Why The Sky Is Dark Black To Us

Why The Sky Is Dark Black To Us

by Dr. MMM

Dr. MMM has taught astronomy to over 10,000 higher education students over 15 years at a Carnegie Level R1 type University in the USA. The author continues to teach more than 10 classes of astronomy and physics each year at different colleges.

Read on to learn why space looks black to our human eyes.

From the vantage point of Earth’s surface on a dark, clear night, space looks black. From a vantage point of less light pollution, more celestial objects can be seen in the clear night sky. However, with stars, planets, the Moon, nebulae, and galaxies at nearly every point in the sky, the sky should be lit with light. So why is the sky dark at night?

The simple answer is that our eyes cant truly see the full color spectrum nor could they perceive the vast amount of lights coming from space. However to fully understand why space appears black to us we need to know about Olber’s Paradox.

This article provides a clear answer to why space appears black to us. If you liked this type of article you can also check out the Youtube video where I discuss how Olber’s Paradox impacts our perception of the black color of space.

Without further ado, here is the reason why space appears black to us.

Olber’s Paradox and the Black Sky

Although Olber’s Paradox addresses why Earth’s sky is dark at night from a paradoxical point of view, we will approach this answer more pragmatically. After all, a human asked the question and thus this question is biased! A human asked the question and therefore the question is biased to human eyes. A human eye sees the space between celestial objects as black.

image courtesy of ESO/H. Dahle under CC BY 4.0

Figure 1 shows shades of black between bright celestial objects as viewed by a human using the naked eye. Through evolution on planet Earth, human eyes have been trained to see in the visual (V) band of the electromagnetic (EM) spectrum due to intense sunlight, which has its peak emission in the V band. Human eyes do not see in the radio band, infrared band, UV band, X-ray band, or gamma ray band of the EM spectrum. Human eyes only see in the narrow V band. Human eyes do not see any features of space in any other band other than V. Human eyes see space as black as space does not emit light in the V band; black space is an absence of V band light.

Black Space Is Actually Full Of Colors

The blackness of space emits light! Although human eyes cannot see the light space emits, the black parts between the celestial objects does emit light. Light has many different types: radio band light, infrared light, V band light, UV band light, X-ray light, and gamma ray light. The radio band contains wavelengths and frequencies that AM and FM radio stations transmit and receive in; the radio band also contains microwaves used by microwave ovens to warm food; and the radio band also contains wavelengths and frequencies that our cell phones emit and receive in.

Image courtesy of National Nuclear Security Administration / Nevada Site Office under Public Domain

The warmth of the Sun humans feel on their skin is from the Sun’s light emission in the infrared band. Sunburns and suntans of human skin are from the Sun’s light emission in the UV band. Humans go to the dentist to get X-rays of their teeth; X-rays is light emission within the X-ray band of the EM spectrum. Gamma rays are the highest energy rays of the EM spectrum such as those emitted from the BADGER atom bomb explosion in 1953 (see Figure 2). Of all the bands of the EM spectrum, the blackness of space emits in the radio band of light; specifically, space emits microwaves.

Why We Cant See These Colors

Can humans hear space emitting in the radio band? As most humans are on the surface of Earth and not in space, most humans will not likely have the chance to test this answer firsthand.

Space has about one molecule per cubic centimeter, which is not enough molecules for a source to vibrate and for human ears to hear. Although space emits it the radio band of the EM spectrum, any human in space today will not hear sound from this emission. However, this answer is for humans listening today. Although humans were not around when the Universe was 380,000 years old (i.e., when this cosmic microwave background (CMB) emission was first emitted) through to when the Universe aged to 760,000 years, which is a time range when the Universe was emitting this light in the radio band, humans today can simulate this sound of space. John G. Cramer, Professor of Physics at the University of Washington, simulated sound from the CMB during the Universe’s age from 380,000 – 760,000 years, which can be download from here or listed on YouTube here.

Will These Microwaves Cook Someone In A Spacesuit?

Microwaves cause water molecules to vibrate, and vibration produces heat. Humans are about 60% water and thus targets for microwaves if in space unprotected. The CMB emits a frequency of about 160 GHz. At this high of frequency, the unprotected human body would be cooked from outside in, with amount of cooking depending upon time of exposure. Short exposures will mostly likely result in internal organs of a human body being protected but not the surface of the body: The skin will likely experience reddening and hurt like a sunburn, with longer exposures likely leading to third degree burns. Human eyes are mostly water and highly susceptible to significant damage as they are at the surface of the body. Spacesuits with visors are the best protection for humans from this radiation as well as from extreme temperatures and dust; spacesuits are also good for supplying necessary oxygen for humans to breathe while in the vacuum of space. In short, humans will not survive space without special suits!


Although space is black to humans, space is filled with light. This light is in the radio band, namely in microwaves. Today’s human ears cannot hear these cosmic microwaves that permeate space, but space at one time in its history did emit a continuous sound for a period of thousands of years. Space is now of such low density that any sound that is emitted cannot be heard by humans without instruments. Even though humans cannot hear this CMB background noise when in space, humans need to wear special space suits for protection from this CMB radiation to prevent from being cooked from outside-in.

In summary, space appears black to human eyes as humans only see in ROYGBIV and white/black, whereas celestial objects emit light in several, and sometimes all, light bands of the electromagnetic spectrum. If human eyes can see in all wavelength bands, humans would see light in all areas of the sky! But let us humans be thankful we have instruments that can see in all wavelength bands so that we may sleep more easily on planet Earth when the Sun goes down each night. 

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About the author

Michele M. Montgomery earned a B.S. Degree in Nuclear/Mechanical Engineering from the Pennsylvania State University, an M.S. Degree in Physics from The University of Alabama with a concentration in Solar Physics, and a Ph.D Degree in Physics from Florida Institute of Technology with a concentration in close binary star systems. She joined the faculty at The University of Central Florida Physics Department in 2004 where she regularly taught astronomy, astrophysics, and cosmology. In 2006, she noticed that a large, urban college nearby to UCF did not teach astronomy at one of their largest campuses. She began teaching astronomy at this East Campus of Valencia College, a college that has more than 60,000 students; she still teaches four courses of astronomy each fall, spring, and summer semesters. The astronomy program atValencia College East has grown significantly with several more faculty added who teach astronomy.

By 2019, Dr. Montgomery has taught astronomy to more than 10,000 college and university students, both online and face-to-face. Many of her students have gone on to take her astrobiology, astrophysics, and space physics courses. 

By 2016, Dr. Montgomery had co-authored several astronomy texts and quiz/exam banks. Her work appears in several domestic and international astronomy text books (e.g., Horizons by Cengage, Universe by Cengage, Foundations of Astronomy by Cengage) that are used both at the higher education as well as at the high school levels. Starting in Fall 2019, Dr. Montgomery switched gears to authoring digital textbooks and research full time, while still teaching 12 courses of astronomy and up to eight conceptual, algebra, and/or calculus-based physics courses each year. Her research interests are numerical simulations using Smoothed Particle Hydrodynamics of close binary star systems. She also regularly is granted telescope time on the NASA's Kepler space telescope for observing eclipsing binary star systems. She has also observed using Gemini South, Keck, and Kitt Peak ground-based telescopes. Her major teaching areas are Astronomy, Astrobiology, Astrophysics, Cosmology, Space Weather/Space Physics.