Determine Longitude

this video is another story of human innovation and accomplishment the story of the early explorers and the incredible system they created to enable global exploration you are probably familiar with latitude and longitude the coordinate system used to define locations on the surface of the earth this system has ancient roots it appears the early Greeks were the first to propose of global coordinate system their system evolved over thousands of years into the one we use today lines of latitude are drawn parallel to the equator defining north-south coordinates the equator is zero degrees lines of longitude radiate from the poles defining east-west coordinates zero degrees longitude passes through Greenwich England intersecting lines of latitude and longitude define a location for instance the Great Pyramid at Giza is located at latitude twenty nine point nine eight degrees north longitude thirty-one point one three degrees east determining your latitude and longitude today is pretty simple even most smartphones carry GPS technology displaying your location with precision let's look at how the early explorers and surveyors were able to determine these coordinates with only a clock and a knowledge of the solar system and the night sky determining latitude is relatively simple in the northern hemisphere your latitude is the angle between the horizon and Polaris the North Star Polaris it's apparently motionless directly above the Earth's North Pole there is an equivalent celestial method for the southern hemisphere the first Portuguese explorers with an eye on Polaris crossed the Atlantic following lines of latitude you can easily create a device to measure this angle viewer inclinometer video for instructions on measuring the angle to Polaris using an inclinometer at my location reveals an angle to Polaris of 45 degrees my latitude is 45 degrees north with latitude determined we will now turn to determining longitude this is not easy humankind has struggled with the longitude question for thousands of years the problem is the rotating earth unlike latitude there is no apparent motionless star no Polaris to define longitude with all the stars appear to be in motion from the east-west perspective of longitude by the 18th century all seafaring nations were focused on the longitude problem if the Stars were to play a role in determining longitude some method or device for determining the earth's position at any time in its rotation was needed in the late 1700s just such a device appeared with the unveiling of the world's first precision marine chronometer John Harrison an English clock maker had accomplished what many of the most celebrated minds of the era couldn't he had solved the longitude problem I won't dwell on the story of Harrison and his clock but it is a dramatic and inspiring story if you get a chance Reed Davis obols book longitude an amazing story his clock was the first timekeeper to reliably and precisely keep time on a pitching rolling ship his chronometer revolutionized navigation one way to think of a clock like Harrison's is that it is a mechanical model of the rotating earth accurately following the 24-hour period of a day with this device it now became possible to determine longitude from any location on the planet here's how this works we'll use a clock and the Sun the first line of longitude 0 degrees and also called the Prime Meridian passes through Greenwich England this is an arbitrary location agreed upon by a majority of nations this line of longitude extends from the north to the South Pole passing through Greenwich when solar noon arrives at Greenwich England the Sun is at its highest altitude and at this moment it sits directly above the Prime Meridian 0 degrees longitude this is solar noon or Sun transit time some of you all know that solar time and clock time are sometimes not the same we will discuss that anomaly later for this demonstration we will assume that solar noon and clock noon at Greenwich are the same clock time and solar time at Greenwich agree they both display 12:00 noon as the Earth rotates the Sun no longer sits above the prime meridian it is now sweeping past other lines of longitude the Earth rotates 360 degrees in 24 hours that's 15 degrees per hour one hour after noon and Greenwich the noontime Sun now sits directly above 15 degrees of longitude two hours later it sits above thirty degrees of longitude five hours later solar noon is approaching my neighborhood here in eastern North America we are in a time zone Eastern Standard Time that puts us five hours behind Greenwich Time 12:00 noon here translates to five pm at Greenwich you may see where we are going with this if I can determine the time of solar noon the moment when the Sun is highest in the sky at my location I will know how many degrees west of Greenwich I am located my longitude I can use a watch to determine time and here is how we will determine solar noon at solar noon the Sun sets at its highest above the horizon for that day this means it casts the shortest shadow at that time we are going to determine o'clock time for solar noon by monitoring the length of a shadow cast by the Sun this is the equipment I used the short post casts a shadow onto a sheet of card the card needs to be secured and level to collect data note and record the time then mark the end of the shadow I started at 10 a.m. plenty early the first few recordings were 20 to 30 minutes apart as we got closer to noon o'clock time I increased data collection to every 5 minutes this is the important data I finished data collecting at 2:00 p.m. but concentrating on 20 minutes before and after clock noon would have sufficed next step is to determine the time for the shortest shadow I drilled a hole in the zero mark of this metal ruler and slid it over the post the length from post and of shadow is easy to determine my best estimate here shows the shortest shadow occurred at 1210 to 5-minute marks past 12 now to calculate our longitude we have to relate our local time for the solar noon calculation to the time and Greenwich we know that our time here Eastern Standard Time is 5 hours behind Greenwich Time so solar noon here occurred at 5:10 p.m. Greenwich Time this means the earth rotated for 5 hours and 10 minutes from zero longitude and Greenwich at 15 degrees per hour 5 hours and 10 minutes translates to 77 point five degrees of longitude according to my GPS my longitude is actually seventy six point nine three degrees west my calculation missed by half a degree that's an error of approximately 80 kilometers on the surface of the earth here is the source of some of this error it turns out that solar time varies from plot time throughout the year if you're interested in why this happens viewer video the equation of time on the day I collected this solar data that's December 30th solar time was 2 minutes and 15 seconds behind clock time this means that solar noon at Greenwich actually occurred at 12:02 15 p.m. not 12 this means the earth actually rotated for five hours 7 minutes and 45 seconds from Greenwich doing the math at 15 degrees per hour this time we get seventy six point nine four degrees very close within point zero one degrees of our actual longitude an error of less than one kilometer not bad for watching shadows in recording time the early navigators and surveyors not only use the Sun they were knowledgeable astronomers working at night they used overhead stars and the best timekeepers of the period to determine longitude again the time of transiting stars was compared to the time at Greenwich to calculate longitude the maps produced by the early surveyors using these methods are remarkable visit hila Road comm slash longitude for more information about the people and math behind this story and for a complete list of our science and technology videos visit hila Road comm follow the videos link you

Channel: ScienceOnline

Views: 829,386

Rating: 4.8868427 out of 5

Keywords: longitude, navigation, explorers, polaris, John Harrison, Greenwich, prime meridian, latitude, solar noon, time zones, board of longitude, Dava Sobel, true south

Id: b7yoXhbOQ3Y

Length: 11min 10sec (670 seconds)

Published: Wed Jan 07 2015