GREENWICH MEAN TIME By: Chloe O’Connor 8gg

SOME HISTORY: So what is GMT (Greenwich Mean Time)? GMT goes back to before 1884, when Britain was a maritime nation. When on the sea, British (and other) sailors used a device called a marine chronometer, which was a portable and accurate clock you used whilst away. The chronometer was used to calculate longitude from the Greenwich meridian. It was a convention considered to have longitude, this was later internationally adopted by the International meridian Conference of They synced the chronometer to solar time, which was later discovered as GMT.

AND MORE… Eventually, the practise of the sync to solar time with chronometer, became combined with other nations, drawing from Nevil Maskelyne’s theory of lunar distances – based on observations at Greenwich. GMT was adopted across the United Kingdom (by the Railway Clearing House) in 1887, and almost all Railway’s the following year. This is where the term Railway time derived.

SO WHAT’S HAPPENING UP THERE…? The daily rotation of earth is slightly irregular, and is slowing down slightly. Atomic clocks (an electronic transition frequency) provide a much more stable accuracy. In 1972, GMT was replaced by UTC (Coordinated Universal Time) due to the use of atomic clocks. All is based on the rotation of the earth, and it is a modern continuation of GMT. In addition, it is also based on the earths rotation in relation to distant celestial (stars) objects. It also defines date and the exact time. Nasa: When in space, the astronauts set their watches to GMT as this defines a more accurate measure of the time; this is what world time is based on.

NEVIL’S THEORY Lunar distances: Nevil’ theory is based on the relatively quick movement of the moon across the sky. Completing a circuit of 360 degrees in 27.3 days. It has 3 steps t its method. Step one: Preliminaries: Almanac tables predict lunar distances between the centre of the Moon and the other body (see any nautical almanac from 1767 to c.1900). However, the observer cannot accurately find the centre of the Moon (and Sun, which was the most frequently used second object). Instead, lunar distances are always measured to the sharply lit, outer edge ("limb") of the Moon and from the sharply defined limb of the Sun. The first correction to the lunar distance is the distance between the limb of the Moon and its centre. Since the Moon's apparent size varies with its varying distance from the Earth, almanacs give the Moon's and Sun's semidiamter for each day (see any nautical almanac from the period).Additionally the observed altitudes are cleared of dip and semi diameter. Step two: Clearing: Clearing the lunar distance means correcting for the effects of parallax and atmospheric refraction on the observation. The almanac gives lunar distances as they would appear if the observer were at the centre of a transparent Earth. Because the Moon is so much closer to the Earth than the stars are, the position of the observer on the surface of the Earth shifts the relative position of the Moon by up to an entire degree. [ The clearing correction for parallax and refraction is a relatively simple trigonometric function of the observed lunar distance and the altitudes of the two bodies. Navigators used collections of mathematical tables to work these calculations by any of dozens of distinct clearing methods Step three: Finding the time: The navigator, having cleared the lunar distance, now consults a prepared table of lunar distances and the times at which they will occur in order to determine the Greenwich time of the observation.