An orbiting body can remain in continuous gravitational free fall.
Key knowledge
An orbiting satellite moves under gravity while its sideways motion can keep it from reaching the surface. In the Newtonian two-body model with inverse-square attraction, a bound non-radial relative orbit is an ellipse. Such an orbit does not require continuous rocket thrust.
Conditions and scope
The ellipse statement assumes point masses, or the exterior field of a spherical central mass, and nonzero orbital angular momentum.
It excludes atmospheric drag, other bodies, non-spherical gravity, thrust and relativistic corrections; real orbits can change.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
NASA/JPL's free-fall account and the textbook's Newtonian inverse-square two-body orbital model; no trajectory was newly calculated.
2026-10-10 · English free-fall and bound-orbit scope checked against NASA/JPL and University Physics section 13.5; radial motion and general potentials are excluded.
Foundational review
2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.
Sources from other places
Scope or convention differs
Planetary elliptical-orbit teaching and satellite free fall.
This comparison does not independently derive the general two-body ellipse or its sideways-motion conditions.
National Council of Educational Research and Training (NCERT)
A dark opaque planet crossing a uniformly bright stellar disk removes light in proportion to its projected area while fully superimposed on the disk. The ideal fractional depth is δ=(Fout−Fin)/Fout=(Rp/Rstar)². This measures a radius ratio; an absolute planet radius requires an independently constrained stellar radius. Repeated dimming needs validation against alternative causes.
Orbital geometry and period determine whether and when a transit occurs.
Away from eclipse, sunlight illuminates about half of the Moon. Its phase describes the illumination pattern of the apparent lunar disk as the Sun–Earth–Moon geometry changes, including the waxing or waning orientation. The phase cycle is about 29.5 days, longer than its roughly 27.3-day orbit relative to distant stars. Approximately synchronous rotation keeps broadly the same lunar hemisphere facing Earth, while libration reveals somewhat more than half over time.
The phase cycle is relative to the changing Sun–Earth direction; an orbit relative to distant stars is a different period.