The whole objective of a satellite in a geostationary orbit is so that the satellite will be fixed in one spot in the sky so that a user of the system gets full time coverage with one satellite and doesn't have to have a tracking antenna. A geostationary orbit is not synchronous with the stars. To a ground observer the stars appear to move slowly across the sky so a satellite that would stay fixed relative to the stars would also appear to move across the sky, thus requiring a tracking antenna. The relationship between the sidereal day (23:56:04) versus the solar day (24:00:00) is explained by the fact that in the one day that it takes to make one earth revolution, the earth has moved in it's orbit of about 0.9863 degrees in it's one year (365 days) orbital (360 deg) revolution around the sun. The point of reference for a sidereal day is an imaginary point way out in deep space which is considered to be fixed because it is so far away. It is also called the First Point of Aires - along a line from the center of the earth through the equatorial plane at the point when the Sun passes from the southern hemisphere to the northern hemisphere on the first day of spring (vernal equinox). The point of reference for a solar day is the center of the sun. The math is available at http://scienceworld.wolfram.com/astronomy/SiderealDay.html There's also a diagram and explanation available at http://www.storm.ca/~tjones/sidereal.html Jeff Barker -----Original Message----- From: Robert G Fenske Jr [mailto:fenske@rgfpc.electro.swri.edu] Sent: Wednesday, April 03, 2002 3:25 PM To: SeeSat-L@satobs.org Subject: Re: 23h 56m vs 24h On Wed, 3 Apr 2002, Jonathan T Wojack wrote: > But what is the point in making a geosat synchronous with the stars? > It's not going to stay over an area of Earth (more or less). What is the > purpose for a geosat to track the stars? As others have essentially pointed out, being synchronous with the stars is equivalent to being synchronous with the Earth's rotation, which is 23h 56m. The reason we all use 24 hours for a day is that it takes the Earth an extra 4 minutes to rotate back to its position relative to the Sun as this exaggerated diagram will (I hope) show: /-\ 1 E------------|Sun| /\-/ / / / / (takes 4 more minutes to rotate to this line) / 2 E--------------- At position 1 the Sun is overhead for somebody. 23h 56m later the Earth has rotated around once so the person is again pointing in the same spatial direction (and a geo sat with a 23h 56m period will stay with this person). But the Earth has moved to position 2 in its solar orbit so it takes 4 more minutes (on average) for it to rotate so that the Sun is overhead for the person. Robert Fenske, Jr. rfenske@swri.edu Sw |The Taming the C*sm*s series: Southwest Research Institute /R---\ | Signal Exploitation & Geolocation Div | I | |"The Martian canals were the San Antonio,Texas USA ph:210-522-3931 \----/ | Martians' last ditch effort." ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www.satellite.eu.org/seesat/seesatindex.html
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