This program computes the line-of-sight distance and the curved surface distance to the horizon of a sphere at any distance of the eye as reckoned from the surface or center of the sphere. The radius and distance can be in any convenient linear units.

The initial defaults radius is the mean lunar radius and the distance is 1/100th of the mean geocentric distance of the eye from the center of the moon, both in statute miles.

**Where the Distance to the Eye Is Reckoned From the Center of the Sphere:**
Given the radius (

`R`

) of the sphere and the distance (

`D`

) of the eye from the center of the sphere, the general equation for the line-of-sight distance (

`H`

) to the sphere horizon viewed from that point is:

The horizon distance (

`S`

), corresponding to (

`H`

), but measured along the curved surface of the sphere directly below H and corresponding to the foot or sailing distance is:

Where:

**D** >

**R** and

**R** >

**0** and the angle is expressed in radians

**Where the Distance to the Eye Is Reckoned From the Surface of the Sphere:**

Given the radius (

`R`

) of the sphere and the distance (

`d`

) (height of) of the eye above the surface of the sphere, the general equation for the line-of-sight distance (

`H`

) to the sphere horizon viewed from that point is:

The horizon distance (

`S`

), corresponding to (

`H`

), but measured along the curved surface of the sphere, corresponding to the foot/sailing distance is:

Where: **R** > **0** and **d** > **0** and the angle is expressed in radians