Difference between revisions of "Terminology/zh-cn"

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(Created page with ";<span id="TWR">Thrust-Weight-Ratio</span> :<math style="float:right;padding:.5em;">\text{TWR} = \frac{T}{W} = \frac{T}{m \cdot g}</math>The Ratio between the total mass of th...")
 
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KSP中, 有许多可能会使非专业人员感到迷惑的关于物理和轨道的术语。并且,许多其他科学术语和简写被用来描述常用术语。
 +
 +
此表格被设计成一个包含了使你走上专业宇航员之路所必须的术语的简洁的表单。
 +
 +
== 有用的术语 ==
 +
 +
=== 数学和几何 ===
 +
 +
<div style="float:right;">[[File:cartesian.svg|thumb|upright|left|Cartesian coordinate system - Uses rectangular coordinates]][[File:polar.svg|thumb|upright|Polar coordinate system - Uses only angles and one length]]</div>
 +
;椭圆(Elliptical)
 +
:椭圆形的,一般用于你的轨道的参考。
 +
;法向量(Normal vector)
 +
:一条与平面垂直的向量。
 +
;标量(Scalar)
 +
:一个没有方向的数值。标量一般会有测量单位的后缀,表明这个标量的量纲。 比如 3&nbsp;米每秒(m/s), 3&nbsp;米(m),3&nbsp;秒(s) 都是标量:他们有不同的表示速度,长度/距离和时间的单位,但是都没有方向。
 +
;向量(Vector)
 +
:一个方向和大小的集合。比如前进方向和速率放在一起就是速度。一个向量如何被表示取决于采用的坐标系,以及多少个维度被使用。  <35°, 12>是一个二维的极坐标向量,而<14, 9, -20>则是一个三维笛卡尔(直角)坐标向量。也有其他的坐标系,但这些是最常用的
 +
 +
:<35°, 12>看起来像是一个12个单位长的箭头,从原点(零点,角度在这里没有意义,因为这是一个没有长度的点)出发,结束在一个与偏离极轴(通常是x轴,逆时针旋转为正角度)35°的点。
 +
 +
:<14, 9, -20>看起来像是一个从原点(<0,0,0>)出发,在一个x坐标为14,y坐标为9,z坐标为20的点终止的箭头。
 +
 +
:使用直角坐标系的优点是你可以准确的知道向量的终止位置在哪里,但是计算向量的长度则比较麻烦。而在极坐标系中,向量长度十分易得,终止位置却很难得出。
 +
 +
:这些物理量都是向量:'''速度''', '''加速度''', '''力'''
 +
 +
一个三维坐标系需要:
 +
* 一个原点。
 +
* 3个单位向量。这些向量定义了沿着坐标轴的基本量度和坐标轴的方向。
 +
* 3个标量,这可以是角度或坐标,用来描述坐标空间中的位置。
 +
 +
{{clear}}
 +
----
 +
 +
=== Orbital Terms ===
 +
[[File:Orbit.svg|thumb|upright=2.25|Visualization of the most common orbital parameters]]
 +
;Apoapsis
 +
:In an elliptical orbit, the point of the orbit farthest from the planet.
 +
;<span id="pe">Periapsis</span>
 +
:In an elliptical orbit, the point of the orbit closest to the planet.
 +
;Peri-* and Apo-*
 +
:When speaking of orbits, oftentimes you replace the "-apsis" of Periapsis and Apoapsis with something signifying which planet you are speaking of. In this game, -kee and -kerb are both commonly used to describe orbits around Kerbin resulting Perikee/Perikerb and Apokee/Apokerb.
 +
;<span id="an">Ascending Node</span>
 +
:The point at which the orbit crosses the reference plane moving north.  Here, "north" means the direction of the orbit normal of the reference plane.
 +
;Descending Node
 +
:The point at which the orbit crosses the reference plane moving south.
 +
;<span id="ecc">Eccentricity</span>
 +
<math style="float:right;padding:0.5em;">\text{ecc} = 1 - \frac{2}{\frac{Ap}{Pe}+1}</math>
 +
:A scalar describing how non-circular an orbit is.
 +
:* ecc = 0 → circular orbit.
 +
:* 0 < ecc < 1 → elliptical orbit.
 +
:* ecc = 1 → parabolic orbit - this is an escape orbit.
 +
:* ecc > 1 → hyperbolic orbit - this is an escape orbit.
 +
;<span id="inc">Inclination</span>
 +
:The angle between an orbital plane relative to a reference plane (e.g. an orbit with 90° inclination to an equatorial reference plane would be called polar).
 +
;Low Kerbin Orbit
 +
:Also known merely as LKO. A "low orbit" exists for most planetoids with a gravity well. It describes an orbit that is low, but stable (above atmosphere). Low orbits are used as stepstones, after ascent and before burning to another rendezvous object (planet/vessel), as it allows to perform the exit burn at any point of the orbit. The upside is that it requires the least possible amount of fuel to reach a low orbit.
 +
;<span id="zenith">Zenith</span>
 +
:"Up" in ground-relative coordinates, i.e. the direction your rocket points at when at the launchpad, aka: "the highest point in the sky".
 +
;Nadir
 +
:opposite direction of "Zenith".
 +
;Orbital Nodes
 +
:Specific points of reference in any orbit such as Apoapsis, Periapsis, intersect points with other orbits etc.
 +
;Orbit Normal
 +
:A normal vector of the Orbital Plane. Produced by cross multiplying the ship's velocity and gravity. Since this follows the right hand rule, from a perspective where the ship is orbiting counter-clockwise it will point "up", while for a ship orbiting "clockwise" it will point down. "Up" is also often labeled as "North" or "N+", and in tandem with that "Down" is labeled "Anti-Normal", "South" or "N-".
 +
;Orbital Plane
 +
:The imaginary disk described by the path of an orbit around a body (commonly used when describing inclinations).
 +
;<span id="prograde">Prograde</span>
 +
:The "Direct" direction of your orbital path. Since orbits are elliptical, it's actually the tangent of the orbit where the ship is.
 +
;<span id="retrograde">Retrograde</span>
 +
:The reverse of Prograde, "backwards".
 +
;Reference Plane
 +
:Any plane used as a reference for describing your current orbit. For local orbits around a planet, this is often the equatorial plane. When multiple bodies in a solar system are involved, the ecliptic plane can be used.  For intercepting another orbiting body, the orbital plane of the body to be intercepted is used. An orbital plane can be fully described by giving the inclination and the longitude of the ascending node relative to a reference plane.
 +
;<span id="sma">Semi-major axis</span>
 +
:The major-axis is the long axis of an ellipse, and the semi-major axis is half of this. In KSP the semi-major axis is calculated with <math>\text{sma} = \text{radius} + \frac{Ap+Pe}{2}</math>. It's the average of the Apoapsis and Periapsis computed relative to the center of the body. As both are relative to the bodies surface, the radius must be added. All orbits with the same semi-major axis have the same period, regardless of their eccentricity.
 +
;Sub-Orbital
 +
:Describes an orbit where the periapsis is below the surface of a planetary body, leading to an impact on said body.
 
