Deux unités de mesure de l’angle sont majoritairement utilisées en physique et en mathématique : le radian et le degré. Il existe également des sous-unités du degré, très utilisées en astrophysique notamment : la minute d’arc et la seconde d’arc. Prenez un angle d’1 degré, divisez-le en 60 et vous aurez des minutes d’arc. Prenez une de ces minutes d’arc et divisez-la elle-même en 60, et vous aurez des secondes d’arc.

Donc dans 1 degré vous avez 60 minutes d’arc, et dans 1 minute d’arc vous avez 60 secondes d’arc. Autrement dit, dans 1 degré vous avez 3600 secondes d’arc.

C’est très utile en astrophysique parce que l’unité de distance la plus utilisée faisait intervenir la seconde d’arc dans sa définition.

La définition du mille marin est basée sur ce principe : tracez un cercle de la taille de la Terre et découpez le en minutes d’arc, le tracé sur le cercle correspondant à une minute d’arc vaut 1 mille marin.

Is that one galaxy or three? Toward the right of the featured Hubble image of the massive galaxy cluster Abell 3827 is what appears to be a most unusual galaxy -- curved and with three centers. A detailed analysis, however, finds that these are three images of the same background galaxy -- and that there are at least four more images. Light we see from the single background blue galaxy takes multiple paths through the complex gravity of the cluster, just like a single distant light can take multiple paths through the stem of a wine glass. Studying how clusters like Abell 3827 and their component galaxies deflect distant light gives information about how mass and dark matter are distributed. Abell 3827 is so distant, having a redshift of 0.1, that the light we see from it left about 1.3 billion years ago -- before dinosaurs roamed the Earth. Therefore, the cluster's central galaxies have now surely all coalesced -- in a feast of galactic cannibalism -- into one huge galaxy near the cluster's center.

The Ring Nebula (M57), is more complicated than it appears through a small telescope. The easily visible central ring is about one light-year across, but this remarkably deep exposure - a collaborative effort combining data from three different large telescopes - explores the looping filaments of glowing gas extending much farther from the nebula's central star. This composite image includes red light emitted by hydrogen as well as visible and infrared light. The Ring Nebula is an elongated planetary nebula, a type of nebula created when a Sun-like star evolves to throw off its outer atmosphere to become a white dwarf star. The Ring Nebula is about 2,500 light-years away toward the musical constellation Lyra.

Étoiles filantes

Lors d’une rentrée atmosphérique (navette, astéroïde, etc.), la majeure partie de l’échauffement n’est pas due à la friction, mais à la compression de l’air. La chaleur générée est suffisante pour chauffer l’air à plusieurs milliers de degrés, alors que la friction a un effet négligeable.

Une jolie vidéo du survol de Ganymède et Jupiter :-)

Joli :-)

Today the Sun reaches its northernmost point in planet Earth's sky. Called a solstice, many cultures mark this date as a change of seasons -- from spring to summer in Earth's Northern Hemisphere and from fall to winter in Earth's Southern Hemisphere. Precisely, the single time of solstice occurs today for some parts of the world, but tomorrow for other regions. The featured image was taken during the week of the 2008 summer solstice at Stonehenge in United Kingdom, and captures a picturesque sunrise involving fog, trees, clouds, stones placed about 4,500 years ago, and a 4.5 billion year old large glowing orb. Even given the precession of the Earth's rotational axis over the millennia, the Sun continues to rise over Stonehenge in an astronomically significant way.

This moon appears multiply strange. This moon was a full moon, specifically called a Flower Moon at this time of the year. But that didn't make it strange -- full moons occur once a month (moon-th). This moon was a supermoon, meaning that it reached its full phase near its closest approach to the Earth in its slightly elliptical orbit. Somewhat strange, a supermoon appears a bit larger and brighter than the average full moon -- and enables it to be called a Super Flower Moon. This moon was undergoing a total lunar eclipse. An eclipsed moon can look quite strange, being dark, unevenly lit, and, frequently, red -- sometimes called blood red. Therefore, this moon could be called a Super Flower Blood Moon. This moon was seen through thin clouds. These clouds created a faint corona around the moon, making it look not only strange, but colorful. This moon was imaged so deeply that the heart of the Milky Way galaxy, far in the background, was visible to its lower right. This moon, this shadow, this galaxy and these colors were all captured last month near Cassilis, NSW, Australia -- with a single shot. (Merged later with two lower shots that better capture the Milky Way.)

Explanation: Yes, but have you ever seen a sunrise like this? Here, after initial cloudiness, the Sun appeared to rise in two pieces and during partial eclipse, causing the photographer to describe it as the most stunning sunrise of his life. The dark circle near the top of the atmospherically-reddened Sun is the Moon -- but so is the dark peak just below it. This is because along the way, the Earth's atmosphere had an inversion layer of unusually warm air which acted like a gigantic lens and created a second image. For a normal sunrise or sunset, this rare phenomenon of atmospheric optics is known as the Etruscan vase effect. The featured picture was captured in December 2019 from Al Wakrah, Qatar. Some observers in a narrow band of Earth to the east were able to see a full annular solar eclipse -- where the Moon appears completely surrounded by the background Sun in a ring of fire. The next solar eclipse, also an annular eclipse for well-placed observers, will occur later this week on June 10.

How does the Moon's appearance change during a total lunar eclipse? The featured time-lapse video was digitally processed to keep the Moon bright and centered during the 5-hour eclipse of 2018 January 31. At first the full moon is visible because only a full moon can undergo a lunar eclipse. Stars move by in the background because the Moon orbits the Earth during the eclipse. The circular shadow of the Earth is then seen moving across the Moon. The light blue hue of the shadow's edge is related to why Earth's sky is blue, while the deep red hue of the shadow's center is related to why the Sun appears red when near the horizon. Tomorrow, people living from southeast Asia, across the Pacific, to the southwest Americas may get to see a Blood Supermoon Total Lunar Eclipse. Here the term blood refers to the (likely) red color of the fully eclipsed Moon, while the term supermoon indicates the Moon's slightly high angular size -- due to being relatively close to the Earth in its slightly elliptical orbit.

Magnifique !