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# Longitude and Latitude: Why Does the Prime Meridian go Through Greenwich, England? (PowerPoint Presentation Sample)

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this was a presentation prepared for longitude and latitude for submitting to phd teacher.

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1974850-575945THE
LONGITUDE AND LATITUDE
On this image of the Earth, the horizontal (blue) lines represent lines of latitude and the vertical (pink) lines represent lines of longitude. The horizontal gray line represents the equator and the vertical gray line represents the Prime Meridian; both of these lines represent 0 degrees. The North Pole, at the top of the image, is where all of the lines of longitude come together.Click on image for full sizeImage Courtesy of Dennis Ward/UCAR
The most common way to locate points on the surface of theÂ EarthÂ is by standard, geographic coordinates called latitude and longitude. These coordinates are measured in degrees and represent angular distances calculated from the center of the Earth. On maps and globes, these are drawn as imaginary lines of latitude and longitude and help us determine locations.
LatitudeÂ describes the distance from the Earth's equator, with 0 degrees being the equator. The North Pole is +90 degrees and the South Pole is -90 degrees. Latitude is also described as being either â€œNorthâ€ or â€œSouth,â€ depending on the position in relationship to the equator. The equator divides the Earth into Northern and Southern Hemispheres. A line connecting all the points with the same latitude value is called a line of latitude.
LongitudeÂ is the measurement either east or west from the Prime Meridian, a line of longitude which runs between the poles and through Greenwich, England. Longitude increases as you leave the Prime Meridian (0 degrees) going east (0 to 180 degrees) and decreases as you head west (0 to -180 degrees), until they meet at 180 degrees. The Prime Meridian divides the Earth into Eastern and Western Hemispheres.
* Why does the Prime Meridian go through Greenwich, England?
* It could be anywhere, but in the mid-1800s the Royal Observatory in Greenwich, England was well known for keeping time, and because the time is the same all along that line of longitude, it was decided that the Prime Meridian would go through Greenwich. TheÂ antipodalÂ meridian of Greenwich is both 180Â°W and 180Â°E. The zero/zero point is located in the Gulf of Guinea about 625Â km south of Tema, Ghana.
* Here's an example of the latitude and longitude of a specific location. New York, NY, USA has a latitude of 40.75793 degrees (north) and a longitude of -73.98551 (west). This means New York, NY is approximately 41 degrees north of the equator and approximately 74 degrees west of the Prime Meridian.
A region'sÂ latitude has a great effect on its climateÂ because latitude determines the amount ofÂ solar energyÂ a region receives. Low latitude locations, on or near the equator, are called the tropics. It's warm there because roughly the same amount of sunlight is received year-round. High latitude locations, at or nearÂ the poles, have a cold, polar climate. The closer you are to one of Earth's poles, the less sunlight there is during winter days. Mid-latitude climates in the areas between the tropics and polar regions have several distinct seasons throughout the year because the amount of sunlight changes from summer to winter.
The Earth was split into lines of latitude and longitude in order to help withÂ navigation. Latitude lines are parallel lines around the Earth moving from north to south. A person's latitude determines how far north or south they are in degrees. At the equator the latitude is 0 degrees and it is 90 degrees at the poles. Longitude lines, or meridians, run around the Earth from east to west. The meridian that runs through Greenwich in London is set at 0 degrees longitude for historic reasons.
Latitude is equal to the angle of theÂ SunÂ relative to a stick placed vertically in the ground at noon, when the Sun is highest in the sky. At the equator the Sun will be perpendicular to the ground, but as you move away, the Sun will seem tilted because you are on a surface which is tilted. At the pole you are standing at a 90 degree angle to how you would be standing on the equator and the Sun's orbit is tilted as such.
This is not strictly true however, because the Earth is titled at an angle of just over 20 degrees and the side of the Earth that is tilted towards the Sun changes over the course of a year. The seasons change because the light of the Sun comes from different angles. Summer happens on the side of the Earth that is tilted towards the Sun and winter on the side that is tilted away. It is hotter in the summer for the same reason that it is hotter at the equator, the the light looses less energy -Â meaning it has a shorter wavelength and is therefore bluerÂ - when it travels through less atmosphere.
