In geometry, a spherical coordinate system specifies points by three numbers: the *radial distance* of that point from a fixed origin, its *polar angle* measured from a fixed zenith direction, and the *azimuth angle*, which is the horizontal angle between the origin to the point of interest.

A spherical coordinate system fits well with length space (3+1), with the temporal direction ignored for scalar time. That is, every point event in duration space (‘3D time’) is projected onto a temporal sphere centered on the origin instant. The scalar time is the radial time of each instant.

Let the spherical coordinates of length space be (*r, θ, φ*) with *r* representing the radial distance, and *θ* and *φ* representing the zenith and azimuth angles, respectively. Let the spherical coordinates of duration space (3D time) be (*t, χ, ψ*) with *t* representing the radial time, and *χ* and *ψ* representing the temporal zenith and azimuth angles, respectively.

To represent the fullness of length space and duration space requires six dimensions (3+3), three for length and three for duration: ((*r, θ, φ*); (*t, χ, ψ*)) or (*r, θ, φ*; *t, χ, ψ*). Then length-time (3+1) can be represented by the coordinates [*r, θ, φ*; *t*] and freelength-duration (1+3) by the coordinates 〈*r; t, χ, ψ*〉.

If rectilinear coordinates are used for duration space, say (*ξ, η, ζ*), then the time, i.e., radial distime, *t* = √(*ξ*² + *η*² + *ζ*²). The corresponding length space concept, *freelength*, is the radial distance. If rectilinear coordinates are used for length space, say (*x, y, z*), then the *freelength r* = √(*x*² + *y*² + *z*²). For 2D applications such as mapping, polar coordinates would be used instead of spherical, in which case *r* = √(*x*² + *y*²) and *t* = √(*ξ*² + *η*²).

To transform a (3+1) function to a (1+3) function requires expansion to its (3+3) function, inversion by interchange of length and duration, and then contraction to (1+3). In symbols with ↑ as expand, ↓ as contract, and ↔ as interchange: (3+1) ↑ (3+3) ↓ (1+3), or (*r, θ, φ*; *t*) ↑ (*r, θ, φ*; *t, χ, ψ*) ↓ (*r*; *t, χ, ψ*). In this way length and duration, time and freelength are interchanged.

In more detail, a parametric length space function is converted to a parametric duration function: **r** = [*r, θ, φ*] = **r**(*t*) = [*r*(*t*), *θ*(*t*), *φ*(*t*)] ↑ [*r´*(*t´, χ´, ψ´*), *θ´*(*t´, χ´, ψ´*), *φ´*(*t´, χ´, ψ´*)] ↔ (*t´*(*r´, θ´, φ´*), *χ´*(*r´, θ´, φ´*), *ψ´*(*r´, θ´, φ´*)) ↓ [*t*(*r*), *χ*(*r*), *ψ*(*r*)] = **t**(*r*) = [*t, χ, ψ*] = **t**.

Take for example the definition **v** = d**r**/d*t*. We have: **v** = d**r**/d*t* = [d*r*/d*t*, d*θ*/d*t*, d*φ*/d*t*] = [*r*(*t*), *θ*(*t*), *φ*(*t*)]) ↑ [*r´*(*t´, χ´, ψ´*), *θ´*(*t´, χ´, ψ´*), *φ´*(*t´, χ´, ψ´*)] ↔ (*t´*(*r´, θ´, φ´*), *χ´*(*r´, θ´, φ´*), *ψ´*(*r´, θ´, φ´*)) ↓ [*t*(*r*), *χ*(*r*), *ψ*(*r*)] = [d*t*/d*r*, d*χ*/d*r*, d*ψ*/d*t*] = d**t**/d*r* = **u**. The result is that length space and duration space are interchanged, with length space vectors becoming duration space vectors and times becoming freelenghts.

Functions may be converted by differentiation, then inversion, then integration. Take for example, the function **s**(*t*) = **s**_{0} + **v**_{0}*t *+ ½**a***t*². Differentiating twice leads to **s**(*t*)´´ = **a** = d**v**/d*t*. Expanding to spherical coordinates, inverting, and contracting to the radial component results in **t**(*s*)´´ = d**u**/d*s* = **b**. Integrating twice produces **t**(*s*) = **t**_{0} + **u**_{0}*s *+ ½**b***s*², which has the same form as the original function.