Earth and Moon Viewer -
(every time You refresh the site -
Credits: by John Walker
Views from Satellites: Keeping Elements Current
The Living Earth
The EARTH from the SUN
The EARTH from the MOON
by John Walker: Quotation -
When requesting a view from an Earth satellite, the name of the satellite and its orbital elements are specified in the text box below the “From satellite” check box, with “From elements below” selected. For satellites in Earth and Moon Viewer's satellite database, you may simply specify the name: current orbital elements will be filled in automatically from the database. For satellites not in the database, you can paste the orbital elements in NASA/NORAD “two line element” format into the box, but this is a poor idea for dynamic views you intend to use for an extended period of time. Orbits of Earth satellites are specified as “osculating elements” which “kiss” the actual orbit only for a limited period of time around the date they are published. Even if a satellite's orbit doesn't change, out of date orbital elements will result in errors computing its position and show it in the incorrect location. Errors accumulate faster for satellites in low Earth orbit, but even geosynchronous satellites will eventually be shown at the wrong latitude if obsolete elements are used.
By specifying just the name of a satellite in Earth and Moon viewer's database, you're guaranteed up to date orbital elements will be used to compute its position; the database is updated daily from the Celestrak orbital element repository. For example, here's a request which shows the current view from one of the satellites in the Iridium constellation, specifying just the name to cause Earth and Moon Viewer to fetch its orbital elements from the database at the time of the request:
<a href="http://www.fourmilab.ch/earthview/" target="_blank">
width="320" height="320" border="0"
alt="View from Iridium 77" />
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Dynamic Earth from Space