Throughout history, one of the great driving forces for increasingly precise astronomical observations has been the need to improve navigation. Even with GPS the astronomy is still there, but it is now deeply buried in the system, so we don’t notice it.
The earliest seafarers found their way by following the coast. If storms or tides took them out of sight of land, they could be in serious trouble. However, our ancestors were also astronomers, and it was the astronomers who provided solutions. They noticed that there is one particular star, named Polaris, in the northern sky that seems fixed, while all the other stars circle around it each day. That is because Polaris is almost directly overhead at the North Pole, so does not move as the Earth rotates. For us at lower latitudes, it is a beacon telling us which way is north. The star is called the Pole Star because it is overhead at the North Pole and the North Star because it tells us which way is north.
Polaris also tells us our latitude. We just measure how many degrees the Pole Star is above the northern horizon and we have our latitude. This provides a nice, convenient navigation technique. Say we want to sail from Bristol, England (latitude 51.5 degrees) to Halifax, Nova Scotia (latitude 44.6 degrees). So we set out sailing in a south-westerly direction until the Pole Star tells us we are at the latitude of Halifax, and then we sail westward. Every night we make sure the Pole Star is 44.6 degrees above the northern horizon. We then keep sailing west until Halifax appears ahead of us.
One problem with this is that you don’t know when you will arrive until you do. It is very hard to know your speed. If there are off-shore rocks or shoals and you reach them in the middle of the night, this could be very inconvenient. We need to know our longitude, that is, how far around the Earth we are compared with our home port. There is no easy way to get that from observing the stars alone.
The Earth rotates one turn – 360 degrees – each day. That is fifteen degrees an hour, and a quarter of a degree a minute. This means we can use time to determine our longitude. We have agreed internationally that the zero degrees of longitude line passes through the Royal Greenwich Observatory in the U.K. If we have a precise clock, we can set it to Greenwich Mean Time (GMT), now called Universal Time (UT). Then, each day as we travel, we measure the time local noon happens. Each degree we move will find local noon coming four minutes later than noon in Greenwich. Halifax is 63.6 degrees west of Greenwich, so when we see local noon coming 4h, 14 minutes later than Greenwich noon, as shown by our clock, we start scanning our horizon for our destination.
Even in these days of GPS, we need a backup, so keeping those navigation clocks accurate is critically important. That is why we have time standard radio stations like Canada’s CHU, Britain’s MSF, or America’s WWV. Next time you hear, “At the Beginning of the Long Dash”, imagine all those navigators checking their clocks.
Venus still dominates the western sky after sunset. Mars is high in the South; Saturn is in the eastern sky. The Moon will be new on the 20th.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton, B.C.