Welcome to Foundations of Astronomy
An ephemeris cannot produce a single number until certain settings are defined. A planet's position is always stated relative to something: the point it is seen from, the circle it is measured along, and the zero the count starts from. Until those are fixed, there is nothing yet to compute.
Four settings do most of that work:
| Setting | The choices |
|---|---|
| Location | A latitude, longitude, height and time zone |
| Zodiac | Tropical or Sidereal |
| Coordinate origin | Geocentric, Topocentric or Heliocentric |
| Reference plane | Ecliptic or Equatorial |
Between them they shape every number the ephemeris returns, and this site explains what each one means.
What this is
The site is a set of short pages, each covering a single idea, grouped into four categories, one for each setting. The pages document no software; they explain the astronomy an ephemeris assumes, so that each of these settings can be understood rather than guessed at.
An ephemeris has other settings besides these four, such as the time scale its positions are given in, and whether those positions are true or apparent. Each of them will have its own category here once it is written.
Geometry does the explaining here. Every idea on this site is a matter of geometry, and geometry can be drawn. Where another text would give a spherical trigonometry identity, this one gives a picture: a sphere with the relevant circles drawn on it, or a fence with a planet hovering above 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 runs from the concrete to the abstract:
- 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 you came for one setting in particular, the sidebar will take you straight to it, though its pages may use terms defined earlier on.
A note on the diagrams
Nearly every page carries at least one diagram, and they are worth reading slowly. Much of this subject comes down to looking at a flat drawing and seeing the sphere it represents, and that comes with practice, from looking at a good many 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.