;<span id="TWR">Thrust-Weight-Ratio</span>
 
;<span id="TWR">Thrust-Weight-Ratio</span>
 
:<math style="float:right;padding:.5em;">\text{TWR} = \frac{T}{W} = \frac{T}{m \cdot g}</math>The Ratio between the total mass of the vehicle and the available thrust of all propulsion devices of the vehicle/current stage. A TWR > 1 means the craft will have enough thrust to accelerate and ascend. A TWR below 1 won't be able to counteract gravity and drag at low altitudes, though a low TWR might totally suffice in space. Because the weight (W) of the craft depends on the current gravitational acceleration (g) the TWR depends on the position of the craft. The acceleration at Mün's surface is only 16.6 % of Kerbin's acceleration so at the surface a TWR<sub>Kerbin</sub>&nbsp;=&nbsp;1 would be a TWR<sub>Mün</sub>&nbsp;=&nbsp;6.
 
:<math style="float:right;padding:.5em;">\text{TWR} = \frac{T}{W} = \frac{T}{m \cdot g}</math>The Ratio between the total mass of the vehicle and the available thrust of all propulsion devices of the vehicle/current stage. A TWR > 1 means the craft will have enough thrust to accelerate and ascend. A TWR below 1 won't be able to counteract gravity and drag at low altitudes, though a low TWR might totally suffice in space. Because the weight (W) of the craft depends on the current gravitational acceleration (g) the TWR depends on the position of the craft. The acceleration at Mün's surface is only 16.6 % of Kerbin's acceleration so at the surface a TWR<sub>Kerbin</sub>&nbsp;=&nbsp;1 would be a TWR<sub>Mün</sub>&nbsp;=&nbsp;6.
 +
----
 +
 +
=== Space Maneuvers ===
 +
 +
;Atmospheric Braking
 +
:→ Main article: [[Aerobraking]]
 +
:Lowering the periapsis so it is inside a planetary atmosphere. This will lead to the vessel being slowed by atmospheric drag. Can lead to reentry (see below), but also is used to reduce the necessary burn time for significant orbit alterations.
 +
;Burn
 +
:firing of the engines, usually to alter trajectory in some way.
 +
;Circularizing
 +
:a maneuver (firing of the engines) that makes an orbit's eccentricity 0, or close to zero. This is usually achieved by a burn close to an apsis.
 +
;Re-entry
 +
:Re-entering atmosphere and using drag to decelerate a vessel to a groundwards trajectory. This usually causes intense heat stress on the object as the vessel requires sufficient speed to not "bounce" back from the atmosphere into space. Currently re-entry is only partially implemented with effects but heat and bounce are not yet implemented. (There are mods however)
 +
;Retroburn
 +
:A burn performed "backwards", e.g. with the engines facing towards prograde and nose towards retrogade (hence the name). This is a common maneuver to used to lower the height of the orbit without altering any other orbital parameters.
 +
 +
----
 +
 +
=== Physics ===
 +
 +
;Acceleration
 +
:Rate of change to your velocity. Acceleration is a vector, measured in "m/s<sup>2</sup>".
 +
;Ballistic trajectory
 +
:A falling object's trajectory is ballistic. In rocketry it usually indicates that the object in question is only influenced by gravity and does not exert any force (ie. thrust) of its own.
 +
;Delta-v (Δv)
 +
:The change in velocity that has or can be exerted by your spacecraft. This is measured in meters per second (m/s). More mass can reduce the delta-v, while more propulsion can increase it. This makes it a useful value to calculate efficiency of launch vehicles. For example, a launch vehicle requires about 4,000 m/s of delta-v to escape Kerbin's atmosphere and achieve a stable orbit.
 +
;Energy
 +
:The energy of an object in an orbit is the sum of its potential and kinetic energy. Potential energy is <math>E_p = -\frac{G \cdot M \cdot m}{R}</math> and Kinetic Energy <math>E_k = -\frac{1}{2} \cdot m \cdot v^2</math> where ''G'' is the [[w:Gravitational constant|gravitational constant]], ''M'' is the mass of the planet, ''m'' is the mass of the vehicle, ''R'' is the distance from the center of the planet and ''v'' is the velocity. So <math>E_k = -\frac{1}{2} \cdot m \cdot v^2 - \frac{G \cdot M \cdot m}{R}</math>. When the energy is greater than zero the vehicle is on an escape trajectory.  All orbits with the same semi-major axis have the same energy.
