New Page 2

The Royal Observatory at Greenwich on the river Themes is one of the oldest astronomical institutions in Europe. Its construction was ordered by King Charles II, and it was designed by England's most famous architect, Christopher Wren. The observatory's express purpose was to solve the "longitude problem" in celestial navigation*. Construction was begun in 1675 and the entire structure, including observing rooms and living quarters, was completed one year later. The first Astronomer Royal (basically, the observatory director) was John Flamsteed. The star maps he and his staff produced were the most accurate in the world at the time, and he devised a system of stellar nomenclature which is still used today. Other famous astronomer royals include Edmond Halley, James Bradley, and George Airy. Today the observatory complex is a museum open to the public.


Prime Meridian of the World: The line of longitude which passes through the Greenwich Observatory is by common agreement longitude 0 degrees, the Prime Meridian. The day officially begins when it is midnight here. Further, this line officially divides the world into eastern and western hemispheres. In the first picture (left) we are looking north, so the western hemisphere is to the left, the eastern hemisphere to the right. The second picture (taken before the "line" was refurbished for the millennium) is a view toward the south.

A bevy of instruments spanning two centuries. The first three frames are of the Octagon Room, the original facility designed by Christopher Wren for astronomical observations. It offers a large, comfortable space with unobstructed views of the night sky. Here Flamsteed and his staff utilized the long refracting telescopes of his day (second picture) to pinpoint faint stars. The first picture in the series is of a more sophisticated refractor developed in the 18th century, while a quadrant, an instrument used to determine the elevation of a star above the horizon (the altitude), is shown in the third frame. The final picture is in James Bradley's Transit Room built in 1749. These instruments can move only in altitude, and their function is to determine the precise moment when a star reaches the meridian. These are the telescopes with which Bradley spent a good part of his career searching for stellar parallax. He never found it, but he did discover the "aberration of starlight", which was the first direct proof that the earth is moving.

As the earth turns, every star in the sky will pass through the local meridian once each day. The precise local time at which a particular star reaches the meridian depends on the time of year, and for a given day will be the same for all observers. These facts form the basis for celestial latitude determination. Suppose that a navigator at sea has determined that the bright star Vega has reached the meridian as seen from his ship at 8:00 p.m. local time. Further, suppose he has a clock on board which is set to Greenwich time. If this "Greenwich" clock reads 10:30 p.m. while his local clock reads 8:00 p.m., he can deduce that Vega was on the meridian in Greenwich two and one half hours earlier. Since It is known that the earth is rotating at a rate of 15 degrees per hour, a straightforward calculation gives the position of the ship as 2.5 x 15 = 37.5 degrees west of Greenwich. For this technique to be successful the navigator needs two things: (1) a very accurate clock and (2) a very accurate star map. Neither of these necessities existed in the late seventeenth century, and the Greenwich Observatory was founded to address both of these problems.