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# Why Can't We See Stars During the Day?

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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)

Jack Taylor, Getty Images
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Big Questions
How Are Royal Babies Named?
Jack Taylor, Getty Images

After much anticipation, England's royal family has finally received a tiny new addition. The birth of the Duke and Duchess of Cambridge's second son was confirmed by Kensington Palace on April 23, but the name of the royal newborn has yet to be announced. For the heir to the British throne and his wife, choosing a name for their third child—who is already fifth in line to the throne—likely won't be as easy as flipping through a baby name book; it's tradition for royals to select names that honor important figures from British history.

According to ABC WJLA, selecting three or four names is typical when naming a royal baby. Will and Kate followed this unwritten rule when naming their first child, George Alexander Louis, and their second, Charlotte Elizabeth Diana. Each name is an opportunity to pay homage to a different British royal who came before them. Some royal monikers have less savory connotations (Prince Harry's given name, Henry, is reminiscent of a certain wife-beheading monarch), but typically royal babies are named for people who held a significant and honorable spot in the family tree.

Because there's a limited pool of honorable monarchs from which to choose, placing bets on the royal baby name as the due date approaches has become a popular British pastime. One name that keeps cropping up this time around is James; the original King James ruled in the early 17th century, and it has been 330 years since a monarch named James wore the crown.

If the royal family does go with James for the first name of their youngest son, that still leaves at least a couple of slots to be filled. So far, the couple has stuck with three names each for their children, but there doesn't seem to be a limit; Edward VIII, who abdicated the throne to George VI in 1936, shouldered the full name of Edward Albert Christian George Andrew Patrick David.

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

CHRIS JACKSON, AFP/Getty Images
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Big Questions
Why Does the Queen Have Two Birthdays?
CHRIS JACKSON, AFP/Getty Images

On April 21, Her Majesty Queen Elizabeth II will turn 92 years old. To mark the occasion, there are usually a series of gun salutes around London: a 41 gun salute in Hyde Park, a 21 gun salute in Windsor Great Park, and a 62 gun salute at the Tower of London. For the most part, the monarch celebrates her big day privately. But on June 9, 2018, Her Majesty will parade through London as part of an opulent birthday celebration known as Trooping the Colour.

Queen Elizabeth, like many British monarchs before her, has two birthdays: the actual anniversary of the day she was born, and a separate day that is labeled her "official" birthday (usually the second Saturday in June). Why? Because April 21 is usually too cold for a proper parade.

The tradition started in 1748, with King George II, who had the misfortune of being born in chilly November. Rather than have his subjects risk catching colds, he combined his birthday celebration with the Trooping the Colour.

The parade itself had been part of British culture for almost a century by that time. At first it was strictly a military event, at which regiments displayed their flags—or "colours"—so that soldiers could familiarize themselves. But George was known as a formidable general after having led troops at the Battle of Dettingen in 1743, so the military celebration seemed a fitting occasion onto which to graft his warm-weather birthday. Edward VII, who also had a November birthday, was the first to standardize the June Trooping the Colour and launched a tradition of a monarchical review of the troops that drew crowds of onlookers.

Even now, the date of the "official" birthday varies year to year. For the first seven years of her reign, Elizabeth II held her official birthday on a Thursday but has since switched over to Saturdays. And while the date is tied to the Trooping the Colour in the UK, Commonwealth nations around the world have their own criteria, which generally involve recognizing it as a public holiday.

Australia started recognizing an official birthday back in 1788, and all the provinces (save one) observe the Queen's Birthday on the second Monday in June, with Western Australia holding its celebrations on the last Monday of September or the first Monday of October.

In Canada, the official birthday has been set to align with the actual birth date of Queen Victoria—May 24, 1819—since 1845, and as such they celebrate so-called Victoria Day on May 24 or the Monday before.

In New Zealand, it's the first Monday in June, and in the Falkland Islands the actual day of the Queen's birth is celebrated publicly.

All in all, just another reason it's great to be Queen.

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

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