 +
;Escape Velocity
 +
:The velocity needed to escape a given planet's gravity well, as given by <math>v_e = \sqrt{\frac{2GM}{r}},</math> where G is the [[w:Gravitational constant|gravitational constant]], M is the mass of the planet, and r is the radius of the planet.
 +
;g-force (G)
 +
:A measurement of acceleration as expressed in the sea-level force of Earth's gravity with 1&nbsp;G being about 9.81&nbsp;m/s². An object at Earth's surface is accelerated at 1&nbsp;G. The object weighs twice as much when at 2&nbsp;G acceleration and is weightless when accelerated with 0&nbsp;G. In orbit and without an engine running all objects experience no acceleration which can be expressed as 0&nbsp;G.
 +
;Gravity
 +
:The force exerted by all objects with mass. Very weak. Usually only objects with very high mass - ie. planets, moons - have any noticeable effect. Diminishes with the square of distance from the center of mass. So for an object twice as far, experiences only 1/2<sup>2</sup> = 1/4 of the gravity.
 +
;Gravity Well
 +
:The area around a planet affected by gravity. Actually extends to infinity, but as gravity decreases exponentially with distance, it is only significant within the planet's sphere of influence.
 +
;[[Orbit]]
 +
:When an object has sufficient tangential velocity (and is outside the atmosphere, so drag won't slow it down) so that it will keep falling "next" to the planet (never touching ground) its trajectory is called an orbit. Stable orbits are elliptical (a circle is an ellipse with zero eccentricity). If the objects tangential speed exceeds escape velocity it's orbit will be either para- or hyperbolic.
 +
;<span id="isp">Specific Impulse (I<sub>sp</sub>)</span>
 +
:<math style="float:right">I_{sp}= \frac{T}{\dot m}, [I_{sp}] = \frac{m}{s}</math>The I<sub>sp</sub> defines how effective a propulsion system is. The higher the I<sub>sp</sub> the more powerful is the thrust applied to the rocket with the same fuel mass. The I<sub>sp</sub> is usually given in ''seconds'' but actually the physically correct unit is ''distance per time'' which is usually given in ''meters per second'' or ''feet per second''. To avoid confusion which unit of speed is used, the physical correct I<sub>sp</sub> (in ''distance/time'') is divided by the surface gravity of Earth/Kerbin (9.81&nbsp;m/s²). This results in a value given in ''seconds''. To use this I<sub>sp</sub> in formulas it must to be converted back into ''distance per time'' which requires multiplying with the surface gravity of Earth/Kerbin again. As this value is only used to convert between those two units, the '''specific impulse doesn't change when the gravity changes'''.
 +
:As the specific impulse is the ratio of thrust and fuel flow <math>\frac{Ns}{kg}</math> is sometimes given as the unit. This is mathematically another form of <math>\frac{m}{s} = kg \cdot \frac{m}{s^2} \cdot \frac{s}{kg}</math> because force is the multiplication of mass and acceleration defining <math>N = kg \cdot \frac{m}{s^2}</math>. So <math>1 \frac{Ns}{kg} = 1 \frac{m}{s}</math> with the later begin simply only in SI base units.
 +
;Sphere of influence
 +
:The radius around a celestial body where its gravity well is non-neglectable. Commonly known as SoI/SOI.
 +
;Tangential velocity
 +
:The component of your velocity that's perpendicular to your trajectory. Instantaneous velocity - velocity when the time of measurement approaches zero - is always tangential to your trajectory.
 +
;Trajectory
 +
:A trajectory is the path that a moving object follows through space as a function of time.
 +
;Velocity
 +
:Rate of change of your position. Your speed and direction. Velocity is a vector, measured in meters per second (m/s).
 +
 +
==See also==
 +
* [[Tutorials]]

