Hey babe, want to get horizontal..slap!!..is that a no?

[Buck_Naked]

What is the vertical equivalent of horizon?

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I feel more like I do now than I did when I got here.

[tanstaafl]

A vertical.

[aseymayo]

A plumb line.

[AuraSeer]

Vertex.

Vertices is the plural. The adjective form is vertical.

[Keeves]

“zenith” is when the sun is directly overhead. That’s not a line like the horizon is, but still, that’s my suggestion.

[tanstaafl]

Under some circumstances. Here is the definition:

Main Entry: ver·ti·cal
Pronunciation: 'v&r-ti-k&l
Function: adjective
Etymology: Middle French or Late Latin; Middle French, from Late Latin verticalis, from Latin vertic-, vertex
Date: 1559
1 a : situated at the highest point : directly overhead or in the zenith b of an aerial photograph : taken with the camera pointing straight down or nearly so
2 a : perpendicular to the plane of the horizon or to a primary axis : UPRIGHT b (1) : located at right angles to the plane of a supporting surface (2) : lying in the direction of an axis : LENGTHWISE
3 a : relating to, involving, or integrating economic activity from basic production to point of sale <vertical monopoly> b : of, relating to, or comprising persons of different status <the vertical arrangement of society>

• vertical noun
• ver·ti·cal·i·ty /"v&r-t&-'ka-l&-tE/ noun
• ver·ti·cal·ly /'v&r-ti-k(&-)lE/ adverb
• ver·ti·cal·ness /-k&l-n&s/ noun
  synonyms VERTICAL, PERPENDICULAR, PLUMB mean being at right angles to a base line. VERTICAL suggests a line or direction rising straight upward toward a zenith <the side of the cliff is almost vertical>. PERPENDICULAR may stress the straightness of a line making a right angle with any other line, not necessarily a horizontal one <the parallel bars are perpendicular to the support posts>. PLUMB stresses an exact verticality determined (as with a plumb line) by earth’s gravity <make sure that the wall is plumb>.

Definition 2 is the one I was using. (Note that vertical can also be a noun.) Plumb is also mentioned.

[AuraSeer]

Whoops, I answered the wrong question.

The “vertical equivalent of the horizon” is a line perpendicular to the plane of the horizon. Mathematically, this is called the normal. In construction, as aseymayo said, it’s a plumb line.

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Laugh hard; it’s a long way to the bank.

[pluto]

Just a quibble, Aura, but normal can refer to any line at right angles to another line or plane. It’s just as correct to refer to the horizon as the normal of the vertical.

You are guilty of gravity chauvinism, which we free-floating thinkers abandoned long ago, on our way to the fourth dimension.

Speaking of dimensions, I suppose the third member of our triad of lines (side to side, up and down, front to back) would be the heading or bearing, that is, the line straight ahead of us. Any other thoughts on this?

“If you had manifested fatigue upon noticing that you had been an ass, that would have been logical, that would have been rational; whereas it seems to me that to manifest surprise was to be again an ass.”
Mark Twain
Personal Recollections of Joan of Arc

[Cap_n_Crude]

Throwing out a couple of ideas to asnwer the “vertical equivalent of horizon.” Normally I’d just put down the right answer (either from knowledge or research) but I’m weary and very sick today.

Azimuth
Right Ascension

Do either of these work?

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–Da Cap’n

[pluto]

(BTW, I spend a lot of time each day writing navigation software.)

Azimuth and right ascension are measures of distance (angular distance) along the horizon. As such I don’t think they meet the criteria of the OP. Corresponding measures along the vertical (didn’t we decide that was the right word?) are altitude and declination. To be precise (and what are we if not precise?) right ascension and declination are equatorial coordinates rather than horizon coordinates, i.e., they refer to angles along and above the celestial equator, rather than the horizon.