to rise to the West , then returns to bed from where it comes, then gets up again East , crosses the sky to lie in the West , but rises again once in the West to lie down again at the same place .

In the case of Mercury, the movement of the Sun in the sky does not only depend on the rotation period of the planet and its period of revolution, but also on the variation of its orbital velocityIndeed, Mercury has a very eccentric orbit, and the orbital angular velocity increases as Mercury approaches the Sun (and decreases as its distance increases), just as comets, for example, accelerate as they get closer to the sun. (see the gravitational slingshot effect ). Mercury oscillates between a perihelion of 46 million km and aphelion of 70 million km. And when Mercury is closer to the Sun, the orbital angular velocity manages to exceed the rotational speed, giving the impression that the Sun stops and goes back. A show certainly very significant, provided you have a sunscreen adequate.