What a Reference Plane Is
The last of the four dropdowns offers two values, Ecliptic and Equatorial, under a label that gives very little away.
It is the setting with the longest reach. It decides which circle in the sky a position is measured along — and by the end of this category you will have seen it move the date of a transit by years.
Along, and away
Every coordinate pair on this site works the same way. Pick a great circle. Then:
- 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.
Two numbers, and any 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 identical in all three rows. Only the circle changes.
Why it has to be a great circle
This is where great circles earn the page they were given.
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 says Ref. Plane, 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 dropdown offers two. The horizon is missing, and its absence is worth a sentence.
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 applications use the horizon constantly — the Ascendant, the Midheaven and every house cusp are horizon-frame constructions — but they do not offer it as a reference plane, because a body's catalogued position cannot live there.
The two on the menu 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 already appeared, several pages apart and in different roles. Setting them side by side is the next job.
Next: The Two Planes.