The Two Planes
Both options have appeared already. The celestial equator was built in the Locations category, and the ecliptic opened the Zodiacs category. This page sets them side by side as two candidates for the same job.
Two different things to be built on
| Celestial equator | Ecliptic | |
|---|---|---|
| Comes from | the Earth's rotation | the Earth's orbit |
| Constructed by | extending the Earth's equator outward | extending the Earth's orbital plane outward |
| Its poles | the celestial poles | the ecliptic poles |
| Measured along it | right ascension | celestial longitude |
| Measured away from it | declination | celestial latitude |
| Zero point | the vernal equinox | the vernal equinox |
The last row is the one to notice. Both scales start from the same point — the vernal equinox, where the two circles cross — and then run along different circles. They start from the same point and immediately part company.
That shared zero is convenient and it is also the source of a persistent confusion. Agreeing on where to start counting is not the same as agreeing on what you are counting along.
What each is good at
The equatorial plane matches the sky's motion. The whole sphere turns about the celestial poles, so a body's declination does not change as the night passes, and its right ascension is directly a statement about when it will cross your meridian. Every star catalogue and every telescope is built on this frame, because it is the one the Earth's rotation makes natural.
The ecliptic plane matches the bodies' motion. The Sun, Moon and planets all travel close to the ecliptic, so measured against it their second coordinate — celestial latitude — stays small and nearly constant while the first does all the work. That is why an astrological position is a single number. Try quoting a single number in the equatorial frame and it falls apart: declination swings across ±23.4° for the Sun alone over a year, and dropping it would throw away most of the position.
One frame suits the sky's daily turn; the other suits the objects moving through it. Neither is more correct, and both are on offer because different questions want different ones.
How far apart they are
The two planes are tilted by about 23.4°, and the size of that angle causes trouble in two ways.
It is small enough that the two frames give similar answers — similar enough that a reader can glance at a right ascension, see a number in the same range as a longitude they were expecting, and carry on without noticing which frame produced it.
It is large enough that the answers are different, and that the differences build up, in timing most of all. Most of this category is about those differences.
What comes next
The next page is about the tilt between the two planes, the obliquity of the ecliptic: where it comes from, and what depends on it.