The Hour Angle is equivalent to longitude – only instead of degrees, angular distance is given as the time difference between Greenwich and another point on the globe. The Lindbergh Hour Angle Watch was designed by Lindbergh to work with Weems' system of avigation, and to make calculating the hour angle from observation simpler. It’s based on simple math: since the Earth rotates once in 24 hours, and since there are 360 degrees in a circle, every hour represents fifteen degrees. To understand how this is useful to navigators, consider this: suppose it’s noon at your location. All you have to do is figure out the Greenwich Hour Angle for a celestial body, and you have your longitude.

If using the Sun, the example is simple. Let’s say it’s 4:30 in the afternoon at Greenwich. That means the Hour Angle – the equivalent to the time difference expressed in degrees – can be read right off the Hour Angle watch; it’s 60 degrees (note that 60 and IV correspond on the dial) plus another 7 degrees and 30 minutes, read off the outer bezel. You don’t need to add anything for the seconds, although you could – the inner dial rotates to allow you to line the zero point, at 60/15, up with the last pip of a radio time signal (this method of setting the seconds to time was actually invented by Weems, and is found in the Longines Weems Second Setting Watch). That means – if the Sun is directly over your head – you are exactly 67 degrees and 30 minutes west of Greenwich – if you’re at, say, 45 degrees north latitude, you are somewhere over Maine. (Latitude is a much easier problem – for instance, if you're in the Northern Hemisphere and navigating at night it pretty much corresponds exactly to the altitude above the horizon of Polaris.) While this is not the exact method in every detail, nor comprehensive, it illustrates the basic principle of the Hour Angle as used in navigation, and the Hour Angle watch.

It can easily be seen, for instance, that this system can be used to determine the Greenwich Hour Angle for other celestial bodies than the sun, and with the aid of an almanac, used to find the geographic point of that object at the time of observation. The idea of simplifying celestial navigation by using Hour Angle and declination instead of altitude and azimuth was first suggested by Weems, and first appeared, according to Whitney's Military Timepieces, in the Lunar Ephemeris of 1929. In 1933, the first Air Almanac was published, in which Weems gave Greenwich Hour Angle and declination for the Sun, Moon, and important navigational stars as well – it became the cornerstone of modern air navigation.