Revision as of 04:56, 26 May 2013

KSP中, 有许多可能会使非专业人员感到迷惑的关于物理和轨道的术语。并且,许多其他科学术语和简写被用来描述常用术语。

此表格被设计成一个包含了使你走上专业宇航员之路所必须的术语的简洁的表单。

有用的术语

数学和几何

Cartesian coordinate system - Uses rectangular coordinates
Polar coordinate system - Uses only angles and one length
椭圆(Elliptical)
椭圆形的,一般用于你的轨道的参考。
法向量(Normal vector)
一条与平面垂直的向量。
标量(Scalar)
一个没有方向的数值。标量一般会有测量单位的后缀,表明这个标量的量纲。 比如 3 米每秒(m/s), 3 米(m),3 秒(s) 都是标量:他们有不同的表示速度,长度/距离和时间的单位,但是都没有方向。
向量(Vector)
一个方向和大小的集合。比如前进方向和速率放在一起就是速度。一个向量如何被表示取决于采用的坐标系,以及多少个维度被使用。 <35°, 12>是一个二维的极坐标向量,而<14, 9, -20>则是一个三维笛卡尔(直角)坐标向量。也有其他的坐标系,但这些是最常用的
<35°, 12>看起来像是一个12个单位长的箭头,从原点(零点,角度在这里没有意义,因为这是一个没有长度的点)出发,结束在一个与偏离极轴(通常是x轴,逆时针旋转为正角度)35°的点。
<14, 9, -20>看起来像是一个从原点(<0,0,0>)出发,在一个x坐标为14,y坐标为9,z坐标为20的点终止的箭头。
使用直角坐标系的优点是你可以准确的知道向量的终止位置在哪里,但是计算向量的长度则比较麻烦。而在极坐标系中,向量长度十分易得,终止位置却很难得出。
这些物理量都是向量:速度, 加速度,

一个三维坐标系需要:

