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# lecture07 - Earth-centered earth xed(ECEF The rotation of...

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Earth-centered earth fixed (ECEF) The rotation of the earth means that all of inertial space appears to be spinning around us at approximately one revolution per day. This means earth-relative positions and obser- vations will have to be converted to inertial space. The earth-relative coordinate system is called Earth-centered earth fixed (ECEF) . In order to discuss earth rotation, we need to investigate time and earth rotation in detail. L. Healy – ENAE404 – Spring 2007 – Lecture 7 (Feb. 15) 1

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Meridian line and transit of position A circle that passes through both poles is called a meridian . From an earth observer’s point of view, this line goes from the North horizon di- rectly overhead to the South horizon. Meridi- ans: An important particular meridian is the prime or Greenwich meridian , which passes over Greenwich, England. A local meridian is the meridian at a par- ticular point on the earth. An astronomical or space object transits an ob- server’s position when it crosses the observer’s meridian. Sun transit is called noon : A.M. (= Ante Meridian) and P.M. (= Post Meridian). Of course, when a star transits your position, it’s not the star that’s moving, it’s you. Satel- lites (and even planets) are a little more com- plicated because both they and you are mov- ing. L. Healy – ENAE404 – Spring 2007 – Lecture 7 (Feb. 15) 2
A day: sidereal and solar A day is one complete revolution of the earth on its axis. The question is, what’s a complete revolution? The sun transit (noon) to transit (noon) defines the solar day = 86400 seconds (24 hours exactly), The distant stars being in the same posi- tion from one day to the next defines the sidereal day = 86164.090524 seconds or slightly more than 23:56:04 (about 4 min- utes shorter than a solar day). There is a difference because the earth is mov- ing in its orbit as well as rotating, so it has to rotate a little extra to get the sun overhead again vs. getting the stars overhead again. L. Healy – ENAE404 – Spring 2007 – Lecture 7 (Feb. 15) 3

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Time zones The phase of the solar day cycle, or time of day, is dependent on longitude of your position on earth. Two points with different longitude have different meridian lines, so noon happens at different times. Making the local time exactly dependent on longitude would be quite inconvenient, so we divide the earth up into longitude “chunks” of approximately 15 called time zones . The civil time in each of these zones is the same. Typically, the center of the zone matches the solar time, so that from east boundary of the zone to the west, the civil and the solar times can differ by ± half an hour. L. Healy – ENAE404 – Spring 2007 – Lecture 7 (Feb. 15) 4
Universal Time The Universal Time (UT, or UTC) zone is the local time zone on the prime meridian. Time zones are named with their offset from the UT zone. Our time zone is Eastern Standard Time (EST), and is - 5:00 (5 hours behind UT) so that for example 10:00 EST is 15:00 UTC.

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