Our summer almanac 25.06.2010 – 01.07.2010

Knowing where we are and what the time is
In the constellation Gemini the star known as Pollux, named after one of the twin sons of Zeus, is No 21 among the “navigational stars”. Telling the time and finding out where we are connected in the first clock faces, based as they are on the astrolabe, an instrument that can be used to work out our latitude position by measuring the angles of sun, moon and stars.

Astroclocks and Astrolabes
The first mechanical astronomical clocks were influenced by the astrolabe; in many ways they could be seen as clockwork astrolabes designed to produce a continual display of the current position of the sun, stars, and planets. For example, Richard of Wallingford’s clock (c. 1330) consisted essentially of a star map rotating behind a fixed rete, similar to that of an astrolabe.

Many astronomical clocks, such as the famous clock at Prague, use an astrolabe-style display.

Celestial Spheres
The design of astrolabes, and the first astronomical clocks, are made up the many rings that show the movements in the heavens. In Greek antiquity the ideas of celestial spheres and rings first appeared in the cosmology of Anaximander in the early 6th century BC. Following Anaximander, his pupil Anaximenes (c. 585–528/4) held that the stars, sun, moon and the planets are all made of fire. He thought the stars are fastened on a revolving crystal sphere, the but the sun, moon and planets, and also the Earth, all just ride on air like leaves because of their breadth. After Anaximenes, Pythagoras, Xenophanes and Parmenides all held that the universe was spherical. And much later in the fourth century BC Plato’s Timaeus proposed that the body of the cosmos was made in the most perfect and uniform shape, that of a sphere containing the fixed stars. So the celestial spheres, or celestial orbs, became the fundamental entities of the cosmological models developed by Plato, Eudoxus, Aristotle, Ptolemy, and others. In these celestial models the stars and planets are carried around by being embedded in rotating spheres made of an aetherial transparent fifth element (quintessence), like jewels set in orbs.

The astronomer Ptolemy (fl. ca. 150 AD) defined geometrical predictive models of the motions of the stars and planets in his Almagest and extended them to a unified physical model of the cosmos in his Planetary hypotheses.

In the middle ages in Europe the ideas went back to the idea that the Earth was the centre of the universe. Medieval Christians identified the sphere of stars with the Biblical firmament and sometimes posited an invisible layer of water above the firmament, to accord with Genesis.

Around a thousand years ago, the Arabic astronomer and polymath Ibn al-Haytham (Alhacen) developed the idea of Ptolemy’s geocentric epicyclic models as being like nested spheres. In chapters 15-16 of his Book of Optics, Ibn al-Haytham also discovered that the celestial spheres do not consist of solid matter.


The Earth orbits the Sun
We take our modern knowledge for granted, and yet it isn’t hard to imagine believing the world is in fact a flat disc, and that the sun rises over this disc and sets beneath it. And even if people knew for certain that the Earth was a sphere, it isn’t hard to understand how people might think that the Earth was at the centre of the Universe.

The name of Nicolaus Copernicus Warmiensis, called Copernicus, is synonymous with a cosmological revolution. Mikolaj Kopernik, as he is known in Poland, spent much of his life from 1506 making observations of the stars and the planets, using relatively simple astronomical instruments.

By 1512, while still making observations from the tower in which he lived, set in the thick defensive walls of Frauenburg cathedral (today Frauenburg is known as the city of Frombork) he had begun to develop a new theory about the movements of the planets. In 1530 he published a book called De Revolutionibus Orbium Coelestium, which explained that it was possible to calculate that the Earth was not at the centre of the universe, but like the other planets, the Earth went around the Sun.

This theory challenged all those ideas about the way universe worked that were handed down over the centuries by people from ancient times. Plato (ca.428 BC-ca.348 BC) believed that the stars were living beings, eternal and divine. He thought that the motions of these heavenly bodies must be perfectly circular, as such a perfect and beautiful geometrical form as the circle would be a natural form for heavenly things.

According to Aristotle (384 BC-322 BC), the Earth was spherical, and remained quite still at the centre of the universe. He thought that the boundary of the universe was a sphere of fixed stars, and that between this sphere and the Earth there were a series of 55 spherical shells made of a heavenly element called aether. This aether was invisible and weightless, and purer than fire. The planets, the Sun and the Moon were part of these aetherial shells, with the Moon being the shell-like sphere closest to Earth.

Nowadays we know that though the stars may look fixed in the sky and frozen in time, they are in fact in movement. If we could speed up time we would see all stars move, while the planets would each be a speeding blur. The stars would all move in different directions and at different speeds.

