
Index 5c. Coordinates 6. The Calendar 6a. Jewish Calendar 7.Precession 8. The Round Earth 8a. The Horizon 8b. Parallax 8c. Moon dist. (1) 8d. Moon dist. (2) 9a. Earth orbits Sun? 9b. The Planets 9c. Copernicus to Galileo 
Imagine you are standing at an elevation of h meters above the ocean and looking out across the water. What is the distance D to the horizon? It can be calculated, but you need to know the radius R of the Earth.
Your line of sight to the horizon is a tangent to the Eartha line which touches the sphere of the Earth at just one point, marked B in the drawing here. If O is the center of the sphere of the Earth, by a wellknown theorem of geometry such a tangent is perpendicular to the radius OB, that is, it makes a 90^{o} angle with it. It follows that the triangle OAB obeys the theorem of Pythagoras, which here can be written (OA)^{2} = (AB)^{2} + (OB)^{2} or if the length of each line is spelled out (R + h)^{2} = D^{2} + R^{2} By an algebraic identity (derived in the "mathematical refresher"), the lefthand side equals R^{2} + 2Rh + h^{2}, giving R^{2} + 2Rh + h^{2} = D^{2} + R^{2} 
If now R^{2} is subtracted from both sides and the remaining terms on the left are rearranged h(2R + h) = D^{2} The diameter 2R of the Earth is much bigger than h, and therefore the error introduced if (2R+h) is replaced by 2R is very, very small. Carrying out this replacement gives 2Rh = D^{2} where √ stands here for "square root of". This equation lets one calculate Din kilometers, if h and R are given in kilometersand one then can simplify it even furtherfurther: √(2Rh) = ( √(2R)) x (√h) with the two square roots multiplied. Using R = 6371 km, SQRT (2R) = 112.88, giving D = 112.88 km √ (h) If you are standing atop a mountain 1 km high, h = 1 km and your horizon should be 112.88 km away (we neglect the refraction of light in the atmosphere, which may increase this value). From the top of Mauna Kea on Hawaii, an extinct volcano about 4 km high (also the site of important astronomical observatories), the horizon should be about twice as distant, 226 km. On the other hand, standing on the beach with your eyes 2 meters = 0.002 km above the water, since SQRT(0.002) = 0.04472, the horizon is only 5 km distant. The calculation should also hold the other way around. From a boat on the ocean you should begin seeing the top of Mauna Kea after you pass a distance of 226 km (again, not accounting for refraction). On November 15, 1806, Lieutenant Zebulon Pike of the US Army, leading an exploration party across the plains of the midwestern US, saw through his spyglass the top of a distant peak, just above the horizon. It took his party a week to cover the 100 miles to the mountain, which is now known as Pike's Peak, one of the tallest in Colorado. Pike actually tried to climb to its top, but the snow and the unexpected height of the mountain forced him back.
Questions from Users: What is the distance to the horizon on an asteroid? Also: To settle a bet about the distance to the horizon. One more: Distance to the horizon on Mars. *** Rope stretched across a long lake... 
Further Exploring: #8b. Parallax
Next Regular Stop: #9a Is Earth Revolving around the Sun?
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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 1 April 2014
Curators: Robert Candey, Alex Young, Tamara Kovalick
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