  • 一个原点。
  • 3个单位向量。这些向量定义了沿着坐标轴的基本量度和坐标轴的方向。
  • 3个标量,这可以是角度或坐标,用来描述坐标空间中的位置。

Orbital Terms

Visualization of the most common orbital parameters
Apoapsis
In an elliptical orbit, the point of the orbit farthest from the planet.
Periapsis
In an elliptical orbit, the point of the orbit closest to the planet.
Peri-* and Apo-*
When speaking of orbits, oftentimes you replace the "-apsis" of Periapsis and Apoapsis with something signifying which planet you are speaking of. In this game, -kee and -kerb are both commonly used to describe orbits around Kerbin resulting Perikee/Perikerb and Apokee/Apokerb.
Ascending Node
The point at which the orbit crosses the reference plane moving north. Here, "north" means the direction of the orbit normal of the reference plane.
Descending Node
The point at which the orbit crosses the reference plane moving south.
Eccentricity

A scalar describing how non-circular an orbit is.
  • ecc = 0 → circular orbit.
  • 0 < ecc < 1 → elliptical orbit.
  • ecc = 1 → parabolic orbit - this is an escape orbit.
  • ecc > 1 → hyperbolic orbit - this is an escape orbit.
Inclination
The angle between an orbital plane relative to a reference plane (e.g. an orbit with 90° inclination to an equatorial reference plane would be called polar).
Low Kerbin Orbit
Also known merely as LKO. A "low orbit" exists for most planetoids with a gravity well. It describes an orbit that is low, but stable (above atmosphere). Low orbits are used as stepstones, after ascent and before burning to another rendezvous object (planet/vessel), as it allows to perform the exit burn at any point of the orbit. The upside is that it requires the least possible amount of fuel to reach a low orbit.
Zenith
"Up" in ground-relative coordinates, i.e. the direction your rocket points at when at the launchpad, aka: "the highest point in the sky".
Nadir
opposite direction of "Zenith".
Orbital Nodes
Specific points of reference in any orbit such as Apoapsis, Periapsis, intersect points with other orbits etc.
Orbit Normal
A normal vector of the Orbital Plane. Produced by cross multiplying the ship's velocity and gravity. Since this follows the right hand rule, from a perspective where the ship is orbiting counter-clockwise it will point "up", while for a ship orbiting "clockwise" it will point down. "Up" is also often labeled as "North" or "N+", and in tandem with that "Down" is labeled "Anti-Normal", "South" or "N-".
Orbital Plane
The imaginary disk described by the path of an orbit around a body (commonly used when describing inclinations).
Prograde
The "Direct" direction of your orbital path. Since orbits are elliptical, it's actually the tangent of the orbit where the ship is.
Retrograde
The reverse of Prograde, "backwards".
Reference Plane
Any plane used as a reference for describing your current orbit. For local orbits around a planet, this is often the equatorial plane. When multiple bodies in a solar system are involved, the ecliptic plane can be used. For intercepting another orbiting body, the orbital plane of the body to be intercepted is used. An orbital plane can be fully described by giving the inclination and the longitude of the ascending node relative to a reference plane.
Semi-major axis
The major-axis is the long axis of an ellipse, and the semi-major axis is half of this. In KSP the semi-major axis is calculated with . It's the average of the Apoapsis and Periapsis computed relative to the center of the body. As both are relative to the bodies surface, the radius must be added. All orbits with the same semi-major axis have the same period, regardless of their eccentricity.
Sub-Orbital
Describes an orbit where the periapsis is below the surface of a planetary body, leading to an impact on said body.
Thrust-Weight-Ratio
The Ratio between the total mass of the vehicle and the available thrust of all propulsion devices of the vehicle/current stage. A TWR > 1 means the craft will have enough thrust to accelerate and ascend. A TWR below 1 won't be able to counteract gravity and drag at low altitudes, though a low TWR might totally suffice in space. Because the weight (W) of the craft depends on the current gravitational acceleration (g) the TWR depends on the position of the craft. The acceleration at Mün's surface is only 16.6 % of Kerbin's acceleration so at the surface a TWRKerbin = 1 would be a TWRMün = 6.

