On this chart, that pole is the centre — Page 453, Lesson 441

I want to walk you through a very specific piece of navigation equipment now — this is the kind of thing you'll see on a real flight deck, and it's a classic exam favourite. What we have here is a star chart, and not just any star chart — this is the polar stereographic projection of the sky as seen from the northern hemisphere, centred on the North Celestial Pole. Let me explain what you're actually looking at, because the numbers on this page are the whole point. The chart is built around the North Celestial Pole — that's the point in the sky directly above the Earth's North Pole, around which all the stars appear to rotate. On this chart, that pole is the centre. The concentric circles radiating outward from the centre are declination circles. Declination is the celestial equivalent of latitude — it's measured in degrees north or south of the celestial equator. So the circle marked N 50 is the line of 50° north declination, N 20 is 20° north declination, N 10 is 10° north, then we cross the celestial equator at 0°, and continue into the southern hemisphere with S 10, S 30, S 52, and S 60 — those are lines of south declination. So the chart extends from the pole right down to 60° south. Now, the radial lines — the ones spreading out from the centre like the spokes of a wheel — those are hour circles, or lines of right ascension. Right ascension is the celestial equivalent of longitude, but it's measured in hours, minutes and seconds of time, not degrees, because the sky rotates once every 24 hours. Each hour of right ascension corresponds to 15 degrees of rotation. Here's the clever part — and this is the heart of the chart. Look at the numbers running along the outer edge: 18 00, 18 02, 18 03, 18 05, 18 06, 18 08 and so on. Those are Local Hour Angle values, in hours and minutes. Local Hour Angle, or LHA, is the angular distance measured westward from your meridian to the celestial body. On this chart, the LHA is printed along the outer rim, and it's used to set the chart for your time of observation. Now look at the numbers printed inside the chart, in the little columns — values like 20 05 19 57, 20 04 19 57, 20 03 19 56, and further down 19 55, 19 49, 19 43, 19 36 — those are Greenwich Hour Angle values, in hours and minutes. Greenwich Hour Angle, or GHA, is the angular distance measured westward from the Greenwich meridian to the body. The difference between GHA and LHA is your longitude — that's the fundamental relationship in celestial navigation. So here's how you actually use this chart. You take your time of observation — your chronometer reading — and you look up the Greenwich Hour Angle of the star for that moment. Then you rotate the chart — the movable overlay — until the GHA on the chart aligns with the LHA on the outer rim. That rotation effectively sets the chart for your longitude. Once the chart is set, the declination circles give you your latitude directly — you read off the declination of the star you've observed, and that tells you where you are north or south. Let me trace one of these columns so you can see the pattern. Take the column starting 20 05 19 57 — that's a GHA of 20 hours 05 minutes, followed by 19 hours 57 minutes. The numbers descend: 49, 41, 32, 24, 16, 08 — those are minutes of declination, decreasing as you move outward along that hour circle. Then the next column: 20 04 19 57, with 49, 42, 34, 26, 19, 11, 03 — again, declination values in minutes, stepping down as you move away from the pole. And the next: 20 03 19 56, with 49, 42, 35, 28, 21, 14, 07. Do you see the pattern? Each column is a star track — the path of a particular star across the chart, with its GHA listed at the top and its declination in minutes at each point along the track. Now look at the bottom of the chart — the rows marked S 52 and S 60. These are the southern declination bands, and you'll see the GHA values there are different: 17 31, 17 36, 17 40, 17 45, 17 49, 17 54, 17 58, 18 03, 18 08, 18 12, 18 17, 18 22, 18 27, 18 32, 18 37, 18 42, 18 47 — and then the declination minutes: 54, 27, 32, 37, 42, 47, 52, 57, 02, 07, 13, 18, 23, 29, 34, 40, 45, 51, 56. And for S 60: 17 14, 17 21, 17 27, 17 33, 17 40, 17 46, 17 53, 18 00, 18 06, 18 13, 18 20, 18 27, 18 34, 18 41, 18 49, 18 56, 19 04. So the whole chart is a grid of declination and right ascension, with GHA values printed for each star track and LHA values around the rim. To fix your position, you set the chart for your time of observation, align GHA to LHA, and read off your latitude from the declination circles and your longitude from the difference between GHA and LHA. One thing I want to stress, because it's a classic trap: the declination values here are in minutes of arc, not hours — so when you see 49, 41, 32, those are 49 minutes, 41 minutes, 32 minutes of declination, not 49 hours. And the hour angle values — 20 05, 19 57 — those are in hours and minutes of time. Keep those two units separate in your head, and the chart reads cleanly. That's the polar stereographic star chart — the tool that lets you turn a star sight into a position line. Take a moment to let the layout sink in: pole at the centre, declination circles radiating out, hour circles as spokes, GHA printed on the tracks, LHA on the rim. That's the whole instrument.