Why Can't We See Stars During the Day?


What causes our inability to see stars during the day? I always thought sunlight would bounce off the particles in the air, thus illuminating them. And the stars would no longer stand out. However people argue that the reason there are no stars in moon landing pictures is because the pictures are taken in lunar days. But the moon has no atmosphere. So I'm wrong.

Rebecca Pitts:

Your thinking is not wrong, merely incomplete. Rather, you’re applying the same principles to two different situations: Sunlight can scatter off of any substance between a light source and a detector—including all parts of your eyeball in front of your retinas—but in the absence of that, it’d still be hard to see the stars. The Sun, and bodies that reflect its light, are just too darn bright compared to their surroundings.

To quantify just how much brighter the Sun and the daytime sky are than the stars, let me start by introducing the wonky way astronomers gauge how bright things are relative to each other or to a standard star. It’s called the Magnitude system, and barely makes sense today because it’s a 2000-year-old hand-me-down from Hipparchus/Ptolemy (it’s so old we can’t even agree on who’s responsible). The relevant details are summed up in the following images:

(By the way, that infographic is overly optimistic in one regard: the naked-eye limit in most cities is more like 3rd magnitude.)

To put the Sun and Moon on that scale and show you just how far the magnitude system can go into the negatives, look at this:

How the Size of a Star Relates to Brightness

The daytime sky is bright enough that it outshines anything fainter than magnitude -4. So, yes, on Earth, the atmosphere is in fact the problem, because of Rayleigh Scattering.

Now what about situations where the atmosphere isn’t a factor?

Combining information from the two figures, the full moon is at least 25,000 times brighter than Sirius. The sun is 400,000 times brighter than that—10,000,000,000 times brighter than the brightest star in the night sky. The brightness of a candle, not coincidentally, is about 1 candela (SI unit of brightness). What’s something 10,000,000,000 times brighter than a candle? Try something like the Luxor Sky Beam in Las Vegas, which shines at 42.3 billion candela. Seeing a star with the sun in your field of view will never be less hard than spotting a handful of candles while staring down the beam of the most powerful spotlight on Earth.

The ratio of signal intensity (brightness in the case of light) between the faintest detectable signal and the point where your instrument maxes out (saturation) is called dynamic range, essentially the maximum contrast ratio. So to photograph the sun and have another star show up in the same image, your detector needs a dynamic range of 10 billion. The dynamic ranges of existing technologies are as follows:

  • Charge Coupled Devices (CCDs, the detectors for digital cameras): 70,000–500,000 depending on the grade (16-bit Analogue-to-Digital converter software that typically accompanies consumer- and education-grade CCDs will cut this to about 50,000)
  • Charge-Injection Devices (the fancier cousin of the CCD where pixels are handled individually rather than by rows and columns): 20 million, as this PDF demonstrates.
  • Human Eye: widely variable, but tops out around 15,000
  • Photographic Film: a few hundred. Yep—that’s it.

To add insult to injury, film doesn’t even react to 98 to 99 percent of the light that hits it. Your eye is every bit as inefficient, but at least it has a dynamic range closer to that of a CCD than to film. CCDs will register upwards of 90 percent of the incident light. You can read about other advantages of CCDs here (their stat on the dynamic range of film is a tad low). But back in the 1960s, CCDs didn’t exist. NASA had to make do with film. (Here’s a whole article on NASA’s film supplies and their specs during the Apollo Program.)

At the Earth’s (and moon’s) distance from the sun, the average square meter of surface receives about 342 watts per square meter (W/m^2) of power from the sun (see Solar Radiation at Earth). If the sun is directly overhead, that number is closer to 1368 W/m^2, but let’s stick with 342 W/m^2 because that’s the average over the sun-facing hemisphere and most of the surface is at some angle to the sun. The Moon reflects about 12 percent of the light that hits it. That doesn’t seem like a lot, but for the Apollo astronauts, that’s like standing on a surface where every square meter is, on average, as bright as a typical desk lamp. The astronauts’ white suits and the highly reflective landing modules were even brighter. As far as the film was concerned, the Apollo astronauts were flood lights standing in a lamp shop. That kind of light pollution doesn’t make for good astrophotography.

Regardless of the technology used, the correct exposure time is important to get a good picture of what you want and as little as possible of what you don’t want. The background stars were not important to the Apollo crews’ studies of the Moon, so their exposure times were calculated to get the best images of Moon rocks, astronauts, landing sites, etc. The upshot is that exposure times for most Apollo photographs were so short that the photo emulsion never received enough light from the background stars to react.

However, there are images taken by the Apollo crews with stars in them. But stars were never their targets, so they don’t look very good, as these UV images from Apollo 16 show:

NASA (*Note - false color UV photo of Earth’s Geocorona in 3 filters, rather poorly aligned judging by the stars)

This post originally appeared on Quora. Click here to view.

