What if you could see stars in the daytime?

At night, we see lots of stars. In the day, we see none.

Do the stars go away? No! They are still up there. So why can't we see them?

On a clear night, the sky sparkles with stars. When the Sun comes up, they seem to vanish. But NASA explains that the stars are still up there during the day; we just can't see them because the sky is too bright.

So what if you could see stars in the daytime? What would have to change? This is a thought experiment built on real places and moments where daytime stars do appear. Choose a path and explore.

The stars visible at night are still above the horizon during the day. NASA's Cosmicopia puts it simply: you can see one star during the day, the Sun, and because its light makes the sky so bright, the others are not visible.

This thought experiment asks what would let daytime stars show: removing the bright sky (as on the airless Moon), blocking the Sun (as in a total solar eclipse), or both. It also asks why our sky is bright in the first place.

On a sunny day, the whole sky is bright blue.

The stars are much dimmer. Their light gets lost in the bright blue sky.

In the daytime, sunlight pours into our air. NASA Space Place explains that the gases in the air scatter that sunlight in all directions, and blue light gets scattered the most. So light reaches your eyes from every part of the sky, not just from the Sun.

That bright glow is far brighter than the light of a distant star. The starlight is still arriving, but it is drowned out, like trying to see a flashlight beam in a sunny room. The Moon, though, is bright enough to see in the day.

Sunlight entering the atmosphere is scattered by gas molecules and particles, with shorter blue wavelengths scattered most strongly. The result is a sky that sends light toward an observer from every direction, a bright background against which starlight has too little contrast to be seen.

NASA notes that the stars are still in the daytime sky but the sky is too bright for us to see them, while the Moon reflects enough sunlight to be visible by day or night when it is in the right part of the sky.

On the Moon, there is no air to make the sky bright. So the sky is black, even in the day.

If you block out the Sun there, you can see stars!

Take away the air, and the sky stops glowing. NASA's Cosmicopia says that on the Moon, if you block the Sun with your hand and let your eyes get used to the dark, you can see stars during the lunar 'day'.

On Earth, there's another way: a total solar eclipse. When the Moon completely covers the Sun, NASA says, the sky grows dark, you may see a sunset-like glow all around the horizon, and you may be able to spot some particularly bright stars or planets in the middle of the day.

Two routes give daytime stars. Without an atmosphere, there is no scattered skylight: on the Moon, Cosmicopia notes, shielding the Sun and dark-adapting the eyes reveals stars in daytime. With the Sun's face hidden, the direct sunlight that the air would scatter is gone: during totality, NASA says, you may see a 360-degree sunset and bright stars or planets may be visible, as Venus and Jupiter were for viewers in Dallas in April 2024.

A third factor is the eye. Cosmicopia notes that photos from space often show no stars because the stars are much dimmer than the brightly lit subject and the exposure is set for it; the human eye handles different light levels better than a camera.

Air bounces sunlight all over the sky. That makes the whole sky glow.

The glow is much brighter than the stars. So the stars hide.

The sky is bright because air scatters sunlight. Each tiny bit of air sends a little sunlight off in a new direction, so the whole sky glows. Blue light is scattered most, which is why that glow is blue.

Stars are so far away that their light is very faint. Next to the glowing sky, there's too little difference for your eyes to notice them. Take away the glow, with no air or no sunlight, and the stars appear. Something similar happens at night in cities: bright lights make the sky glow and hide the fainter stars.

Seeing a faint point source depends on contrast against the background. Daytime skylight, scattered sunlight, raises the background so much that even bright stars fall below the eye's contrast threshold, while the Moon, which NASA notes shines brightly enough, can still be seen when it is in the right part of the sky.

The same principle governs night viewing: NASA's Cosmicopia notes that how well you can see stars depends mostly on nearby light pollution, usually from artificial lights. Eclipse totality and airless skies remove the background in different ways, which is why both reveal stars.

During a total solar eclipse, the Moon covers the Sun. The sky gets dark for a short time.

Then you might see bright stars and planets in the middle of the day!

During the total solar eclipse of April 8, 2024, NASA reported that viewers in Dallas, Texas, saw Venus and Jupiter shining brightly in the darkened midday sky. NASA says totality can last only a minute or two in some places, and during it you may see a 360-degree sunset glow around the horizon.

Out in space, above all of our air, a NASA scientist explains that the stars look wonderful, bright and very clear, though they do not twinkle.

NASA's eclipse guidance notes that totality may last only a minute or two, that you may see a 360-degree sunset, and that particularly bright stars or planets may be visible; for the 8 April 2024 eclipse, NASA reported Venus and Jupiter very bright over Dallas. Eclipse glasses may come off only once no direct sunlight remains, and must go back on before totality ends.

Above the atmosphere there is no scattered skylight at all. NASA's Cosmicopia describes stars seen from space as bright and very clear, though not twinkling, while photographs often miss them because exposures are set for brightly lit subjects.