Angular distance




In mathematics (in particular geometry and trigonometry) and all natural sciences (e.g. astronomy and geophysics), the angular distance (angular separation, apparent distance, or apparent separation) between two point objects, as viewed from a location different from either of these objects, is the angle of length between the two directions originating from the observer and pointing toward these two objects.




Contents






  • 1 Use


  • 2 Measurement


  • 3 Equation


  • 4 See also


  • 5 References





Use


The term angular distance (or separation) is technically synonymous with angle itself, but is meant to suggest the (often vast, unknown, or irrelevant) linear distance between these objects (for instance, stars as observed from Earth).



Measurement


Since the angular distance (or separation) is conceptually identical to an angle, it is measured in the same units, such as degrees or radians, using instruments such as goniometers or optical instruments specially designed to point in well-defined directions and record the corresponding angles (such as telescopes).



Equation


In order to calculate the angular distance θ{displaystyle theta }theta in arcseconds for binary star systems, extrasolar planets, solar system objects and other astronomical objects, we use orbital distance (semi-major axis), a{displaystyle a}a, in AU divided by stellar distance D{displaystyle D}D in parsecs, per the small-angle approximation for tan⁡(aD){displaystyle tan({frac {a}{D}})}tan({frac aD}):


θ≈aD{displaystyle theta approx {dfrac {a}{D}}}theta approx {dfrac aD}

Given two angular positions, each specified by a right ascension (RA), α∈[0,2π]{displaystyle alpha in [0,2pi ]}{displaystyle alpha in [0,2pi ]}; and declination (dec), δ∈[−π/2,π/2]{displaystyle delta in [-pi /2,pi /2]}{displaystyle delta in [-pi /2,pi /2]}, the angular distance between the two points can be calculated as,


θ=cos−1⁡[sin⁡(δ1)sin⁡(δ2)+cos⁡(δ1)cos⁡(δ2)cos⁡(α1−α2)]{displaystyle theta =cos ^{-1}left[sin(delta _{1})sin(delta _{2})+cos(delta _{1})cos(delta _{2})cos(alpha _{1}-alpha _{2})right]}{displaystyle theta =cos ^{-1}left[sin(delta _{1})sin(delta _{2})+cos(delta _{1})cos(delta _{2})cos(alpha _{1}-alpha _{2})right]}


See also



  • Milliradian

  • Gradian

  • Hour angle

  • Central angle

  • Angular diameter

  • Angular displacement

  • Great-circle distance



References


  • Weisstein, Eric W. "Angular Distance". MathWorld..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}



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