Chip Somodevilla, Getty Images
Big Questions
What Does the Sergeant at Arms Do?
House Sergeant at Arms Paul Irving and Donald Trump arrive for a meeting with the House Republican conference.
House Sergeant at Arms Paul Irving and Donald Trump arrive for a meeting with the House Republican conference.
Chip Somodevilla, Getty Images

In 1981, shortly after Howard Liebengood was elected the 27th Sergeant at Arms of the United States Senate, he realized he had no idea how to address incoming president-elect Ronald Reagan on a visit. “The thought struck me that I didn't know what to call the President-elect,'' Liebengood told The New York Times in November of that year. ''Do you call him 'President-elect,' 'Governor,' or what?” (He went with “Sir.”)

It would not be the first—or last—time someone wondered what, exactly, a Sergeant at Arms (SAA) should be doing. Both the House and the Senate have their own Sergeant at Arms, and their visibility is highest during the State of the Union address. For Donald Trump’s State of the Union on January 30, the 40th Senate SAA, Frank Larkin, will escort the senators to the House Chamber, while the 36th House of Representatives SAA, Paul Irving, will introduce the president (“Mister [or Madam] Speaker, the President of the United States!”). But the job's responsibilities extend far beyond being an emcee.

The Sergeants at Arms are also their respective houses’ chief law enforcement officers. Obliging law enforcement duties means supervising their respective wings of the Capitol and making sure security is tight. The SAA has the authority to find and retrieve errant senators and representatives, to arrest or detain anyone causing disruptions (even for crimes such as bribing representatives), and to control who accesses chambers.

In a sense, they act as the government’s bouncers.

Sergeant at Arms Frank Larkin escorts China's president Xi Jinping
Senat Sergeant at Arms Frank Larkin (L) escorts China's president Xi Jinping during a visit to Capitol Hill.
Astrid Riecken, Getty Images

This is not a ceremonial task. In 1988, Senate SAA Henry Giugni led a posse of Capitol police to find, arrest, and corral Republicans missing for a Senate vote. One of them, Republican Senator Bob Packwood of Oregon, had to be carried to the Senate floor to break the filibustering over a vote on senatorial campaign finance reform.

While manhandling wayward politicians sounds fun, it’s more likely the SAAs will be spending their time on administrative tasks. As protocol officer, visits to Congress by the president or other dignitaries have to be coordinated and escorts provided; as executive officer, they provide assistance to their houses of Congress, with the Senate SAA assisting Senate offices with computers, furniture, mail processing, and other logistical support. The two SAAs also alternate serving as chairman of the Capitol Police board.

Perhaps a better question than asking what they do is pondering how they have time to do it all.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at

Big Questions
What Makes a Cat's Tail Puff Up When It's Scared?

Cats wear their emotions on their tails, not their sleeves. They tap their fluffy rear appendages during relaxing naps, thrash them while tense, and hold them stiff and aloft when they’re feeling aggressive, among other behaviors. And in some scary situations (like, say, being surprised by a cucumber), a cat’s tail will actually expand, puffing up to nearly twice its volume as its owner hisses, arches its back, and flattens its ears. What does a super-sized tail signify, and how does it occur naturally without help from hairspray?

Cats with puffed tails are “basically trying to make themselves look as big as possible, and that’s because they detect a threat in the environment," Dr. Mikel Delgado, a certified cat behavior consultant who studied animal behavior and human-pet relationships as a PhD student at the University of California, Berkeley, tells Mental Floss. The “threat” in question can be as major as an approaching dog or as minor as an unexpected noise. Even if a cat isn't technically in any real danger, it's still biologically wired to spring to the offensive at a moment’s notice, as it's "not quite at the top of the food chain,” Delgado says. And a big tail is reflexive feline body language for “I’m big and scary, and you wouldn't want to mess with me,” she adds.

A cat’s tail puffs when muscles in its skin (where the hair base is) contract in response to hormone signals from the stress/fight or flight system, or sympathetic nervous system. Occasionally, the hairs on a cat’s back will also puff up along with the tail. That said, not all cats swell up when a startling situation strikes. “I’ve seen some cats that seem unflappable, and they never get poofed up,” Delgado says. “My cats get puffed up pretty easily.”

In addition to cats, other animals also experience piloerection, as this phenomenon is technically called. For example, “some birds puff up when they're encountering an enemy or a threat,” Delgado says. “I think it is a universal response among animals to try to get themselves out of a [potentially dangerous] situation. Really, the idea is that you don't have to fight because if you fight, you might lose an ear or you might get an injury that could be fatal. For most animals, they’re trying to figure out how to scare another animal off without actually going fisticuffs.” In other words, hiss softly, but carry a big tail.


More from mental floss studios