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2.3 Navigation

How could you use the sky to find your way? The first orienteering skills were discovered over 10,000 years ago. If you were out hunting during the day you would watch the path of the Sun carefully, because you would have to turn around when it reached its highest point in the sky to avoid being out at night. At its highest point in the sky, the Sun indicates the direction south. The Sun can act as a clock. If you lived in a hot climate, you might need to travel at night and use the stars to guide you. Ocean voyagers must also use the stars to navigate at night. The star patterns can be a map.
 


Description of relations between Axial tilt (or Obliquity), rotation axis, plane of orbit, celestial equator and ecliptic. Earth is shown as?viewed from the Sun; the orbit direction is counter-clockwise (to the left). Click here for original source URL.


Perspective of Earth and?celestial sphere, showing the ecliptic (plane), the celestial equator (overhead the Earth's equator) and the Earth's polar axis (which also points to the celestial poles). The intersection shown between the equator and the ecliptic (the vernal equinox point) is not associated with any particular location on the Earth (despite the diagram), because the Earth rotates daily, while the celestial sphere does not. Click here for original source URL.

Most people are vaguely aware that they can find their way at night by the stars. They also know that there is a North Star, called Polaris, which is always in the north. But there are many other features of the sky that are familiar and useful. Night walks or camping trips offer a great opportunity to learn your way around the sky. Knowing some basic sky terminology is a useful first step. The point directly overhead is called the zenith. An imaginary line overhead, from the due south point on the horizon through the zenith, to the due north point on the horizon, is called the meridian. The Sun, Moon, and all the stars and planets rise on the east side of the meridian, cross the meridian, and set on the west side of the meridian. The ancient term A.M. (from the Latin phrase, ante meridiem, or before the meridian) refers to the first half of the daylight period, before the Sun crosses the meridian. The term P.M. (post meridiem) refers to the second half of the day, after the Sun crosses the meridian.
 


Polaris as seen by the Hubble Space Telescope. Click here for original source URL.

It seemed obvious to ancient people that Sun, the Moon, and the stars all orbited around the Earth. There was no reason to suppose that the Earth was moving — after all, we don't feel any motion! We now know that the Earth spins once per day. The rising and setting motions of the stars are due to the Earth's rotation. The line that connects the North Pole and the South Pole is the Earth's rotation axis. For navigation, the most interesting star is the North Star or Polaris. Because the Earth's North Pole happens to be aimed at it, it neither rises nor sets but sits at the same spot above the northern horizon all night. All the other stars in the sky move slowly in circles around it. This motion is imperceptible from minute to minute, but you can see it over a period of hours or with a long exposure photograph. Polaris is a fairly bright star and thus serves as a beacon, pointing north. The Earth is like a spinning top, with the stars wheeling around two fixed points in the sky.
 


The orientation of the Earth at the December solstice showing the relation of the particular circles of latitude ie the tropics of Cancer and Capricorn and the Arctic and Antarctic circles. Click here for original source URL.


A typical star trail with Polaris in the center. Click here for original source URL.

Polaris reveals your latitude. The angle of Polaris above the northern horizon (measured in degrees) equals your latitude (the number of degrees you are north of the equator). For example, in most of the United States, the North Star is about 30° to 45° above the northern horizon. However, at the North Pole, Polaris is directly overhead and the stars move in concentric circles parallel to the horizon. Anywhere on the equator, Polaris is low on the northern horizon and stars rise straight out of the east and set straight into the west. The Earth's South Pole doesn’t point at any bright star, so navigation south of the equator uses other star patterns.

Ancient navigators sighted the North Star to determine their latitude, even in the middle of the ocean or in a trackless wilderness. For example, ancient Polynesian navigators sailing from Tahiti to Hawaii could sail north until the North Star confirmed the latitude of Hawaii, then head west until they came to Hawaii. Once you have memorized the main features of the night sky, star patterns can be used for navigation too. It is too hot to travel during the day in the desert regions of the Middle East. For this reason, and because the Arabs preserved in translation the original names from ancient cultures, many of the bright stars have Arabic names (Aldebaran, Mizar, Alcor, Deneb, and others).