What Is a Transit Instrument?

A transit instrument is used to collect astronomical data for the purpose of navigation and making star charts. In combination with a highly accurate clock, it can provide information about where someone is on the Earth, which can be critical for navigation. Basic variants on such devices were used by a number of ancient cultures and continued to be vital through the 20th century, when new navigational methods became available. Satellites and related technology provide modern navigational information to help people stay on track and feed new information to map-makers.

This device consists of a telescope mounted on an axis so it can be aligned with a meridian, one of the north-south lines running along the surface of the Earth. It can swing freely along the meridian, but doesn’t move from side to side. An observer uses the transit instrument to note the point at which a star crosses the meridian, taking note of the angle on the instrument and the time recorded by the clock. This allows people to perform calculations to determine where they are.

As the Earth moves, the positions of the stars change relative to an observer on the surface. Tracking these positions can allow people to know where they are, on the basis of a star chart with information about the movements of the stars. Map makers and chart creators could use a transit instrument to create accurate star charts for different regions and times of the year, while navigators relied on these charts to help them get locational fixes.

Precision positioning is critical with a transit instrument. A small error can become large when magnified over the distances involved with astronomical observations. Tight engineering tolerances on historic instruments ensured that operators could rely on their measurements. They also needed highly accurate clocks, down to the second, so they could precisely time the movement of stars across the meridian. Developing an accurate clock that would perform on board ships turned out to be a significant challenge to early inventors.

Modern companies continue to make transit instruments. They are still used for some observations, as well as in education, where people may want to be able to make celestial observations to learn more about earlier navigation techniques. Surveyors and people in related professions can use a transit instrument for some of the observations they need to make in the field as they measure and record geographic information for maps, site preparation, and other activities.