Welcome to Foundations of Astronomy
Open a study in Aquila, or a chart in Compass, and before anything is calculated you are asked for four things:
| Setting | What you can choose |
|---|---|
| Location | Any location from your database |
| Zodiac | Tropical or Sidereal |
| Coord. Origin | Geocentric, Topocentric or Heliocentric |
| Ref. Plane | Ecliptic or Equatorial |
Four dropdowns. Between them they decide every number the application will hand back. This site explains what each one means.
What this is
Short pages, each covering a single idea, grouped into four categories — one per setting. Nothing here documents a product. It explains the astronomy the products assume, so that choosing from those dropdowns is a decision rather than a guess.
For the applications themselves, see the documentation for Aquila, Compass, Cygnus, or Ephy.
Geometry does the explaining here. Every idea on this site is fundamentally geometric, and geometry can be shown — so where another text would give you a spherical trigonometry identity, this one gives you a picture of a sphere with the relevant circles drawn on it, or a fence with a planet hovering over it.
How to read it
Categories are sequences, not reference lists. Each page assumes the ones before it, and each category assumes the ones above it in the sidebar. The order matches the order of the dropdowns themselves:
- Locations — where you are standing, and everything that fixes
- Zodiacs — twelve equal signs, and where to start counting them
- Coordinate Origins — the point a measurement is taken from
- Reference Planes — the circle a position is measured along
If the subject is new to you, read straight through from Why a Chart Needs a Place. If you came looking for one specific setting, the sidebar will take you there — you may just find the page leaning on a term defined earlier.
What you will be able to do
By the end of Locations you will know why a chart cast for a mountain summit and one cast for the valley below it agree, and why moving a few hundred miles north changes the sky permanently.
By the end of Zodiacs you will know why Tropical and Sidereal positions differ by about 25°, why that gap grows, and why switching between them leaves every aspect in your chart exactly where it was.
By the end of Coordinate Origins you will know why no planet is ever retrograde when seen from the Sun.
By the end of Reference Planes you will know why the same planet reaches the same degree at two different times depending on which plane you selected — sometimes hours apart, and for the slow outer bodies, years apart.
A note on the diagrams
Nearly every page carries at least one diagram. They are worth slowing down for. The single most useful skill in this subject is being able to look at a flat drawing and see the sphere it represents, and that skill comes only from looking at a lot of them.
Throughout, a solid line lies on the near side of the sphere, facing you. A dashed line of the same colour is the same circle continuing around the far side, seen through the sphere.