Official War Department Training Film. Restricted. 1– 204. Produced by The Signal Corps, in collaboration with The Chief of Air Corps. Celestial Navigation. Position Finding on the Earth. This 1940s era, black and white film covers the complex but straightforward calculations a navigator needs to determine his position on the earth and how to use celestial objects to do so. The film opens with a man tracking star charts. An animation shows the Earth and points to its different zeniths and geographical positions 1:16. If the bodies geographical position on the Earth is known then its meridian is also known, as shown in this animation 1:40. Using the Greenwich meridian which is the zero meridian for longitude, the Greenwich hour angle can be measured, it is the angle at the pole between the meridian and the celestial body on the Greenwich meridian. It is measured to the West through 360° 2:07. Another type of hour angle is shown. It is the angle at the pole between the observer’s meridian and the meridian of the celestial body 2:23. It is also called the Local Hour Angle or LHA 2:38. The Air Almanac is consulted 2:50. Navigator determines his longitude 3:03. The Greenwich hour angle of a celestial body represents the longitude of its geographical position 3:12. Greenwich hour angles and local hour angles are calculated 3:39. Zenith distance is calculated. It is the measurement between the geographical position to the observers position 4:05. The angle from the line through the center of the earth to the observers Zenith is always 90° as this animation shows 4:23. Animated starlight hits the earth at an angle 4:40. In this particular case the angle of the starlight is 60° making the Zenith distance measurement 30° or a complement of the altitude 5:00. 90° minus the altitude always equals the Zenith distance 5:05. Linear value of the Zenith distance is calculated 5:20. The circle of position is demonstrated 5:40. Military man uses a navigational aid 5:53. The pole represents a star and in order to see the top of the star at the same angle from any direction you need to stay on the circumference of the circle 6:15. If the radius is lengthened, the angle of observation is decreased 6:32. If the radius is shortened, the altitude is increased 6:44. The circle of position is measured once again 7:05. A star’s altitude is measured from the circle of position 7:27. The navigator’s circle of position can be measured with another circle of position, creating 2 intersecting points. One may be disregarded and the other on track with his dead reckoning 8:00. A globe is shown 8:15. The navigator uses minute circles of position called lines of position (LP) as this animation shows 8:33. A third line of position based on a third celestial body may be added to get an additional check 8:50. The American Practical Navigator is consulted 8:53. The astronomical triangle is shown 9:00. Mathematical equations for the triangle fill the screen 9:28. Solving the triangle is now quite simple, filling out forms with precomputed data 9:55. A man fills out the form and does the math for the astronomical triangle 10:15. The navigator starts with an assumed position – the dead reckoning position – or, it might be a position on either side of the dead reckoning position 10:48. The navigator determines local hour angle 11:00. Algorithmic tables are consulted 11:15. Navigator finds his position by using an intercept in determining his actual position 12:11. Actual position and assumed positions are compared by determining the observer’s altitude 12:20. A review of the measurements covered in the film, focusing on circle of position and line of position 13:05. Books of tables are displayed 13:23. Navigator consult his books and his maps 13:37. End of Training Film. 1 – 204.

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