Space Maneuvers

Atmospheric Braking
→ Main article: Aerobraking
Lowering the periapsis so it is inside a planetary atmosphere. This will lead to the vessel being slowed by atmospheric drag. Can lead to reentry (see below), but also is used to reduce the necessary burn time for significant orbit alterations.
Burn
firing of the engines, usually to alter trajectory in some way.
Circularizing
a maneuver (firing of the engines) that makes an orbit's eccentricity 0, or close to zero. This is usually achieved by a burn close to an apsis.
Re-entry
Re-entering atmosphere and using drag to decelerate a vessel to a groundwards trajectory. This usually causes intense heat stress on the object as the vessel requires sufficient speed to not "bounce" back from the atmosphere into space. Currently re-entry is only partially implemented with effects but heat and bounce are not yet implemented. (There are mods however)
Retroburn
A burn performed "backwards", e.g. with the engines facing towards prograde and nose towards retrogade (hence the name). This is a common maneuver to used to lower the height of the orbit without altering any other orbital parameters.

Physics

Acceleration
Rate of change to your velocity. Acceleration is a vector, measured in "m/s2".
Ballistic trajectory
A falling object's trajectory is ballistic. In rocketry it usually indicates that the object in question is only influenced by gravity and does not exert any force (ie. thrust) of its own.
Delta-v (Δv)
The change in velocity that has or can be exerted by your spacecraft. This is measured in meters per second (m/s). More mass can reduce the delta-v, while more propulsion can increase it. This makes it a useful value to calculate efficiency of launch vehicles. For example, a launch vehicle requires about 4,000 m/s of delta-v to escape Kerbin's atmosphere and achieve a stable orbit.
Energy
The energy of an object in an orbit is the sum of its potential and kinetic energy. Potential energy is and Kinetic Energy where G is the gravitational constant, M is the mass of the planet, m is the mass of the vehicle, R is the distance from the center of the planet and v is the velocity. So . When the energy is greater than zero the vehicle is on an escape trajectory. All orbits with the same semi-major axis have the same energy.
Escape Velocity
The velocity needed to escape a given planet's gravity well, as given by where G is the gravitational constant, M is the mass of the planet, and r is the radius of the planet.
g-force (G)
A measurement of acceleration as expressed in the sea-level force of Earth's gravity with 1 G being about 9.81 m/s². An object at Earth's surface is accelerated at 1 G. The object weighs twice as much when at 2 G acceleration and is weightless when accelerated with 0 G. In orbit and without an engine running all objects experience no acceleration which can be expressed as 0 G.
Gravity
The force exerted by all objects with mass. Very weak. Usually only objects with very high mass - ie. planets, moons - have any noticeable effect. Diminishes with the square of distance from the center of mass. So for an object twice as far, experiences only 1/22 = 1/4 of the gravity.
Gravity Well
The area around a planet affected by gravity. Actually extends to infinity, but as gravity decreases exponentially with distance, it is only significant within the planet's sphere of influence.
Orbit
When an object has sufficient tangential velocity (and is outside the atmosphere, so drag won't slow it down) so that it will keep falling "next" to the planet (never touching ground) its trajectory is called an orbit. Stable orbits are elliptical (a circle is an ellipse with zero eccentricity). If the objects tangential speed exceeds escape velocity it's orbit will be either para- or hyperbolic.
Specific Impulse (Isp)
The Isp defines how effective a propulsion system is. The higher the Isp the more powerful is the thrust applied to the rocket with the same fuel mass. The Isp is usually given in seconds but actually the physically correct unit is distance per time which is usually given in meters per second or feet per second. To avoid confusion which unit of speed is used, the physical correct Isp (in distance/time) is divided by the surface gravity of Earth/Kerbin (9.81 m/s²). This results in a value given in seconds. To use this Isp in formulas it must to be converted back into distance per time which requires multiplying with the surface gravity of Earth/Kerbin again. As this value is only used to convert between those two units, the specific impulse doesn't change when the gravity changes.
As the specific impulse is the ratio of thrust and fuel flow is sometimes given as the unit. This is mathematically another form of because force is the multiplication of mass and acceleration defining . So with the later begin simply only in SI base units.
Sphere of influence
The radius around a celestial body where its gravity well is non-neglectable. Commonly known as SoI/SOI.
Tangential velocity
The component of your velocity that's perpendicular to your trajectory. Instantaneous velocity - velocity when the time of measurement approaches zero - is always tangential to your trajectory.
Trajectory
A trajectory is the path that a moving object follows through space as a function of time.
Velocity
Rate of change of your position. Your speed and direction. Velocity is a vector, measured in meters per second (m/s).

See also