Why Can't We See Stars in the Daytime?
― The Stars Haven't Gone Anywhere
Do you vaguely think that stars only come out at night? In fact, the stars are up there all day, and their light reaches you just the same. They vanish from view not because they have disappeared, but because the sky itself becomes so bright that the starlight is drowned in it. The reason the sky is blue works in exactly the same way.
Look up on a clear midday and you will see not a single star. Then, in the same spot, the sun sets and it grows dark. Stars start appearing, one after another, in a sky that was empty a moment ago.
That is how the idea takes hold that "stars come out at night." It is a natural thing to think.
But it isn't accurate. Like the Sun, stars keep shining in the same place all the time. They don't move, and they don't vanish and reappear.
What changes is not the stars, but the brightness of the sky around them.
Sunlight scatters through the air, and the whole sky becomes one big, softly glowing lamp. That glow washes out the light of the stars.
Even the brightest star is only a few billionths as bright as the Sun. Set against a dazzling light, it gets lost.
"Can't be seen" and "isn't there" are two entirely different things. Let's take it step by step.
A Blue Sky and Invisible Stars Are the Same Thing
As I explained in another article, the daytime sky looks blue because sunlight hits molecules in the air and scatters in every direction. That scattered light makes the whole sky glow like a huge lamp.
Starlight travels through that same sky to reach your eyes. The starlight itself does not weaken or stop arriving just because it is daytime.
The problem is that right behind the starlight lies a far brighter "sky lamp." A faint light can't be told apart in front of a bright background.
The sky lamp switches off.
It's the Candle in the Dark, in Reverse
Let's put this in an everyday setting. In a pitch-dark room, even a tiny candle flame is easy to see. Put the same candle on a windowsill in bright daylight, and the flame barely stands out.
The candle's brightness hasn't changed. What has changed is the brightness around it. When the surroundings are dark, even a small light stands out; when they are bright, the same light fades.
Stars and the sky work exactly the same way. A star is only ever seen relative to the "surrounding brightness" of the sky. The daytime sky is so bright that faint lights like stars are almost all buried.
Not everything in the sky disappears in the daytime. The Sun and Moon look bright even by day, and in good conditions Venus can be seen in daylight too.
What they share is an intrinsic brightness that is far above the rest. Only objects that can outshine the bright sky behind them can be picked out in daylight. In other words, they are further proof of why stars can't be seen. Only those that win the brightness contest break through the daytime sky.
With a Telescope, You Can Sometimes See Stars by Day
The naked eye can't do it, but with a large telescope, bright stars can sometimes be observed in the daytime.
A telescope gathers starlight while narrowing the patch of sky you see (the field of view). A narrower field lets in less total "sky brightness." The starlight is collected by the lens and strengthened, while the background brightness is cut down. The two effects together shrink the difference in brightness and make the star easier to pick out.
So a telescope doesn't "summon" the star. It is a tool that tips the brightness contest between star and background in the star's favour.
Try It Yourself
- On a sunny day, darken a room, then switch on a small torch (ideally a dim one).
- Notice how clearly the light shows up in the dark room.
- Next, open the curtains so the room is bright, and switch on the same torch.
- Check that the light, which should be just as bright, suddenly stands out much less.
- This is a small-scale version of "the starlight hasn't changed, yet it can't be seen by day."
Steps 4 and 5 are the heart of this activity. You can see for yourself that it is the surrounding brightness, not the light itself, that decides whether you can see it. Never point a light directly at anyone's eyes.
Summary
We can't see stars by day, but not because they have gone. Their light arrives unchanged, day and night, but sunlight scatters through the air and lights up the whole sky, burying that faint light. A blue sky and invisible stars are two sides of one and the same phenomenon.
The stars are always there.
Whether you can see them depends on the brightness around them.
For those who want more ― terms, numbers and links to the textbooksFrom middle-school science to open research questions, each part is labelled by level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school Earth Science Basics
- High school+High-school Earth Science, or extension and sidebar material in textbooks
- UniversityUniversity-level astronomy, not taught in high school
- ResearchTopics researchers are still working on, not yet settled enough to be taught even at university
Middle schoolTerms: words about star brightness
- Star: a body that shines by its own light, like the Sun (why stars shine is covered in another article).
- Magnitude: a number for how bright a star is. The smaller the number, the brighter the star, and a difference of 5 magnitudes is exactly a factor of 100 in brightness.
- Scattering: light hitting molecules in the air and spreading in all directions. It is why the sky glows.
- Contrast: the difference between bright and dark. The smaller the difference, the harder it is to tell things apart.
High schoolChecking with a formula: how much brighter is the Sun than a star?
Star brightness is expressed as a magnitude. Once you know how to use these numbers, a calculation shows just how far beyond the stars the Sun is.
