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What Is Greenwich Mean Time?

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In south London, at the Royal Observatory, Greenwich, it’s possible to walk up to a metal strip running along the ground in a courtyard and, stepping over it with one foot, straddle the world. Suddenly, one half of your body is in the Western Hemisphere and the other is in the East. Sort of. More on that later.

This line is the prime meridian, or Greenwich Meridian. In 1851, Sir George Airy established it as 0° longitude. With a fixed line of longitude, or “home meridian,” sailors and explorers were more easily able to nail down their east-west position. All a navigator had to do was compare the time onboard their ship with the local time at the home meridian, and British sailors began keeping a marine chronometer set to the local time at Greenwich.

The practice spread to sailors from other countries, and soon ships all over the world were calculating their positions based on Greenwich time. In 1884, the sailors' custom got legitimized when the International Meridian Conference met in Washington, D.C., and delegates from 25 countries overwhelmingly voted to make the Greenwich Meridian the internationally common point from which to measure time and longitude.

Call it a Day

They also recommended that there should be a Universal Day, counted in 24-hour notation, that would begin at Solar midnight (the point at which the night is equidistant from dusk and dawn) Greenwich Mean Time (the mean being an average that accounts for the uneven speed of the Earth in its rotation).

Of course, not every country at the conference was so gung-ho about adopting a British time and longitude as the world standard. French delegates attempted to convince the others that Paris should be the home of the Prime Meridian, but went ignored. Feeling snubbed, they abstained from the vote and adopted Paris Mean Time as their standard national time and the Paris Observatory meridian as their prime meridian until switching to the Greenwich standard decades later. Even then, some Frenchmen were known to refer to GMT as “the mean time of Paris retarded nine minutes and 21 seconds.”

Just as GMT seemed to have spread over the globe, things began to fall apart. The French reaction demonstrated one big problem with the conference once all the good vibes of international cooperation faded away: the decisions made in Washington had no binding power. They were only recommendations, and it was up to the different national governments to implement them at home.

Progress was slow and confusion rampant. The only nation to do anything concrete within the following decade was Japan, which formally adopted the Greenwich Meridian and a standard national time nine hours in advance of Greenwich (GMT +9) in 1888. Elsewhere, depending on who you were talking to, GMT was used (usually inconsistently) one of two ways — with the hours either numbered starting at midnight, as had been recommended at the conference, or at noon.

To help stem confusion, the International Astronomical Union changed the designation of the standard time of the zero meridian to Universal Time Observed, or UTO, which is more or less equivalent to GMT but more precise and is the mean sidereal time as measured in Greenwich.

In 1972, after the development of super-accurate atomic clocks, Coordinated Universal Time, or UTC, was established. It’s calculated using a weighted average of signals from atomic clocks located in various national laboratories around the world, with leap seconds added at irregular intervals to compensate for the oddities of the Earth’s movement. UTC, like UTO, is synonymous with GMT in common or casual use, but GMT isn’t so precisely defined by the scientific community anymore and isn’t used in technical contexts.

Time Marches On

Even the Greenwich Meridian itself isn’t quite what it used to be. Formerly defined by "the centre of the transit instrument (a specific kind of telescope) at the Observatory at Greenwich,” the line is now defined by a statistical solution resulting from observations of several time-determination stations that the International Bureau of Weights and Measures uses to coordinate the world's time signals. The observatory’s instrument still survives in working order, but is no longer in use, while the actual line in the observatory courtyard, marked by a bronze strip, is actually now a few meters off from the imaginary line of the Prime Meridian.

Flying Telescopes Will Watch the Total Solar Eclipse from the Air

If you've ever stood on the tips of your toes to reach something on a high shelf, you get it: Sometimes a little extra height makes all the difference. Although in this case, we're talking miles, not inches, as scientists are sending telescopes up on airplanes to monitor conditions on the Sun and Mercury during the upcoming total eclipse.

Weather permitting, the Great American Eclipse (as some are calling it) will be at least partially visible from anywhere in the continental U.S. on August 21. It will be the first time an eclipse has been so widely visible in the U.S. since 1918 and represents an incredible opportunity not only for amateur sky-watchers but also for scientists from coast to coast.

But why settle for gawking from the ground when there's an even better view up in the sky?

Scientists at the Southwest Research Institute (SwRI) have announced plans to mount monitoring equipment on NASA research planes. The telescopes, which contain super-sensitive, high-speed, and infrared cameras, will rise 50,000 feet (about 9.5 miles) above the Earth's surface to sneak a very special peek at the goings-on in our Sun and its nearest planetary buddy.

Gaining altitude will not only bring the instruments closer to their targets but should also help them avoid the meteorological chaos down below.

"Being above the weather guarantees perfect observing conditions, while being above more than 90 percent of Earth's atmosphere gives us much better image quality than on the ground," SwRI co-investigator Constantine Tsang said in a statement. "This mobile platform also allows us to chase the eclipse shadow, giving us over seven minutes of totality between the two planes, compared to just two minutes and 40 seconds for a stationary observer on the ground."

The darkness of that shadow will blot out much of the Sun's overpowering daily brightness, giving researchers a glimpse at rarely seen solar emissions.

"By looking for high-speed motion in the solar corona, we hope to understand what makes it so hot," senior investigator Amir Caspi said. "It's millions of degrees Celsius—hundreds of times hotter than the visible surface below. In addition, the corona is one of the major sources of electromagnetic storms here at Earth. These phenomena damage satellites, cause power grid blackouts, and disrupt communication and GPS signals, so it's important to better understand them."

The temporary blackout will also create fine conditions for peeping at Mercury's night side. Tsang says, "How the temperature changes across the surface gives us information about the thermophysical properties of Mercury's soil, down to depths of about a few centimeters—something that has never been measured before."

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What You'll See of the 2017 Solar Eclipse From Your ZIP Code
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Pablo Blazquez Dominguez/Getty Images

On August 21, a total solar eclipse will cross over the continental United States, giving millions of people the exciting experience of watching the Sun briefly disappear, leaving the Earth in darkness. But whether or not you'll be able to experience total darkness depends on where you live. How do you know how much of the Sun you'll see? Check out this infographic from Vox illustrating what the eclipse will look like in each ZIP code in the U.S.

For instance, we at the Mental Floss offices in New York will still be standing in pretty bright light as the eclipse peaks at 2:44:55 p.m. EDT, with 71.4 percent of the Sun covered. We would need to drive 576 miles to see the total eclipse, according to Vox. In Lincoln, Nebraska, though, the Moon will obscure the whole Sun at 1:03:18 p.m. CDT, leaving residents in the dark for about a minute and a half. In Anchorage, Alaska, 1381 miles from the totality zone, residents will see 45.6 percent of the Sun disappear at the eclipse's peak at 9:16:21 a.m. AKDT.

Here's what it will look like in Nashville, according to Vox:

An infographic of the Moon's passage across the Sun over time.

The graphic makes it look like the sky will be quite dark even in Alaska, but that won't really be the case. In the path of the total eclipse, it will get dark and you'll be able to see a few stars, but elsewhere, the partial eclipse will only change the color of the sky slightly. Even a little bit of Sun is still really bright.

Input your own ZIP code over at Vox, and don't forget to grab your eclipse glasses before you look up.


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