Geostationary orbit
● VERBATIM — examined word-for-wordA geostationary orbit is a circular orbit in which a satellite appears stationary when observed from a fixed point on the Earth's surface.
What this actually means
Learn the sentence exactly. The key phrase is 'appears stationary to an observer on the Earth's surface'. The satellite is very much moving, at about 3.1 km s⁻¹, it just keeps pace with the ground beneath it.
The word 'appears' is doing real work. If you write 'the satellite is stationary', you have said something false and the mark goes. Relative to the Earth's centre it is in continuous circular motion.
Geostationary is not the same as geosynchronous. Geosynchronous only requires a 24-hour period; geostationary additionally requires the equatorial plane and the west-to-east sense, so the satellite hovers over one fixed point rather than tracing a figure of eight.
There is exactly ONE geostationary radius, r = 4.23 × 10⁷ m from the Earth's centre, which is an altitude of about 3.6 × 10⁷ m (35 800 km). It is fixed by T = 24 h through Kepler's third law, so it is the same for every satellite regardless of mass.
Application marks usually ask why it is useful: a ground aerial can be pointed once and left alone, giving permanent line of sight for telecommunications and TV broadcasting.
Writing that the satellite 'is stationary' rather than 'appears stationary' to a ground observer.
Prove it — watch it be true
- Open the geostationary panel with all three conditions satisfied.
- Watch the ground track and confirm it stays pinned to a single point on the equator.
- Read the orbit radius and period and check you get 4.23 × 10⁷ m and 24 h.