Brightness ratio = 100 ^ (magnitude difference ÷ 5)
| Brightness ratio | How many times brighter |
| Magnitude difference | The two objects' magnitudes subtracted |
| 100^(…) | 100 raised to the power in brackets |
This formula comes from the rule that "a difference of 5 magnitudes is exactly a factor of 100 in brightness." Magnitude means the smaller the number, the brighter the object, so the Sun and very prominent objects have negative magnitudes.
| Magnitude of the Sun | About −26.7 |
| Magnitude of Sirius (brightest star in the night sky) | About −1.5 |
| Rough limit of the faintest star visible to the naked eye | About +6 |
| Find the magnitude difference | −1.5 − (−26.7) ＝ 25.2 |
| Divide by 5 | 25.2 ÷ 5 ＝ 5.04 |
| Raise 100 to the power 5.04 | 100^5.04 ≒ 1.2 × 10¹⁰ |
| The Sun is | about 12 billion times brighter than Sirius |
Twelve billion times. Even the most striking star in the night sky is, next to the Sun, almost nothing as a source of light. This overwhelming gap is why starlight has no chance against the brightness of the daytime sky.
| Magnitude of Venus at its brightest | About −4.6 |
| Magnitude difference from the Sun | −4.6 − (−26.7) ＝ 22.1 |
| Divide by 5 | 22.1 ÷ 5 ＝ 4.42 |
| Raise 100 to the power 4.42 | 100^4.42 ≒ 6.9 × 10⁸ |
| The Sun is | about 690 million times brighter than Venus |
Now let's compare Venus with Sirius.
| Magnitude difference between Venus and Sirius | −1.5 − (−4.6) ＝ 3.1 |
| Divide by 5 | 3.1 ÷ 5 ＝ 0.62 |
| Raise 100 to the power 0.62 | 100^0.62 ≒ 17.4 |
| Venus is | about 17 times brighter than Sirius |
Venus is a further 17 times brighter than Sirius. Its gap with the Sun (about 690 million times) is far smaller than Sirius's gap (about 12 billion times).
This position of "still a big gap, but better than Sirius" is why on a good day, only someone who knows where to look can spot Venus in daylight. Sirius is very unlikely to show even this way, because the gap is too large.
* Whether you can actually pick out an object by eye in daylight depends on many conditions: the clarity of the air, the observer's eyesight, shading around the eyes and more. The calculation here is a rough guide to the orders of magnitude of brightness.
| For a magnitude difference of 1 | 1 ÷ 5 ＝ 0.2 |
| Raise 100 to the power 0.2 | 100^0.2 ≒ 2.512 |
| Check by multiplying | 2.512 × 2.512 × 2.512 × 2.512 × 2.512 ≒ 100 |
| A difference of one magnitude is | about 2.512 times |
The number "2.512" is the number that gives exactly 100 when multiplied by itself five times. It is the foundation whenever astronomers turn magnitude differences into brightness. A 2-magnitude difference gives 2.512×2.512 ≒ 6.3 times, a 3-magnitude difference about 15.8 times, and so on.
High school+Another condition that decides "visible or not"
So far we have looked at the ratio of star to sky brightness. Whether the human eye can actually pick something out also depends on one more thing: how small (how point-like) the object looks.
Stars are enormously far away, so they reach the naked eye almost as "points." Light that arrives as a point has no spread, which makes it especially easy to lose against a bright background. A telescope, which gathers starlight while narrowing the field of view, turns this property to its advantage.
In observational astronomy, the brightness of a point like a star (apparent magnitude) and the brightness of the sky itself per unit area (background brightness) are treated separately. Comparing the two lets you estimate more accurately whether a given star can be seen under the sky conditions of the moment.
UniversityOn a Mountaintop or in Space, the Story Changes
Sky brightness depends largely on how much air there is to scatter the light. At high altitude there is less air overhead, so even in daytime the sky looks somewhat darker.
Go beyond the atmosphere into space, and there is no air to scatter light at all. That is why astronauts can see a sky full of clear stars even in "daytime," with the Sun shining. A bright sky is something that exists only because Earth has an atmosphere.
This idea bears directly on the practical question of where to build an observatory. Mountaintops with thin air and little light pollution are chosen because they lower the background brightness as far as possible.
ResearchWhat is still unclear
- Predicting daytime sky brightness precisely from place, time and atmospheric conditions is still not easy. Dust, water vapour, the Sun's height and many other factors interact, so model estimates still have room for improvement.
- How faint a light the human eye can detect against a bright background varies from person to person. Age, how well the eyes are shaded and other conditions make a big difference, so it is hard to draw a single line saying "with this much brightness difference, it can never be seen."
- How much city lighting (light pollution) affects the night sky is also still being studied, with measurements and countermeasures region by region. In a growing number of places the naturally dark night sky is being pushed toward a "daytime-like state" by artificial light, so the mechanism described in this article is starting to happen at night too.
"Whether a star can be seen" looks simple, but it involves the physics of brightness, the workings of the eye and the state of the atmosphere. Here again is a pattern we have met often on this site: the more everyday the question, the deeper it goes.
Links to the textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of stars, properties of light | Stars shine day and night alike |
| High school | Earth Science Basics: brightness of stars (magnitudes) | Calculating brightness ratios from magnitude differences |
| High school+ | Earth Science: basics of observation | Point sources versus background brightness |
| University | Astronomy, atmospheric science | Altitude and sky brightness, choosing observatory sites |
| Research | Astronomy, environmental science (unsolved) | Modelling sky brightness, effects of light pollution |
| ― | Activity | Experiencing how contrast changes what you see |
- Explanatory materials on stellar magnitudes and observation basics from the National Astronomical Observatory of Japan (国立天文台).
- General-audience materials on the brightness and visibility of celestial objects from the Astronomical Society of Japan (日本天文学会).
- Bohren, C. F. & Clothiaux, E. E., Fundamentals of Atmospheric Radiation (a specialist book on atmospheric scattering and sky brightness).
- Materials on light pollution from the International Dark-Sky Association (IDA).
- Magnitude data for stars and planets from various astronomical almanacs.
* Magnitude values vary slightly with observing conditions and sources. This article uses representative, commonly cited values.
* This article is a general-audience science explainer. Never look directly at the Sun: it can cause serious eye damage. The figures given are rough guides to help you understand how things work.