水平面

WordNet

make even or smooth, with or as with a carpenters plane; "plane the top of the door"
(mathematics) an unbounded two-dimensional shape; "we will refer to the plane of the graph as the X-Y plane"; "any line joining two points on a plane lies wholly on that plane" (同)sheet
a level of existence or development; "he lived on a worldly plane"
a carpenters hand tool with an adjustable blade for smoothing or shaping wood; "the cabinetmaker used a plane for the finish work" (同)carpenter''s plane, woodworking plane
a power tool for smoothing or shaping wood (同)planer, planing machine
cut or remove with or as if with a plane; "The machine shaved off fine layers from the piece of wood" (同)shave
travel on the surface of water (同)skim
make plans for something; "He is planning a trip with his family"
scale drawing of a structure; "the plans for City Hall were on file" (同)architectural plan
a series of steps to be carried out or goals to be accomplished; "they drew up a six-step plan"; "they discussed plans for a new bond issue" (同)program, programme
have the will and intention to carry out some action; "He plans to be in graduate school next year"; "The rebels had planned turmoil and confusion" (同)be after
make or work out a plan for; devise; "They contrived to murder their boss"; "design a new sales strategy"; "plan an attack" (同)project, contrive, design
parallel to or in the plane of the horizon or a base line; "a horizontal surface"
something that is oriented horizontally

PrepTutorEJDIC

『平面』,水平面 / 『水準』,程度 / 《話》『飛行機』 / (面が)平らな,平たんな / 平面の,平面図形の;平面上にある / 〈飛行機〉滑走する
かんな / …‘に'かんなをかける;…‘を'平らにする / かんなをかける
=plane tree
『計画』,『案』,プラン,設計 / 『設計図』,『図面』;(小区域の)地図,市街図 / …‘を'『計画する』,‘の'案を立てる / …の設計図をかく,‘を'設計する / 計画を立てる
『水平線の』,地平線の;水平の,水平面の / 水平の位置;水平線(面)

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/12/24 13:10:18」(JST)

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A spirit level on a shelf

In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point. Alternatively, a spirit level, which exploits the buoyancy of a bubble, can be used to determine if the plane is horizontal.

In radio science, horizontal plane is used to plot an antenna's relative field strength in relation to the ground (which directly affects a station's coverage area) on a polar graph. Normally the maximum of 1.000 or 0 dB is at the top, which is labeled 0^o, running clockwise back around to the top at 360°. Other field strengths are expressed as a decimal less than 1.000, a percentage less than 100%, or decibels less than 0 dB. If the graph is of an actual or proposed installation, rotation is applied so that the top is 0^o true north. See also the perpendicular vertical plane.

In general, something that is horizontal can be drawn from left to right (or right to left), such as the x-axis in the Cartesian coordinate system.

Discussion

Although the word horizontal is commonly used in daily life and language (see below), it is subject to many misconceptions.

The concept of horizontality only makes sense in the context of a clearly measurable gravity field, i.e., in the 'neighborhood' of a planet, star, etc. When the gravity field becomes very weak (the masses are too small or too distant from the point of interest), the notion of being horizontal loses its meaning.

Verticals at two separate points are not parallel. The same holds for their associated horizontal planes

A plane is horizontal only at the chosen point. Horizontal planes at two separate points are not parallel, they intersect.

In general, a horizontal plane will only be perpendicular to a vertical direction if both are specifically defined with respect to the same point: a direction is only vertical at the point of reference. Thus both horizontality and verticality are strictly speaking local concepts, and it is always necessary to state to which location the direction or the plane refers to. Note that (1) the same restriction applies to the straight lines contained within the plane: they are horizontal only at the point of reference, and (2) those straight lines contained in the plane but not passing by the reference point are not necessarily horizontal anywhere.

field lines for a non homogeneous knobbly planet in motion may be curved. The white, red and hlue bits illustrate the planet's heterogeneity.

In reality, the gravity field of a heterogeneous planet such as Earth is deformed due to the inhomogeneous spatial distribution of materials with different densities. Actual horizontal planes are thus not even parallel even if their reference points are along the same vertical line, since a vertical line is slightly curved.

At any given location, the total gravitational force is not quite constant over time, because the objects that generate the gravity are moving. For instance, on Earth the horizontal plane at a given point (as determined by a pair of spirit levels) changes with the position of the Moon (air, sea and land tides).

On a rotating planet such as Earth, the strictly gravitational pull of the planet (and other celestial objects such as the Moon, the Sun, etc.) is different from the apparent net force (e.g., on a free-falling object) that can be measured in the laboratory or in the field. This difference is the centrifugal force associated with the planet's rotation. This is a fictitious force: it only arises when calculations or experiments are conducted in non-inertial frames of reference, such as the surface of the Earth.

Practical use in daily life

The concept of a horizontal plane is thus anything but simple, although, in practice, most of these effects and variations are rather small: they are measurable and can be predicted with great accuracy, but they may not greatly affect our daily life.

This dichotomy between the apparent simplicity of a concept and an actual complexity of defining (and measuring) it in scientific terms arises from the fact that the typical linear scales and dimensions of relevance in daily life are 3 orders of magnitude (or more) smaller than the size of the Earth. Hence, the world appears to be flat locally, and horizontal planes in nearby locations appear to be parallel. Such statements are nevertheless approximations; whether they are acceptable in any particular context or application depends on the applicable requirements, in particular in terms of accuracy.

The y-axis on the wall is vertical but the one on the table is horizontal.

In graphical contexts, such as drawing and drafting and Co-ordinate geometry on rectangular paper, it is very common to associate one of the dimensions of the paper with a horizontal, even though the entire sheet of paper is standing on a flat horizontal (or slanted) table. In this case, the horizontal direction is typically from the left side of the paper to the right side. This is purely conventional (although it is somehow 'natural' when drawing a natural scene as it is seen in reality), and may lead to misunderstandings or misconceptions, especially in an educational context.

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