Latitudes and Longitudes - CiÃªncia Viva

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10 17 th century Portuguese astrolabe. • Use your quadrant and your protractor to measure the height of the noon sun. Compare the values obtained. You can calculate the noon sun time with the aid of your sundial. Based on experiments with stick shadows and much observation of the Sun and the North Star, instruments were devised to calculate our position on the surface of the Earth. With the astrolabe and the quadrant, we can find out if we are closer to the north or to the south by measuring the angle of the North Star above the horizon or by measuring the angle of the sun above the horizon. It was these instruments that enabled the beginning of the great ocean voyages from the 15th century on. They enabled sailors to return to their point of origin as well as allowing them to make other voyages along the same routes. It was during the 16th century that Portuguese mathematician Pedro Nunes developed instruments which allowed navigators to draw accurate charts of their sea routes. The modern version of the astrolabe and the quadrant is the sextant, which is still used today by all boats that sail the seas. Pedro Nunes was born in Alcácer do Sal in 1502. He lived at the height of the Portuguese Discoveries and in 1529 he was appointed Royal Cosmographer by D. João III. In order to resolve some of the difficulties of navigation, Pedro Nunes invented several astronomical instruments, such as the universal ringdial, the solar compass and the nonius, nowadays called vernier. The first two calculate the height of the Sun in the sky and the third, when fitted to a quadrant can measure to fractions of a degree and accurately establish the height of a star. Some of these instruments were tried out successfully by D. João de Castro on his voyages to Goa and the Red Sea. The nonius was used and adapted by the astronomer Tycho Brahe to build two quadrants. Brahe's observations on astronomy have been the basis of the modern description of planet movement since the end of the 16th century.
We now know how to determine locations further north or further south on the Earth, in other words, we can determine the latitude of a location. To pinpoint our position on Earth, we need to know if we are further east or west. This means that we need to know the longitude. Now the Sun and the stars cannot help us. The idea of establishing our east-west position originated in Egypt with the Greek astronomer Ptolemy, who was born about 2170 years ago. Ptolemy decided to draw vertical circles around the North and South Poles and call them meridians. The position of these meridians was measured relative to the Prime Meridian (Zero Degrees), which passed through a chosen point on the Equator. For Ptolemy, the Prime Meridian passed through the Canary Islands. This, however, is largely a matter of choice and kings and ministers over the years have changed the Prime Meridian to pass through the Azores, Cape Verde, Rome, Paris, Philadelphia and London. Today, it passes through the Greenwich Observatory which is to the east of London. Defining the meridians, however, is not enough to determine the longitude of a location, we also need instruments. One possibility is to measure time with the aid of a clock. Determining the longitude of a location is based on the fact that the Earth spins full circle on its northsouth axis approximately every 24 hours. We need to know the time difference between the noon sun of our position and that of a reference position. These times are determined by the maximum height of the sun at both locations. As the Earth spins a complete circle (360 degrees) every twenty-four hours, for every hour of difference between noon at our location and that of our reference location, the Earth moves 15 degrees east or west. For example, to determine longitude at sea, we need to sail with an accurate time-keeper. Then when it is noon at your current location, establish what time it is at your port of origin. With the aid of navigation tables which show noon times for the port of origin for each day of the year, pilots can calculate the time differences between noon at their current location and at the port of origin. For each hour of difference, they are roughly 15 degrees east or west relative to their port of origin. As we can determine our latitude by looking at the angle of the sun above the horizon at midday, or at night by the angle of the North Star above the horizon, we know which parallel we are on. Knowing the parallel and the meridian, we can now pinpoint our position on the terrestrial globe. 40ºW 20ºW Meridians 0º 20ºE The Greenwich Meridian Zero degrees longitude Equator The longitude scale is marked along the line of the Equator 11
Page 1 and 2: Latitudes and Longitudes AGÊNCIA N
Page 3 and 4: Index The Earth revolves around the
Page 5 and 6: Today using rockets and space ships
Page 7 and 8: Imagine that the Earth’s orbit is
Page 9 and 10: How to travel on Earth without gett
Page 11: By observing the regularity of the
Page 15 and 16: During the time of the Discoveries,
Page 17 and 18: If it was during the solstice, the
Page 19 and 20: It is by using data on the apparent
Page 21 and 22: The GPS Nowadays, longitude and lat
Page 23 and 24: • Here is an aerial photograph of
Page 25 and 26: In order to determine latitude we m
Page 27: Ciência Viva Agência Nacional par

Latitudes and Longitudes - CiÃªncia Viva

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