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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
Earth's First-Recorded Interstellar Visitor Gets Its Closeup—And a Name
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NASA/JPL-Caltech

In October, scientists using the University of Hawaii's Pan-STARRS 1 telescope sighted something extraordinary: Earth's first confirmed interstellar visitor. Originally called A/2017 U1, the once-mysterious object has a new name—'Oumuamua, according to Scientific American—and researchers continue to learn more about its physical properties.

Fittingly, "'Oumuamua" is Hawaiian for "a messenger from afar arriving first." 'Oumuamua's astronomical designation is 1I/2017 U1. The "I" in 1I/2017 stands for "interstellar." Until now, objects similar to 'Oumuamua were always given "C" and "A" names, which stand for either comet or asteroid.

'Oumuamua moved too quickly through space to orbit the Sun, which led researchers to believe that it might be the remains of a former exoplanet. Long ago, it might have hurtled from an unknown star system into our solar system. Far-flung origins aside, new observations have led some researchers to conclude that 'Oumuamua is, well, pretty ordinary—at least in appearance.

'Oumuamua's size (591 feet by 98 feet) and oblong shape have drawn comparisons to a chunky cigar that's half a city block long. It's also reddish in color, and looks and acts like asteroids in our own solar system, the BBC reports. Its average looks aside, 'Oumuamua remains important because it may provide astronomers with new insights into how stars and planets form.

University of Wisconsin–Madison astronomer Ralf Kotulla and scientists from UCLA and the National Optical Astronomy Observatory (NOAO) used the WIYN Telescope on Kitt Peak, Arizona, to take some of the first pictures of 'Oumuamua. You can check them out below.

Images of an interloper from beyond the solar system — an asteroid or a comet — were captured on Oct. 27 by the 3.5-meter WIYN Telescope on Kitt Peak, Ariz.
Images of 'Oumuamua—an asteroid or a comet—were captured on October 27.
WIYN OBSERVATORY/RALF KOTULLA

U1 spotted whizzing through the Solar System in images taken with the WIYN telescope. The faint streaks are background stars. The green circles highlight the position of U1 in each image. In these images U1 is about 10 million times fainter than the faint
The green circles highlight the position of U1 in each image against faint streaks of background stars. In these images, U1 is about 10 million times fainter than the faintest visible stars.
R. Kotulla (University of Wisconsin) & WIYN/NOAO/AURA/NSF

Color image of U1, compiled from observations taken through filters centered at 4750A, 6250A, and 7500A.
Color image of U1.
R. Kotulla (University of Wisconsin) & WIYN/NOAO/AURA/NSF
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Space
8 Useful Facts About Uranus
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Uranus as seen by the human eye (left) and with colored filters (right).
NASA/JPL

The first planet to be discovered by telescope, Uranus is the nearest of the two "ice giants" in the solar system. Because we've not visited in over 30 years, much of the planet and its inner workings remain unknown. What scientists do know, however, suggests a mind-blowing world of diamond rain and mysterious moons. Here is what you need to know about Uranus.

1. ITS MOONS ARE NAMED AFTER CHARACTERS FROM LITERATURE.

Uranus is the seventh planet from the Sun, the fourth largest by size, and ranks seventh by density. (Saturn wins as least-dense.) It has 27 known moons, each named for characters from the works of William Shakespeare and Alexander Pope. It is about 1784 million miles from the Sun (we're 93 million miles away from the Sun, or 1 astronomical unit), and is four times wider than Earth. Planning a trip? Bring a jacket, as the effective temperature of its upper atmosphere is -357°F. One Uranian year last 84 Earth years, which seems pretty long, until you consider one Uranian day, which lasts 42 Earth years. Why?

2. IT ROTATES UNIQUELY.

Most planets, as they orbit the Sun, rotate upright, spinning like tops—some faster, some slower, but top-spinning all the same. Not Uranus! As it circles the Sun, its motion is more like a ball rolling along its orbit. This means that for each hemisphere of the planet to go from day to night, you need to complete half an orbit: 42 Earth years. (Note that this is not the length of a complete rotation, which takes about 17.25 hours.) While nobody knows for sure what caused this 98-degree tilt, the prevailing hypothesis involves a major planetary collision early in its history. And unlike Earth (but like Venus!), it rotates east to west.

