If for some reason you can't use Google Earth, you may try use Great Circle Mapper
www.gcmap.com/.
Prior to the Ring of Fire, it was impractical to sail a great circle route. On such a route, the course (the angle of the ship's track relative to true north) is continuously changing. You must adjust your heading depending on where you are. However, pre-RoF navigation capabilities were gravely limited. Latitude was determined by sightings of the sun at noon, or stars at night, and the accuracy was no better than a quarter-degree. Longitude was determined by dead reckoning and could be wildly in error (tens of degrees!) if you had spent a long time at sea.
I discussed the possible improvements in the art Sof marine navigation in my two part article, "Soundings and Sextants" (Part I, "Navigational Instruments Old and New" in Grantville Gazette 14; Part II, "Celestial Navigation Methods," in 15). Ultimately, of course, we will have sophisticated sextants and accurate chronometers, but it will take years, if not decades, for these to become commonplace.
Moreover, since an airship travels substantially faster than a watership, the time between celestial observations is more of a factor, and there are obvious problems with measuring the elevation of a celestial object above the horizon when you are in the air.
The Hindenburg didn't in fact rely much on celestial navigation. Rather, it used the combination of a gyroscopic compass and dead reckoning. If it were traveling in still air, its position could be accurately calculated from its airspeed and heading. Wind could be assessed by flying a special pattern; every hour, head 45 degrees off course, first to port and then to starboard, and take drift readings. (Dick 60). The zeppelin was equipped with a searchlight and a telescope; water ripples or landmarks below were studied to determine the ships angle of drift (the angle between its heading and its course). (Grossman).
Prior to RoF, the standard navigational practice was either to follow a coastline (or other landmarks), or to sail a rhumb line (loxodrome). The latter means to sail with a constant compass heading. Even that had its difficulties, as it was difficult to correct compasses for magnetic deviation or variation, but at least at night the Pole Star provided a check on the accuracy of your compass reading. Even so, navigators preferred whenever possible to "run down a line of latitude," that is, sail directly east or west, as that way the noon sun sightings could be used to verify that they were still on course.
Even after the introduction of the sextant and chronometer, mariners didn't follow a perfect great circle route even when the winds were not an issue. For one thing, taking a great circle path could force the ship into high latitudes with stormy weather. Hence, even modern sailors sometimes follow a "composite" route in which they truncate the great circle at a particular maximum latitude, thus following a great circle route at the ends and a rhumb line (of constant latitude) in the middle.
For another, it's inconvenient to make all the necessary course changes. A modern sailor might approximate a great circle route by a series of rhumb lines, changed daily. An airship might make hourly changes but the principle is the same.
Winds, of course, offer another reason for deviating from the great circle route. In general, you want to take the shortest path through a region of unfavorable winds, and keep the route as much as possible where the winds are favorable.
If the great circle route is overland, then it may pass over mountains. You have three choices: (1) fly above them, but at the cost of having to carry more hydrogen and less cargo in order to achieve the necessary buoyancy (and there are some mountains you still won't be able to fly over), (2) skirt them, at the cost of increased travel distance, or (3) thread through the same passes that the mule trains do, but at risk of encountering turbulence and mountain storms.
Limits on Route Length
The length of the route is limited by the amount of fuel that the airship can carry, the energy content of that fuel, the efficiency with which the airship transforms fuel into propulsion, and the availability of refueling stops en route. The more fuel the airship carries, the greater its range, but the less its cargo capacity.
The loss of hydrogen, whether through leakage or deliberate venting for altitude correction, can also limit the route. The less hydrogen, the less buoyant the airship is. It eventually needs to stop at a depot with a supply of iron, fuel and water so that it can make more hydrogen by the steam-iron process. (Hydrogen may also be made by the acid-iron process, but sulfuric acid is likely to be harder to find, especially outside Europe.)
Prevailing Winds Navigation
The minimum distance route (always a great circle) is not necessarily the minimum time or minimum energy route. That's because winds affect how quickly an airship can travel and how energy it must expend to move a particular distance. Ornithologists tell us that "birds will wait to embark on a migration until they can fly with a tail wind and minimize the energy they must spend." (Deblieu 77). Airships can take advantage of the wind, too.
Initially, the best that the characters will be able to do is to plan their routes to take advantage of prevailing winds; later, "pressure pattern" navigation, which takes advantage of chance "highs" and "lows," will be possible. Prevailing winds are "typical" winds; on a day-to-day basis, the wind varies in speed and direction. Indeed, the average wind velocity distribution itself varies, at a single location, on a seasonal basis.
While sailors have taken advantage of prevailing winds for millennia (since a sailing ship cannot sail directly upwind, and can only beat obliquely upwind with difficulty, they had no choice), the formal mathematical theory of planning a minimum time path for a sailing ship was developed by Francis Galton in the 1860s and 1870s. Maurice Giblett, in 1924, proposed a similar scheme for use by airships. Unlike sailing ships, airships need fuel, and therefore there has also been interest in identifying the minimum energy route given a particular wind distribution (Munk; Zhao).
In my article, "Untying the Wind," (Grantville Gazette 35), I explain what the characters might reasonably be expected to know, or find out, about the prevailing winds, and guide prospective 1632 universe authors to sources of more detailed information.
Speed Variation
Just as with a sailing ship, an airship can expect to experience both poor and good passages, depending on the vagaries of the wind. In December, 1934, over the Mediterranean, the Graf Zeppelin encountered northwest winds of 45-56 mph, which increased its ground speed to 122 mph. (Dick 52).
For the Hindenburg, the Frankfurt-Lakehurst passage varied from 52h49m to 78h57m, while the return was usually faster, ranging from 43h02m to 60h58m. The latter no doubt resulted from the advantage of flying with the westerlies. For the passage from Frankfurt to Rio, the Hindenburg's times ranged from 85h13m to 111h41m, and the return was 93h17m to 105h57m. (airships.net). There was a fairly wide variation in westbound routes-as far south as the Azores and as far north as the Orkneys-but the return flights were, at mid-ocean, between the latitudes of Bordeaux and Aberdeen.
Not only did airships pick their routes to benefit from favorable winds, they chose their cruising altitudes with the same consideration in mind. The normal cruising altitude of the Graf Zeppelin was 575-820 feet, but it went higher if the upper winds were better. (Dick 67).
What a drag . . .
A balloon rises until the buoyant force lifting it and the gravitational force pulling it back toward the surface are equal. Its horizontal movement is dictated by the wind, which exerts a drag force on it, pushing it downwind. A force, by definition, causes an object to accelerate (gain speed); the less massive the object, the faster it accelerates.