The electromechanical angle computer inside the B-52 bomber's star tracker

Before GPS, navigation relied on celestial navigation. It used the positions of stars, the sun, the moon, and planets to determine direction and position, but doing it by hand was slow and cumbersome.

The Astro Compass for the B-52 automated this. The Astro Tracker on top tracked stars, while the Angle Computer inside handled the navigational calculations. The result was a heading accurate to about 0.1 degrees.

The operation was surprisingly simple.

• On the Master Control Panel, you select values such as time, SHA, and Declination.
• You turn the Set Control knob to match the desired figures.
• The Star Data display looks digital, but it's actually an analog dial driven by a motor.

The astronomical information came from the Air Almanac. This book was newly published every 4 months and provided celestial data at 10-minute intervals for each day.

• GHA Aries: the reference point angle based on Greenwich
• SHA: the sidereal hour angle of a star
• declination: the declination of a star
• positions of the sun, moon, and planets
• since stars are nearly fixed, they were handled with a separate table

The core of the calculation is the navigational triangle. To determine a star's azimuth and altitude relative to the aircraft's position, spherical trigonometry is required. The spherical triangle must be solved using latitude, longitude, LHA, and declination, which determines the star's direction from the observation point.

The Angle Computer implemented this math mechanically. It modeled a hemisphere with a radius of 2 5/8 inch like an actual celestial sphere, and moved a star pointer using complex internal linkages and gears. The position of that pointer was read by a synchro, converting azimuth and altitude into electrical signals.

Ultimately, this device is an example of solving a navigation problem in the 1960s—when digital computers were still unsuitable—by converting mathematical coordinate transformations and spherical trigonometry into a physical structure.