> Tom Wagner asks: > >>Why are some polar orbit satellites moving in one direction and others in >>the same part of the sky going the opposite way? Does it only have to do >>with where they are launched? > >>Also is there a term used to describe these opposite directions? > > > You probably mean why when watching these satellites in the night sky some > of > them go northbound and other are southbound. Of course if a satellite > moves > north and reaches the North Pole there is no other way to go but south > (but > it does so on the opposite side of the Earth). If a particular (polar > orbiting) satellite can be seen northbound (this is called an ascending > pass) > or southbound (this is called a descending pass) depends on its orbit > plane. > The Iridium constellation for example uses six different orbit planes and > you > have always some satellites on ascending and some on descending passes > over > your sky (moving in various planes). > > Earth observing satellites are often in sun-synchronous orbits in which > the > orbit plane keeps its relation to the sun-direction. Such satellites will > always pass in a certain direction at certain times of the day. Envisat > for > example is in a descending morning orbit. It will always pass a particular > site southbound in mid-morning and northbound in mid-evening. So if you > observe Envisat in the evening sky it will always move from North to > South. > > Any polar orbit can be reached from any launch site. It just depends on > which > time of the day you launch (you simply wait until your launch site has > turned > so it is under the desired orbit plane). > I just took a look at an Iridium at Heavens-above. the orbit was listed as 86 degrees. I'm assuming that I won't always see that exact satellite as ascending, sometimes it could be descending for my location, right? Roger ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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