What a Reference Plane Is
The last of the four settings offers two values, Ecliptic and Equatorial, under a label that says little about what it does.
It decides which circle in the sky a position is measured along, and its effects reach further than those of the other three settings: later in this category it moves the date of a transit by years.
Along, and away
Every coordinate pair on this site is built the same way, starting from a great circle:
- Measure along it from an agreed zero. That is the first number — longitude, right ascension, azimuth.
- Measure away from it, perpendicular, towards one of its poles. That is the second — latitude, declination, altitude.
With those two numbers, every point on the sphere has exactly one pair. The great circle supplying them is the reference plane.
You have already used three of these without the name:
| Reference circle | Along | Away |
|---|---|---|
| The horizon | azimuth | altitude |
| The celestial equator | right ascension | declination |
| The ecliptic | celestial longitude | celestial latitude |
The pattern is the same in all three rows, and only the circle changes.
Why it has to be a great circle
This is why great circles were given a page of their own in Locations.
A reference circle needs poles — two points exactly 90° from every point on it — because "measure away from the circle" means "measure towards its pole". Only a great circle has them. A small circle has a centre off the axis and no such pair of points, so there is no perpendicular direction to measure along and no second coordinate to be had.
It also needs to halve the sphere, so that the second coordinate runs cleanly from −90° to +90° with the circle itself at zero.
Why it is called a plane and not a circle
The setting is named for a plane rather than a circle, and the word is the more accurate of the two.
A great circle on a sphere is the trace of a flat plane cutting through the sphere's centre. The circle is what you see; the plane is what defines it. The ecliptic is the Earth's orbital plane, extended outward until it meets the sky; the celestial equator is the Earth's equatorial plane, extended the same way.
Holding on to the plane rather than the circle pays off later. When two frames are compared, what is being compared is the angle between two flat surfaces — and a body's position relative to a plane is something you can picture as a height above it.
The one that is not on the menu
Three reference circles were listed above, and the setting offers two of them. The horizon is the one left out.
The horizon belongs to one observer at one instant. It is not a frame anything can be tabulated in, because the table would be wrong for everyone but you, and wrong for you a minute later. The horizon is used constantly — the Ascendant, the Midheaven and every house cusp are horizon-frame constructions — but it is not offered as a reference plane, because a body's catalogued position cannot live there.
The two on offer are both anchored to the sky rather than to you: one to the Earth's rotation, one to its orbit.
What comes next
Both of them have appeared already, several pages apart and in different roles. The next page sets the celestial equator and the ecliptic side by side.
Next: The Two Planes.