This classic star trail image shows how stars move in our sky as Earth rotates. More than 150 individual one-minute digital images were stacked in Photoshop. A first-quarter moon illuminated the surrounding landscape for the duration of the exposures.
Credit: Peter Michaud, Gemini Observatory
Astronomy is the oldest of the sciences, and quite possibly the oldest use of astronomy is navigating by the stars. This craft dates from prehistoric times among humans, and is even practiced by certain animals.
For example, during the 1960s, a study undertaken by New York's Cornell Lab of Ornithology demonstrated through use of planetarium simulations that the indigo bunting, a brilliantly blue bird of old fields and roadsides, migrates at night using the stars for guidance. It learns its orientation to the night sky from its experience as a young bird observing the stars.
Some primitive tribes accomplished amazing feats of pathfinding using only the sky as their guide. The Māori came to New Zealand from eastern Polynesia, probably in several waves between the years 1280 to 1300. With no instruments or tables to consult, they very carefully observed the night sky as well local weather patterns and ocean currents.
Relying on the stars
In today's modern world, private and commercial aircraft depend on a complex network of radio, satellite, inertial and other navigation systems. But should any or all of these systems fail, the starry sky can serve as the last resort.
As the late Henry Neeley, a popular lecturer at New York's Hayden Planetarium during the 1950s once noted: "The navigational use of the stars will continue to be a valuable asset for many years to come. In spite of all the scientific aids that have been developed to do the navigating by robot science, the ancient stars will still be a 'must' for navigator or pilot." Indeed, celestial navigation is still an important part of a navigator's formal training and while we might immediately think of sailors in this regard, the pilot of an aircraft can also sight on the stars in an emergency (and often with an advantage over sailors, being high above any obscuring clouds). [By day, ancient mariners used sundials to navigate.]
The Nautical Almanac and Air Almanac are special annual publications printed both in the United States and the United Kingdom and describes the positions and movements of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship or aircraft including the sun, moon and planets.
57 important stars
In addition, there is a standard roster of 57 stars used by aviators and navigators worldwide and chosen for their ease of identification and wide spacing. A navigator would try to measure the altitude of one of these stars above the horizon during twilight, when both the star and horizon are visible. This yields a "circle of position" on the Earth's globe; the observer must be somewhere on this circle to see the star at a certain altitude at a given time. Other stars yield other circles of position. The point where they all intersect is the observer's location.
In order to be visible against a twilight sky, the majority of the 57 navigation stars are second magnitude or brighter, although a few third magnitude stars were included on the list simply because they occupied regions where none brighter existed (the lower the figure of magnitude, the brighter the star).
Also, it was agreed that all these stars should have proper names. But some stars in the far-southern sky lacked such monikers, so after World War II names such as Acrux, Gacrux and Atria appeared on navigators' charts ? contractions of the original Greek designations for these stars.
Lastly (and not to sound like Maxwell Smart): Would you believe this official list omits what many might consider to be the most important navigational star in the sky? Yes, it's none other than Polaris, the North Star!
Why? Merely because it lies practically on the celestial pole, so its altitude above the northern horizon alone pretty much indicates the observer's latitude, so there is no need to utilize it for the "circle of position" method described above.
- How Vikings Might Have Navigated on Cloudy Days
- History's Most Overlooked Mysteries
- How Weather Changed History
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.