When you step outside on a clear, moonless night, you might be amazed at how many stars you can see in the sky. However, have you ever wondered just how dark the night sky truly is? Contrary to what you may think, the sky is not completely transparent, even when it appears black.
The air around us contains various molecules, atoms, and particles that reflect light. During the day, sunlight scatters in the atmosphere, making the sky appear bright and blue. This scattering is caused by molecules like nitrogen and oxygen, which reflect more blue light than red light.
At night, the sky darkens as the scattered light from the sun diminishes. Yet, even then, the night sky is not completely dark. City lights, street lamps, and other artificial sources of light contribute to what we call light pollution. The farther away you are from these sources, the darker the night sky will appear.
Moreover, the sky itself emits light due to processes like airglow, where oxygen and nitrogen molecules in the upper atmosphere release energy as a soft glow. This phenomenon results in a faint illumination in the night sky, visible from remote locations far from artificial light sources.
Astronomers, who constantly study the skies to observe faint objects, rely on measuring the brightness of the sky background in units called magnitudes per square arcsecond. In rural areas, a dark sky may have a brightness of about 21 mag/arcsec2, allowing thousands of stars and the Milky Way to be visible. In contrast, the darkest sites on Earth can reach a background brightness as low as 22 mag/arcsec2, providing a truly spectacular view of the night sky.
Even in space, the sky is not entirely dark. The Hubble Space Telescope, which orbits Earth, takes advantage of the dark background to observe incredibly faint objects. Beyond our solar system, space is not empty, with interplanetary dust scattering sunlight and creating a glow known as zodiacal light.
To truly experience a dark sky, one must venture far from the inner solar system. The New Horizons spacecraft, which flew past Pluto, captured images of nearly interstellar skies for the first time, revealing a background glow of just 27.42 mag/arcsec2. This faint light originates from distant galaxies scattered throughout the cosmos, combining their ultradim light to create the cosmic optical background.
Other wavelengths of light, such as gamma rays, x-rays, and infrared, also have background glows originating from different sources in the universe. These faint backgrounds provide valuable insights into the components of the universe and help astronomers understand the vastness of space.
So, the next time you gaze up at the night sky, remember that the darkness you see is relative. To astronomers, the night sky holds a wealth of information waiting to be uncovered, offering a different perspective on the beauty and complexity of the universe.
