Why is Space dark when there is the Sun?

8th Mar 2023
Why is Space dark when there is the Sun?

One might assume that space is very bright since there are billions of stars emitting light in our galaxy, and there are billions of galaxies in the Universe. Then why is space so dark? Let’s find the answer.

Scientists have long wondered why the light from so many stars can’t illuminate the Universe. In the 18-19th centuries, astronomers were firmly convinced that the Universe is infinite, homogeneous and stationary, which means that the sky should be completely dotted with bright luminous stars. But, for some reason, we observe continuous darkness broken by individual stars.

Why is outer Space dark if the Sun is there: Olbers’ paradox

This contradiction formed the basis of what is called Olbers’ photometric paradox. The German astronomer answered the “why is space always dark” question by an assumption that interstellar space is partially filled with matter that absorbs light, for example, interstellar dust clouds. But the first law of thermodynamics casts doubt on this hypothesis, since the interstellar matter, absorbing light, would inevitably warm up and begin to emit light itself.

Olbers’ paradox was finally resolved only in the 20th century. It turned out that the Universe is constantly expanding, and the visible light of galaxies, as they move away, goes into the infrared, ultraviolet range and radio waves, which are invisible to the human eye. That is, if we could see microwaves, the whole space would glow. By the way, according to a similar principle, we can hear the sounds of space.

Take a look at how bright and rich the panorama of the Milky Way centre looks. This image was obtained by combining data from NASA’s Chandra X-ray Observatory and the MeerKAT radio telescope in South Africa.

milky way
Image credit: NASA

But even with the latest telescopes, we can’t see all the stars. As we know, light travels at a speed of 300,000 km/s. This means that compared to stars closer to Earth, light from the most distant stars takes longer to reach our eyes. Sometimes it takes millions of years. In other words, we see the light from the past, and perhaps the star from which it emanates has long gone out.

Why is space dark, but Earth lightened up?

This phenomenon can be explained in two words — the atmosphere. Space is almost a vacuum; there is only a very small amount of gas and cosmic dust for a given volume but no atmosphere. And light needs to bounce off something. It will move in a straight line until it hits an object. Once light hits and reflects off an object, it is the atmosphere that provides the “scatter” in the spectrum visible to the human eye. Since the Earth rotates around its axis, darkness reigns on that side upon which the light of our star does not fall, and we call the period this lasts a night. During the day, atoms, molecules and atmospheric dust interact with photons, causing them to scatter, passing through increasingly dense layers as they approach the surface.

On Earth, the atmosphere mostly scatters blue light, as blue light has a shorter wavelength at the end of the visible spectrum and is more scattered in the atmosphere than red. For this reason, the daytime sky on Earth appears blue. On Mars, the atmosphere is thinner, about 100 times thinner than the Earth’s, but still enough to make the sky appear greyish-blue during the day, and when the frequent Martian winds kick up clouds of dust from the surface, Martian sky takes on a reddish hue.

But if you found yourself on a planet or a satellite that has no atmosphere or an extremely rarefied one (like on the Moon or Mercury), you would observe a black sky both during the day and night. If you look at the photographs taken by the Apollo spacecraft on the Moon, you will see that the sky is black there, even with bright sunlight on the surface. This explains why is space dark if the Sun is up there.

William Shatner: space darkness filled me with ‘Dread’

Even though space darkness does not pose a threat to humanity, it serves as an excellent warning of what can happen if we do not take care of our planet. Climate change caused by air pollution, water pollution, and greenhouse gas emissions, is destroying our atmosphere — that thin shell that separates us from the cosmic vacuum and eternal darkness. Being here on Earth, under the soft blue sky, many of us simply do not realize the full danger.

The star of the original Star Trek TV series, 91-year-old William Shatner picturesquely described his impressions of the meeting with space darkness. On 13th October 2022, he and five other tourists set off for the Karman Line on the Blue Origin New Shepard ship. The flight only lasted 11 minutes, but in his new book, Shatner says that for him, the trip felt more like a “funeral” than a celebration.

“I love the mystery of the Universe…Stars exploding years ago, their light traveling to us years later; black holes absorbing energy; satellites showing us entire galaxies in areas thought to be devoid of matter entirely… all of that has thrilled me for years… but when I looked in the opposite direction, into space, there was no mystery, no majestic awe to behold . . . all I saw was death.”

Only after meeting one-on-one with the blackness that was “unlike” anything on Earth and “all-encompassing,” Shatner especially felt the warm care of the Earth and sadly realized how much humanity does not appreciate this gift with its destructive intervention in nature. “It filled me with dread. My trip to space was supposed to be a celebration; instead, it felt like a funeral,” wrote the actor.

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