The night sky tested by our eyes

7:39 PM, from a randomly chosen, non-fixed point on Earth’s surface. Night falls, and the first and most obvious object we see, which is also the closest to us, is our natural satellite—the Moon. Beautiful, radiant, queen of the night, it resides an average of 350,000 km away. Its craters and dark "seas" are visible with nothing more than our eyes. Image credit : cieletespace.fr

8:02 PM. Shortly after the Moon, or sometimes even before, the "Evening Star" makes its appearance. Despite its misleading name, Venus heralds the arrival of night. At approximately 41 million km away, this planet is none other than Earth’s twin sister and the second brightest object in the sky after the Moon. 

Image credit : cnes.fr & futura-sciences.com (Venus)

8:38 PM. The sky begins to fill with bright points. Among them, Mercury, Mars, Jupiter, Saturn, and Uranus make their debut, provided conditions are extremely favorable, particularly for spotting Mercury and Uranus, as these planets are less conspicuous. These solar system neighbors are located between 62 million km (Mars) and 2.3 billion km (Uranus).

9:00 PM. Among the so-called “nearby” stars—those less than 15 light-years from Earth—only four are visible to the naked eye. (One light-year is approximately 9.5 x 10¹² km.) First, Sirius, the most brilliant star in our night sky, located 8.6 light-years from Earth. The​​​​​​​n Procyon, in the constellation of the Little Dog, at a distance of 11.5 light-years; Alpha Centauri, a double-star system at 4.37 light-years; and finally Vega, in the Lyre constellation, situated 25 light-years away. These four stars are relatively close to Earth.  Image credit : futura-sciences.com (Sirius)

10:00 PM. We lose ourselves in the constellated sky. The nearby objects are gone, and what we now see belongs to our galaxy and beyond. Approximately 6,000 stars of the Milky Way are visible to the naked eye from Earth. These stars range in distance from a few light-years to several thousand light-years. Under excellent conditions, the Milky Way itself is visible as a milky, faintly colored streak—pink, violet, yellow, and pale blue—when completely free from light pollution. It spans about 100,000 light-years.  

Image credit : eso.org 

10:15 PM. More unusual objects catch your eye. Among them, star clusters like the Pleiades—the most famous and spectacular cluster 444 light-years from us. At least six to seven stars are visible to the naked eye under normal conditions, and more under a very dark sky. You might also spot the Hyades, the closest star cluster to Earth, forming a triangular figure around Aldebaran, a red giant star. Finally, the Beehive Cluster, 610 light-years away, and the Double Cluster in Perseus, 7,600 light-years away, appear as hazy patches. Image credit : futura-sciences.com (Pleiades)

​​​​​​To round out your list of rarer and more awe-inspiring celestial objects, keep your eyes peeled for nebulae—clouds of interstellar gas and dust. The most visible one is undou​​​​​​​btedly the Orion Nebula, located at the heart of its namesake constellation, 1350 light-years from earth. Not far behind, the Carina Nebula, 8,500 light-yea​​​​​​​rs away, appear​​​​​​​s as a faint lu​​​​​​​minous patch.  I am directing you to the site archives, where you can find more information about the Orion Nebula in week 21 : Archives​​​​​​​Image credit : wikipedia.org (Orion nebula) 

11:01 ​​​​​​​PM. Your eyes have wandered far and are now struggling to discern more. Yet, they haven’t ventured beyond  the Milky Way, our galaxy. On the edge of visibility, however, you can spot three neighboring galaxies. The Large      Magellanic Cloud, 163,000 light-years away; the Small Magellanic Cloud, 200,000 light-years away; and the Andromeda Galaxy, the Milky Way’s largest neighbor and twin, situated 2.5 million light-years away. These three galaxies appear as small, diffuse luminous patches. This marks the end of your journey through the universe—at 2.5 million light-years. I am directing you to the site archives, where you can find more information about these three galaxies in weeks 13, 25, and 26 : Archives​​​​​​​​​​​​​​​​​​​​​​​​​​​

Ultimately, the vast majority of what we can see with the naked eye in the night sky belongs to the Milky Way. Our eyes don’t allow us to delve into the depths of the universe, nor even to explore nearby galaxies. The secrets of the universe are well-kept. Note also that from any fixed point on Earth, not all these objects are visible simultaneously. By regaining this perspective, we truly grasp the importance of tools like telescopes in exploring our universe, its history, and its origins.