In order to measure your true latitude you need to know the angle the Earth is tilted by and know whether you are on the side of the Earth tilted towards or away from the Sun, which can be worked out if you know the date. Greek astronomerÂ Eratosthenes first measured the tilt of the Earth in about 240 BC. This can be done by measuring the angle of the Sun on the summer and winter solstices. The summer solstice occurs when the part of the Earth you are on is tilted towards the Sun at the highest possible angle. The winter solstice is the opposite; the part of the Earth you are on is tilted away from the Sun at the highest possible angle. The difference between these two values is twice the angle the Earth is tilted by. In the northern hemisphere, latitude can be calculated at night by looking at the angle of the North Star. On the North pole it will be straight above you, 90 degrees from the horizon. At the equator it will be straight ahead, 0 degrees from the horizon.
In about 127 BC Greek astronomerÂ HipparchusÂ showed that the Sun's path around the ecliptic is gradually changing. The constellation the Sun appears in during any particular month moves backwards once about every 2000 years, the exact time depending on the size of the constellation. Hipparchus discovered this by recording the location of several stars and seeing how they had moved with respect to the path of the Sun since they were measured by 3rd century BC Greek astronomers Timocharis and Aristillus. During Hipparchus' time the vernal (spring) equinox - the date when incoming Solar energy is equal in both hemispheres three months after the winter solstice - occurred in Aries but by about 1 AD it has moved to Pieces. At this rate it will move into Aquarius in about 2600.
The four seasons;image
In order to know your full position you need East-West as well as North-South co-ordinates and for this you need to know your longitude.Â
* Longitude is much harder to calculate than latitude. Because the Earth rotates 360 degrees per day, or 15 degrees per hour, there is a direct relationship between the time the sun rises and sets and longitude. The Greenwich meridian is designated 0 degrees longitude and the Sun sets an hour earlier every 15 degrees east of this and will set one hour later every 15 degrees west. If you know the difference between the time of the sunset in your location and another known location, then you can work out how far east or west you are from there.
*
Hipparchus was the first to suggest that longitude could be calculated by comparing the time at different locations but the inaccuracy of timekeeping devices meant that longitude measurements were often very inaccurate.
The first clock was the sundial which measures the movement of the Sun across the sky by marking where its shadow falls. The obvious disadvantage of sundials - that they can only be used on a sunny day - meant that other methods were soon utilised such as water clocks where the time measured corresponds to how long it takes for water to flow from one place to another. Water clocks are known to have existed inÂ Babylon and Ancient Egypt, India and China. Sand clocks, candles and incense sticks were also used to measure constant time durations.
FINDING OUR POSITION ON THE EARTH
The position of the Sun, stars and Moon in the sky can all give information about a person's location on the Earth in other words their latitude and longitude.
* The latitude is the angle between the observers location, the centre of the Earth and the equator.
* As the Earth rotates the Sun, stars and Moon appear to move in the sky. The position of the Sun and Moon are further complicated because of the earth's movement around the Sun and also the Moons own motion. By far the simplest way of working out the latitude is with the stars.Â
* The Earth rotates around an axis going through the south and north pole. An observer on the Earth therefore sees the stars moving relative to him or her around this axis. It just so happens that in the northern sky there is a star very close to this imaginary axis and therefore this star does not appear to move very much. This is called the Pole Star or Northern Star.
Mike Leahy with Quadrant outside the prison filming site
* If the observer was on the North Pole the Pole star would be directly above him. Viewing over several hours he or she would see all the stars apparently rotating about the pole star. An observer situated on the equator (ie. on the 'band' around the Earth widway between the two poles) would just about see the pole star on the distant Northern Horizon and all the stars apparently moving about this point. For latitudes between the Norhern Pole and the equator the pole star has a different corresponding angle with respect to the northern horizon.
* This angle being equal to the latitude of the observer.
* By the way the Pol...
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