We have a very simple question at hand. Why don’t the stars shine during the day like on the image below? The answer lies in our eyes, or more specifically, their dynamic range. But to explain dynamic range, we have to understand the term brightness, and why we perceive only a range of it.
Stars during the day, illustration
In case you don’t have time for the whole explanation, here is a summary.
A brief explanation would involve the atmosphere. During the day, the brightness level of the atmosphere is increased so much by the Sun, it is impossible to see dim objects such as stars. The dynamic range of our eyes is simply not wide enough. Only exceptionally bright objects such as Venus can sometimes be seen.
How can we see anything at all?
The human eye is a mesmerizing natural device. It actually is very similar to a telescope. Light beams that reflect from the objects around us are gathered inside our eyes. After that, information is sent to the brain by our neurons.
The light comes inside from the pupil. Yes, the dark part in the middle of our eyes is actually a hole. This is the reason why in some photographs our eyes look red. The flash is so bright that inside of our eyes appear for a mere moment.
Human eye, closeup
Brightness is Subjective
If you close your eyes for a long time on a sunny day, after you open them, you will feel like everything is incredibly bright. It will take time for your eyes to adjust, meaning your pupils to get smaller. After some time passes, everything will look normal and not as bright.
“Brightness” means the amount of light per area received from an object. We can also call this the luminance of a spot. And it is perceived differently for any animal, telescope, camera or device. Therefore things are brighter or less bright relative to each other. The Sun may look like a table lamp for a rabbit, and it may look like a giant Supernova explosion for the Hubble Telescope. It depends on how much light is gathered from the object and how it is interpreted by the device.
Every seeing object has a certain light gathering ability proportional to their aperture. Therefore there is a range of brightness they can sense. This called the dynamic range. It is the brightness range within which we can perceive light beams, and therefore differentiate objects.
To put it mathematically, it is the ratio between the luminance of the dimmest object we can see and the brightest.
Dynamic range, photography
Our eyes have a maximum dynamic range of 1,000,000:1. The reason we use the term “maximum” is that it changes.
In a dark environment, our pupils grow. They receive more light and therefore gain a wider dynamic range.
In a brighter environment, our pupils get smaller. Our dynamic range narrows down.
So in a normal situation, our eyes have a dynamic range around 10,000:1.
James Webb Space Telescope
James Webb Space Telescope will have a maximum dynamic range of 1,000,000:1. It will launch on 21 March 2021. It is the most advanced observing device ever created by humankind.
James Webb Space Telescope in construction at NASA
So now that we have gained some perspective let’s come back to our main question. Why can’t we see the stars during the day? Why do stars only shine at night?
Why do stars shine only at night?
The luminance ratio between the brightest star and the Sun is 10,000,000,000:1. Now I think you can see the reason. It is a stupidly large ratio. To create a device that could perceive such a dynamic range would be an almost impossible mission. You would need a dynamic range of 10 billion. This is why we can’t see stars during the day. We simply don’t have enough aperture to perceive. Nothing has.
Why didn't stars on the Apollo 11 Mission shine?
Let’s step into some controversy. One of the main arguments Moon Landing deniers use is the absence of stars taken on the Lunar Surface on the Apollo 11 Mission. Now I think we can illuminate(pun intended) some confusion. The stars weren’t visible by the cameras because they simply didn’t have enough dynamic range. The luminance of the Lunar Surface is so bright, for the astronauts that walked on the Moon it was equal to walking on bright desk lamps for every square meter.
Picture from NASA, Apollo 11 Mission
As intricate a device our eyes are, they are nowhere enough to see stars in daylight. The aperture is simply too small. But maybe this renders the incredible view of the starry night sky even more special.