“We can enjoy the show” even “in an urban sky”, explained Friday June 24 on franceinfo Emmanuel Marcq, teacher-researcher in planetology at the University of Versailles-Saint-Quentin, while in the sky a rare celestial spectacle delights lovers of astronomy, the alignment of the planets of the solar system. It is by looking at the sky towards the East that this parade of planets will be offered to the spectator from the middle of the night until dawn, the most favorable moment: the picture is organized from left to right, starting of the sun: Mercury, Venus, Uranus, Mars, Jupiter, Neptune and in the distance Saturn, the Moon intercalating at a time in this pseudo alignment due to a rare moment of conjunction between the orbits of these stars. “It’s just something beautiful,” he says. “The planets all revolve around the sun and every so often they all happen to be in roughly the same sector,” he explains. A spectacle not to be missed, because the next alignment is scheduled for 2124. franceinfo: Can these seven planets be observed with the naked eye? Emmanuel Marcq: We can observe the five bright planets among these seven with the naked eye. And the actually very special configuration is that the planets rose in the reverse order of their distance from the sun. We will first have Saturn rising around one o’clock in the morning, followed by Jupiter, and then Mars, the crescent of the moon comes to be inserted between the rising of Mars and that of Venus, then Mercury and finally the sun which rises between five and six in the morning in metropolitan France. The show lasts the entire second part of the night. It is indeed a very rare thing. They are just like the sun. If we look at the Eastern horizon, we see them gradually rising in the sky starting with Saturn around one o’clock in the morning and then in order until sunrise which closes the show, in a way, since once the sun has risen, inevitably, the visibility of the planets decreases very sharply. We can still, with instruments, observe them even in broad daylight. When is the best time to watch this show? The ideal for both practical and aesthetic reasons is to watch about half an hour, forty-five minutes before sunrise. Dawn is just beginning to break and the visibility of the planets is still very good. They are almost all up. In this case, we can have the panorama in all its majesty, which occupies almost the entire eastern horizon of the sky. Some are extremely bright like Venus and Jupiter. You can’t really miss it, even in an urban sky. It’s a spectacle that is open to everyone as long as the weather conditions are good. Is this of scientific interest? It’s just something beautiful. There is not really any particular scientific interest. It’s a coincidence. The planets all revolve around the sun and from time to time they all happen to be in roughly the same sector. And it’s the configuration we have at the moment that allows us to enjoy this show. We take advantage of the fact that the solar system is globally flat. All the planets revolve in the same plane around the Sun. This provides a preferred alignment direction. In this direction of alignment, it happens that by the chance of the periods of the orbits, we find ourselves with all the planets gathered in a sector which is approximately 90° wide. It’s not an ultra-precise alignment and that’s why we could still enjoy it for a few days.
Is it a very rare phenomenon? Yes, it is quite rare. The last time it happened was in the 1990s. The next time is in the 22nd century. Indeed, it is a spectacle that should be enjoyed now, if possible, for a few more days. The moon will shift. It is the star that will break the alignment the fastest, because it is the one that moves the fastest in the sky. And after, the planets, especially the fast planets close to the sun, like Mercury and Venus will quite quickly pass to the other side and break this alignment a little in the space of a few weeks maximum.