Square and Compass

Begin with two observers a known distance apart, both measuring the same object at the same moment. First, record the baseline between them, since that is the fixed length your geometry will use. Next, have each observer measure the object’s altitude, or another shared angle, from their own location. Then compare the two measurements and find the difference between them; this small angular offset is the parallax signal. After that, convert the angle difference from degrees into radians, because the distance relationship uses radians. Finally, divide the known baseline by that angular difference to estimate the object’s distance: D≈B/Δθ​. The whole method rests on one idea: if two people move apart and the line of sight to the same object changes, that change contains the scale of the system.