Sky science
Hill Sphere
Direct answer
Hill Sphere: A Hill Sphere approximates the region where a smaller body can retain satellites against a larger body’s tidal influence.
It estimates a body’s local gravitational domain.
$0 today, then $12/month after 7 days. Cancel anytime.
At a glance
- Found in
- Astronomy & Skywatching
- Related
- Mean Motion Resonance, Lagrange Point, Roche Limit, Tidal Locking
It estimates a body’s local gravitational domain.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
What does your own chart say about you?
Choose love, work or purpose. Your reading is written from your chart. Your reading uses your own birth details.
Walk through Alex’s whole example first
The Road of AgesSee the turning points of your whole life, from your birth date
See your road's shape
Prefer to explore first?
The Road of AgesThe turning points of your whole life, from your own chart
Walk my road
Free, by email
Your daily Almanac letter
One short letter at 7 each morning: the day’s theme read for your own chart, and one thing to do with it.
The story behind Hill Sphere
George William Hill’s lunar work led to this useful restricted-problem region.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Hill Sphere matters
Hill Sphere helps assess satellite stability and close-encounter dynamics.
Hill Sphere in practice: an example
A moon generally needs to orbit well inside its planet’s Hill sphere for long-term stability.
Where readings of Hill Sphere go wrong
The Hill sphere is not a hard physical shell or exclusive zone of gravity.
Sources and editorial method
Research references used for this entry include:
- NASA Science: Universe Glossary - NASA definitions for common stellar, galactic, orbital, and high-energy astronomy terms.
- NASA Science: Solar System Exploration - NASA overview of planets, moons, small bodies, rings, and the structure of the Solar System.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
Estimate a world’s satellite domain
A definition tells you what it is; your own birth chart tells you what it means for you. The Oracle casts your real chart and answers for your love, your work, and your year ahead, in conversation. It opens with the 7-day Seeker trial.
Speak with the OracleRelated terms
More in Astronomy & Skywatching
- Phase Angle
- Reference Epoch
- Solar Prominence
- True Anomaly
- Apparent Magnitude
- Brown Dwarf
- Elliptical Orbit
From the reference page to your own chart
Every word here
is in your own chart.
- A personal reading about love, work or purpose, written from your chart
- A practical next step to consider
- It opens with 7 days free, with the AI Oracle and daily guidance
$0 today, then $12/month after 7 days. Cancel anytime.
Common questions
What is a Hill Sphere?
A Hill Sphere approximates the region where a smaller body can retain satellites against a larger body’s tidal influence.
What do people get wrong about a Hill Sphere?
The Hill sphere is not a hard physical shell or exclusive zone of gravity.
Free, by email
The Almanac, in your inbox at 7
One short letter at 7 each morning: the day’s theme read for your own chart, and one thing to do with it.
