Transit instruments were pivoted telescopes that allowed astronomers to time the passage of a star over the meridian (an imaginary line through the poles). As the heavens rotated, an astronomer would look through the transit telescope (aligned in the meridian) and note when a particular star crossed his field of vision.

At the precise moment that the star passed, the astronomer noted the time via the ticking of a nearby astronomical regulator, an accurate clock used specifically for timing these transit observations. Without such measurements, the transit instrument wasn't able to do its job.

Pinpointing the exact second the star crossed the meridian was crucial. Hence the biggest hand on the astronomical regulator indicated seconds. Each movement of the second hand was usually accompanied by a loud tick to alert astronomers to the passing time. A smaller interior face displayed minutes, and the hour was read off a rotating inset display.

Since temperature affected the refraction of a star's light through the earth's atmosphere, the astronomical regulator also contained a thermometer to record the temperature. Astronomers could then attempt to correct for the error.

Graham's astronomical regulator contained a special pendulum that compensated for temperature to ensure the accuracy of the clock.