A transit is the line formed when two fixed objects appear one directly behind the other from the observer’s position. If you are on that line, you know you lie somewhere along it, and that is a position line as good as any bearing and usually far more precise. Pilotage transits are pairs of marks, beacons, lights or natural features that guide a boat along a safe channel or clear of a hazard; racing sailors use transits at the start line and to judge laylines; helmsmen use them to see whether they are being set by the tide. In American usage the same thing is called a range. For a beginner, a transit is a simple way to stay in a channel without instruments. For an experienced navigator, it is the most trustworthy fix available and one that needs no electricity.
How a transit works
Imagine a lighthouse and a church spire behind it. From most places on the water the spire appears to one side of the lighthouse or the other. From one particular line, the two are exactly in line. If you are on that line and you hold the two objects in transit, you can steer along it. If the rear object drifts to the right of the front one, you have moved to the left of the line, and you steer to bring them back into line.
The power of a transit comes from geometry. Because the two objects are fixed and their positions are known, the line through them is fixed and known with an accuracy limited only by how well you can see them. There is no compass error, no deviation, no variation to apply. A bearing on a single object taken with a hand-bearing compass is good to a couple of degrees at best; a transit between two well-separated marks is good to a fraction of a degree. The further apart the two objects are along the line and the closer you are, the more sensitive the transit becomes.
Charted transits and leading lines
Harbour authorities exploit this by setting up leading marks: pairs of beacons or lights deliberately placed so that when they are in line they lead a boat along the deep-water channel. On the chart they are shown with a line and a bearing, for example a note giving the transit as bearing 032° true. The front light is set lower than the rear, so a boat left of the line sees the rear light to the right of the front, and the reverse.
Clearing transits work the other way. Rather than defining a line to follow, they define a line not to cross. The chart may show that a headland must be kept open of a distant hill, meaning that as long as you can see daylight between them you are clear of the rocks.
Making your own transits
The real skill is spotting natural transits as you go. Any two fixed objects will do: a buoy and a house, a headland and a tower, a pier end and a tree. When you are entering an unfamiliar harbour, pick a pair ahead that lie along the channel and steer to hold them in line. When you are anchoring, note two transits abeam and ahead; if either changes, you are dragging. When crossing a tidal stream towards a harbour mouth, put the entrance in transit with something behind it on the shore and steer to hold the transit. If the rear mark opens to one side, the tide is setting you that way, and you alter course towards it until the transit holds. This is far quicker and more reliable than working out course to steer on paper, and it corrects continuously as the stream changes.
Transits on the race course
Racing sailors live by transits. At the start, the line between the committee boat and the pin is a transit; the trick is to extend it to the shore behind the pin, find a landmark on that extension and use it to judge exactly where the line is from wherever you are on the approach. On the beat, a transit between the windward mark and the shore behind it tells the tactician whether the tide is helping or hurting, and two boats on opposite tacks can judge who is ahead by watching how the other moves against the background.
Frequently Asked Questions about Transit
What is the difference between a transit and a range?
Nothing. Transit is the British term and range the American one for two objects in line. Chart legends and pilot books use the word appropriate to their country.
How accurate is a transit compared with GPS?
For staying in a narrow channel, a good transit on well-separated marks is often more useful than GPS because it gives an immediate, continuous visual correction with no lag and no chart-datum error. GPS is more convenient offshore; the transit wins in the harbour entrance.
A transit costs nothing, needs no batteries, and gives the most precise position line a navigator can have. Learn to see them everywhere – in the harbour, at anchor, on the start line – and you will steer straighter courses, avoid more rocks and win more starts than any instrument alone will give you.