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What Is Greenwich Mean Time?

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Getty Images

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.

New NASA Satellite Called TESS Could Discover Thousands of New Planets

Since NASA’s Kepler spacecraft launched in 2009, the space agency has found and confirmed a whopping 2343 new planets. Of those, 30 are considered to be situated in a “habitable zone,” an area in which a planet’s surface could theoretically contain water.

A new satellite, set to launch today, is expected to find thousands more planets outside of our solar system, known as exoplanets. TESS, short for the Transiting Exoplanet Survey Satellite, is NASA’s latest effort to plumb the depths and darkness of outer space in search of other Earth-like planets—including those that could potentially support life.

TESS is slated to complete a two-year survey of the “solar neighborhood,” a general region which comprises more than 200,000 of the brightest nearby stars. To find these outlier planets, NASA scientists will be keeping an eye out for temporary changes in brightness, which indicate that a planet is blocking its host star.

According to Martin Still, the program scientist working on the TESS mission, the launch comes “with certainty” that TESS will find many nearby exoplanets. "We expect to find a whole range of planet sizes, between planets the size of Mercury or even the Moon—our Moon—to planets the same size as Jupiter and everything in between,” Still said in a NASA interview.

While the Kepler mission was considered a major success, NASA noted that most of the planets it recorded are those that orbit faint, faraway stars, making it difficult to conduct follow-up observations. The stars that TESS plans to survey will be 30 to 100 times brighter than those observed by its predecessor. This allows for newly detected planets and their atmospheres to be characterized more easily.

“Before Kepler launched, we didn't know for sure if Earth-sized planets existed,” Elisa V. Quintana, a NASA astrophysicist, told Reddit. “Kepler was a statistical survey that looked at a small patch of sky for four years and taught us that Earths are everywhere. TESS is building on Kepler in the sense that TESS wants to find more small planets but ones that orbit nearby, bright stars. These types of planets that are close to us are much more easy to study, and we can measure their masses from telescopes here on Earth.”

The most common categories of exoplanets are Earth- and Super Earth–sized masses—the latter of which are larger than Earth but smaller than Uranus and Neptune.

TESS is scheduled to launch from the Cape Canaveral Air Force Station in Florida on a SpaceX Falcon 9 rocket at 6:32pm EDT today.

For more information about TESS, check out this video from NASA.

J. Malcolm Greany, Wikimedia Commons // Public Domain
An Astronomer Solves a 70-Year-Old Ansel Adams Mystery
Ansel Adams circa 1950
Ansel Adams circa 1950
J. Malcolm Greany, Wikimedia Commons // Public Domain

Ansel Adams was a genius with a camera, but he wasn’t so great about taking notes. The famous 20th century landscape photographer did not keep careful records of the dates he took his photos, leading to some debate over the origin period of certain images, including Denali and Wonder Lake (below), taken in Denali National Park in Alaska sometime in the late 1940s.

A black-and-white photo of Denali as seen from across Wonder Lake
Denali and Wonder Lake
Collection Center for Creative Photography, The University of Arizona, © The Ansel Adams Publishing Rights Trust

To settle a debate about when the photograph (known as Mount McKinley and Wonder Lake until the mountain's name was officially changed in 2015) was taken, Texas State University astronomer Donald Olson looked to the sky, using astronomical hints to determine the exact date, time, and location it was shot. Olson—who has solved other cultural mysteries related to topics such as Edvard Munch's paintings and Chaucer's writing using the night sky—writes about the process in his new book, Further Adventures of the Celestial Sleuth.

Adams did take some technical notes during his photography shoots, writing down the exposure time, film type, filters, and other settings used to capture the image, but he wasn’t as meticulous about the more mundane parts of the shoot, like the date. However, during his research, Olson found that another photo, Moon and Denali, was taken the night before the image in question. Because that one featured the moon, he could use it to calculate the date of both images—once he figured out where Moon and Denali was taken.

The moon hangs in the sky over Denali in a black-and-white photo
Moon and Denali
Collection Center for Creative Photography, The University of Arizona, © The Ansel Adams Publishing Rights Trust

To do so, Olson used topographical features such as cirques, hollowed landforms carved by glaciers, that were visible in Moon and Denali to identify several areas of the park where Adams may have been working. He and his student, Ava Pope, wrote a computer program to calculate the view from each possible location along the park road Adams drove along during his trip, eventually determining the coordinates of the location where the photographer shot Moon and Denali.

He could then estimate, using the position of the waxing gibbous moon in the photo, the exact time —8:28 p.m. on July 14, 1948—that Moon and Denali was taken. Denali and Wonder Lake would have been taken the next morning, and Olson was able to calculate from the shadows along the mountain where the sun would have been in the sky, and thus, when the photo was taken.

The answer? Exactly 3:42 a.m. Central Alaska Standard Time on July 15, 1948.


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