Clocks, chronometers, longitude and knowing where we are
A marine chronometer is a clock which is precise enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation.

Longitude is given as an angular measurement ranging from at the Prime Meridian to +180° eastward and −180° westward. The Greek letter λ is used to denote the location of a place on Earth east or west of the Prime Meridian.

Maps and longitude
Longitude is the geographic coordinate most commonly used in cartography and global navigation for east-west measurement. Constant longitude is represented by lines running from north to south. The line of longitude (meridian) that passes through the Royal Observatory, Greenwich, in England, establishes the meaning of zero degrees of longitude, or the Prime Meridian.


This map of Basingstoke is framed by a rectangle that shows along each of the sides the position of the latitude grid, and along the top and bottom the longitude positions.

Degrees and minutes
Each degree of longitude is sub-divided into 60 minutes, each of which divided into 60 seconds. Basingstoke is west of the Greenwich Meridian by 1 degree and 6 seconds of longitude. So maps are divided spatially into degrees, minutes and seconds, as clocks are divided into hours, minutes and seconds.

Latitude and longitude
A location’s position along a meridian is given by its latitude, which is identified by the north-south angle between the local vertical and the plane of the Equator. Unlike latitude, which has the equator as a natural starting position, there is no natural starting position for longitude. Therefore, a reference meridian had to be chosen. It was a popular practice to use a nation’s capital as the starting point, but other significant locations were also used. While British cartographers had long used the Greenwich meridian in London, other references were used elsewhere, including: El Hierro, Rome, Copenhagen, Jerusalem, Saint Petersburg, Pisa, Paris, Philadelphia, and Washington. In 1884, the International Meridian Conference adopted the Greenwich meridian as the universal Prime Meridian or zero point of longitude.

The importance of the measurement of longitude is vital, both to cartography and to provide safe ocean navigation. Mariners and explorers for most of history struggled to determine precise longitude. Finding a method of determining exact longitude took centuries, resulting in the history of longitude recording the effort of some of the greatest scientific minds.

Latitude was calculated by observing with quadrant or astrolabe the inclination of the sun or of charted stars, but longitude presented no such manifest means of study. Amerigo Vespucci was perhaps the first to proffer a solution, after devoting a great deal of time and energy studying the problem during his sojourns in the New World:

As to longitude, I declare that I found so much difficulty in determining it that I was put to great pains to ascertain the east-west distance I had covered. The final result of my labours was that I found nothing better to do than to watch for and take observations at night of the conjunction of one planet with another, and especially of the conjunction of the moon with the other planets, because the moon is swifter in her course than any other planet. I compared my observations with an almanac. After I had made experiments many nights, one night, the twenty-third of August, 1499, there was a conjunction of the moon with Mars, which according to the almanac was to occur at midnight or a half hour before. I found that…at midnight Mars’s position was three and a half degrees to the east.

By comparing the relative positions of the moon and Mars with their anticipated positions, Vespucci was able to crudely deduce his longitude. But this method had several limitations: First, it required the occurrence of a specific astronomical event (in this case, Mars passing through the same right ascension as the moon), and the observer needed to anticipate this event via an astronomical almanac. One needed also to know the precise time, which was difficult to ascertain in foreign lands. Finally, it required a stable viewing platform, rendering the technique useless on the rolling deck of a ship at sea.

In 1612, Galileo Galilei proposed that with sufficiently accurate knowledge of the orbits of the moons of Jupiter one could use their positions as a universal clock and this would make possible the determination of longitude, but the practical problems of the method he devised were severe and it was never used at sea. In 1714, motivated by a number of maritime disasters attributable to serious errors in reckoning position at sea, the British government established the Board of Longitude: prizes were to be awarded to the first person to demonstrate a practical method for determining the longitude of a ship at sea. These prizes motivated many to search for a solution.

John Harrison, a self-educated English clockmaker then invented the marine chronometer, a key piece in solving the problem of accurately establishing longitude at sea, thus revolutionising and extending the possibility of safe long distance sea travel.

The first true chronometer was the life work of this one man spanning 31 years of persistent trial and error that revolutionized maritime navigation. Though the British rewarded John Harrison for his marine chronometer in 1773, chronometers remained very expensive and the lunar distance method continued to be used for decades. Finally, the combination of the availability of marine chronometers and wireless telegraph time signals put an end to the use of lunars in the 20th century.


Near Castle Hill School 01.07.2010

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: