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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.

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Space
Here's Where You Can Watch a Livestream of Cassini's Final Moments
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It's been a road trip like no other. After seven years and 2.2 billion miles, the NASA orbiter Cassini finally arrived at the Saturn system on June 30, 2004. Ever since, it's been capturing and transmitting valuable data about the distant environment. From sending the Huygens probe to land on the moon Titan to witnessing hurricanes on both of the planet's poles, Cassini has informed more than 3000 scientific papers.

It's been as impressive a mission as any spacecraft has ever undertaken. And tomorrow, Cassini will perform one last feat: sacrificing itself to Saturn's intense atmosphere. Project scientists are deliberately plunging it into the planet in order to secure just a little more data—and to keep the spacecraft, which is running low on fuel, from one day colliding with a Saturnian moon that might harbor life.

Because it won't have time to store anything on its hard drive, Cassini will livestream its blaze of glory via NASA. The information will be composed mostly of measurements, since pictures would take too long to send. Instead, we'll get data about Saturn's magnetic field and the composition of its dust and gas.

"As we fly through the atmosphere, we are able to literally scoop up some molecules, and from those we can figure out the ground truth in Saturn’s atmosphere," Scott Edgington, a Cassini project scientist, told New Scientist. "Just like almost everything else in this mission, I expect to be completely surprised."

The action will kick off at 7 a.m. EDT on Friday, September 15. Scientists expect to say goodbye to Cassini less than an hour later. 

While you wait for Cassini's grand finale, you can check out some essential facts we've rounded up from Saturn experts. And keep your eyes peeled for a full recap of Cassini’s historic journey: Mental Floss will be in the control room at the Jet Propulsion Laboratory in Pasadena, California, to offer a firsthand account of the craft's final moments in space. 

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Big Questions
What Are the Northern Lights?
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Over the centuries, many have gazed up at one of the Earth’s most fascinatingly beautiful natural wonders: the Northern Lights. In the past couple of weeks, some lucky American stargazers have gotten the chance to see them from their very own backyards—and could again this week, according to Thrillist. But what are they?

Before science was able to get a read on what exactly was happening in the night sky, ancient tribes had their own theories for what caused the jaw-dropping light show. Many early beliefs had roots in religion, such as that the light was a pathway souls traveled to reach heaven (Eskimo tribes) or that the light was an eternal battle of dead warriors (Middle-Age Europe). Early researchers were a bit more reasonable in their approximations, and most surrounded the idea of the reflection of sunlight off the ice caps. In 1619, Galileo Galilei named the lights the aurora borealis after Aurora, the Roman goddess of morning, after concluding they were a product of sunlight reflecting from the atmosphere.

Today, scientists have come to the general agreement that the lights are caused by the collision of electrically charged solar particles and atoms from our atmosphere. The energy from the collisions is released as light, and the reason it happens around the poles is because that's where the Earth’s magnetic field is the strongest. In 2008, a team at UCLA concluded that “when two magnetic field lines come close together due to the storage of energy from the sun, a critical limit is reached and the magnetic field lines reconnect, causing magnetic energy to be transformed into kinetic energy and heat. Energy is released, and the plasma is accelerated, producing accelerated electrons.”

"Our data show clearly and for the first time that magnetic reconnection is the trigger," said Vassilis Angelopoulos, a UCLA professor of Earth and Space Sciences. "Reconnection results in a slingshot acceleration of waves and plasma along magnetic field lines, lighting up the aurora underneath even before the near-Earth space has had a chance to respond. We are providing the evidence that this is happening."

The best time to see the Northern Lights is during the winter, due to the Earth’s position in relation to the sun (shorter days means darker night skies). And by the way, it’s not just the North Pole that puts on a show—there are Southern Lights, too. There are also aurora borealis on other planets—including Mars—so rest assured that future generations born “abroad” will not miss out on this spectacular feat of nature.

Haven’t seen them yet? Traditionally, the best places to catch a glimpse of the Northern Lights are in Iceland, Sweden, Norway, Finland, Greenland, northern Canada, and Alaska. Maybe you'll get lucky this week and sneak a peek from your very own window. Check out Aurorasaurus for regular updates on where they are showing.

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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