What is the origin of the term "plane of the spirit level"? I have heard the term "plane of the spirit level" e.g. the sun's shadows is X degrees to the plane of the spirit level to signify the surface of the earth and I see it is a common term used in surveying and as an instrument to show if there is a curvature.
My question is: where is this term coming from?
A spirit level is an instrument used to find a horizontal or vertical plane. In surveying, the spirit level is used as a reference. Surveying instruments like the theodolite have a spirit level built in. This is used to set up the instrument, after which you can measure angles relative to that reference plane.
This makes sure you have a repeatable reference, and not one that depends on the incline of the local surface.
As to the origin of the name:
it's called a level because it is used to find a level surface (i.e. one that's horizontal). It's called a spirit level because the liquid inside is usually a spirit (i.e. a mix of alcohol and water).

Early tubular spirit levels had very slightly curved glass vials with constant inner diameter at each viewing point. These vials are incompletely filled with a liquid, usually a colored spirit or alcohol, leaving a bubble in the tube. They have a slight upward curve, so that the bubble naturally rests in the center, the highest point. At slight inclinations the bubble travels away from the marked center position. Where a spirit level must also be usable upside-down or on its side, the curved constant-diameter tube is replaced by an uncurved barrel-shaped tube with a slightly larger diameter in its middle.
Alcohols such as ethanol are often used rather than water. Alcohols have low viscosity and surface tension, which allows the bubble to travel the tube quickly and settle accurately with minimal interference from the glass surface. Alcohols also have a much wider liquid temperature range, and won't break the vial as water could due to ice expansion. A colorant such as fluorescein, typically yellow or green, may be added to increase the visibility of the bubble.

why &quot;spirit&quot;:

The term &quot;spirit&quot; in reference to alcohol stems from Middle Eastern alchemy. These alchemists were more concerned with medical elixirs than with transmuting lead into gold. The vapor given off and collected during an alchemical process (as with distillation of alcohol) was called a spirit of the original material