Category: Cool Stuff

(via MentalFloss)

Turns out, it isn’t.  That’s the short answer.  The long answer is interesting – it’s violet, but our daylight vision is such that it is seen as blue.

Why is the sky blue? It is a question children ask. Yet it also intrigued Leonardo da Vinci and Isaac Newton, among many other legendary thinkers. As late as 1862, the great astronomer John Herschel called the colour and polarization of skylight “great standing enigmas.” Even today, our perception of sky blue is little understood by laymen.

. . .

In 1873, James Clerk Maxwell realized the truth: The scattering substance must be the molecules of air themselves, none other. Indeed, John Ruskin suggested in 1869 that sky blue comes from light “reflected from the divided air itself.” Lord Rayleigh followed Maxwell’s advice and calculated that the observed scattering of skylight requires molecules whose size accords with that indicated by other physical arguments.

. . .

As you gaze at the clear blue sky, then, you are beholding unambiguous evidence that atoms really exist, something that was widely questioned as recently as the 19th century. This presents a profound but simple answer to our opening question about the sky’s colour. But even so, further explanation is required.

If Tyndall and Rayleigh are right, then the violet wavelengths from the sun, having still shorter wavelengths than blue, should be scattered even more. Given this, shouldn’t the sky be violet, not blue?

Indeed the sky is violet, if you observe not with the naked eye but with an instrument that objectively measures the intensity of the spectrum at different wavelengths. Such a device, a spectrophotometer, shows that, in fact, the highest peak of the intensity of skylight occurs in the violet range.

But why do we see blue, nonetheless? The resolution of the mystery lies in our daytime vision, which happens to be eight times less sensitive to violet than to blue light.

Does that mean it is “incorrect” to call the sky blue? Not really. Our names for colors reflect our common perception, whatever a mechanical instrument might say.

There you have it.  The sky is violet.  But we still call it blue.  Our eyes don’t see violet as well as blue, and the higher intensity of violet is not sufficient to overcome our innate sensitivity to blue.  That means more blue is registered by the cones in our eyes, so our brain just gives us what it has more of to work with.

[tags]Why is the sky blue, Eye color sensitivity[/tags]

(via Engadget)
Apparently, the possibility of being off 1 second every 70 million years was just too much for the National Institute of Standards and Technology.  So NIST recently announced a new ultra-precise clock based on the oscillations of a mercury ion.  The new clock, tested and measured over the past 5+ years, should have an accuracy such that drift will be less than 1 second over 400 million years.  It will still take some time before this clock becomes the new standard, but the extra precision certainly suggests it will happen.

A prototype mercury optical clock was originally demonstrated at NIST in 2000. Over the last five years its absolute frequency has been measured repeatedly with respect to NIST-F1. The improved version of the mercury clock is the most accurate to date of any atomic clock, including a variety of experimental optical clocks using different atoms and designs.

. . .

“We finally have addressed the issue of systemic perturbations in the mercury clock. They can be controlled, and we know their uncertainties,” says NIST physicist Jim Bergquist, the principal investigator. “By measuring its frequency with respect to the primary standard, NIST-F1, we have been able to realize the most accurate absolute measurement of an optical frequency to date. And in the latest measurement, we have also established that the accuracy of the mercury-ion system is at a level superior to that of the best cesium clocks.”

And if you just want to learn more about atomic clocks and how they work, check out the NIST atomic clock page.

[tags]Atomic clock, Ultra-precision[/tags]