3. SO ABOUT THAT NAME …

You might have noticed that every non-Earth planet in the solar system is named for a Roman deity. (Earth didn't make the cut because when it was named, nobody knew it was a planet. It was just … everything.) There is an exception to the Roman-god rule: Uranus. Moving outward from Earth, Mars is (sometimes) the son of Jupiter, and Jupiter is the son of Saturn. So who is Saturn's father? Good question! In Greek mythology, it is Ouranos, who has no precise equivalent in Roman mythology (Caelus is close), though his name was on occasion Latinized by poets as—you guessed it!—Uranus. So to keep things nice and tidy, Uranus it was when finally naming this newly discovered world. Little did astronomers realize how greatly they would disrupt science classrooms evermore.

Incidentally, it is not pronounced "your anus," but rather, "urine us" … which is hardly an improvement.

4. IT IS ONE OF ONLY TWO ICE GIANTS.

Uranus and Neptune comprise the solar system's ice giants. (Other classes of planets include the terrestrial planets, the gas giants, and the dwarf planets.) Ice giants are not giant chunks of ice in space. Rather, the name refers to their formation in the interstellar medium. Hydrogen and helium, which only exist as gases in interstellar space, formed planets like Jupiter and Saturn. Silicates and irons, meanwhile, formed places like Earth. In the interstellar medium, molecules like water, methane, and ammonia comprise an in-between state, able to exist as gases or ices depending on the local conditions. When those molecules were found by Voyager to have an extensive presence in Uranus and Neptune, scientists called them "ice giants."

5. IT'S A HOT MYSTERY.

Planets form hot. A small planet can cool off and radiate away heat over the age of the solar system. A large planet cannot. It hasn't cooled enough entirely on the inside after formation, and thus radiates heat. Jupiter, Saturn, and Neptune all give off significantly more heat than they receive from the Sun. Puzzlingly, Uranus is different.

"Uranus is the only giant planet that is not giving off significantly more heat than it is receiving from the Sun, and we don't know why that is," says Mark Hofstadter, a planetary scientist at NASA's Jet Propulsion Laboratory. He tells Mental Floss that Uranus and Neptune are thought to be similar in terms of where and how they formed.

So why is Uranus the only planet not giving off heat? "The big question is whether that heat is trapped on the inside, and so the interior is much hotter than we expect, right now," Hofstadter says. "Or did something happen in its history that let all the internal heat get released much more quickly than expected?"

The planet's extreme tilt might be related. If it were caused by an impact event, it is possible that the collision overturned the innards of the planet and helped it cool more rapidly. "The bottom line," says Hofstadter, "is that we don't know."

6. IT RAINS DIAMONDS BIGGER THAN GRIZZLY BEARS.

Although it's really cold in the Uranian upper atmosphere, it gets really hot, really fast as you reach deeper. Couple that with the tremendous pressure in the Uranian interior, and you get the conditions for literal diamond rain. And not just little rain diamondlets, either, but diamonds that are millions of carats each—bigger than your average grizzly bear. Note also that this heat means the ice giants contain relatively little ice. Surrounding a rocky core is what is thought to be a massive ocean—though one unlike you might find on Earth. Down there, the heat and pressure keep the ocean in an "in between" state that is highly reactive and ionic.

7. IT HAS A BAKER'S DOZEN OF BABY RINGS.

Unlike Saturn's preening hoops, the 13 rings of Uranus are dark and foreboding, likely comprised of ice and radiation-processed organic material. The rings are made more of chunks than of dust, and are probably very young indeed: something on the order of 600 million years old. (For comparison, the oldest known dinosaurs roamed the Earth 240 million years ago.)

8. WE'VE BEEN THERE BEFORE AND WILL BE BACK.

The only spacecraft to ever visit Uranus was NASA's Voyager 2 in 1986, which discovered 10 new moons and two new rings during its single pass from 50,000 miles up. Because of the sheer weirdness and wonder of the planet, scientists have been itching to return ever since. Some questions can only be answered with a new spacecraft mission. Key among them: What is the composition of the planet? What are the interactions of the solar wind with the magnetic field? (That's important for understanding various processes such as the heating of the upper atmosphere and the planet's energy deposition.) What are the geological details of its satellites, and the structure of the rings?

The Voyager spacecraft gave scientists a peek at the two ice giants, and now it's time to study them up close and in depth. Hofstadter compares the need for an ice-giants mission to what happened after the Voyagers visited Jupiter and Saturn. NASA launched Galileo to Jupiter in 1989 and Cassini to Saturn in 1997. (Cassini was recently sent on a suicide mission into Saturn.) Those missions arrived at their respective systems and proved transformative to the field of planetary science.

"Just as we had to get a closer look at Europa and Enceladus to realize that there are potentially habitable oceans there, the Uranus and Neptune systems can have similar things," says Hofstadter. "We'd like to go there and see them up close. We need to go into the system." 

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