How can polaris be used to find latitude

But what if you are in the Southern Hemisphere where Polaris is not visible? You have to use the Southern Cross, or Crux. The Southern Cross is a constellation with many stars, but five are clearly visible. Mimosa is the bright star that forms the left-hand point of the Crux, Hip forms the right-hand point, Gacrux the top, Crux in the middle, and Acrux the bottom point.

You just have to remember that the False Cross is located further north than the true one. Photo credit: pixabay. No Googling required! Before the use of the cross-staff pictured, mariners and others who needed to determine latitude used flat pieces of wood with holes to sight through to locate Polaris and with pieces of cord attached to measure angles.

F or much of the early era of global exploration, longitude had been difficult to calculate because either an elaborate set of astronomical tables and measurements were required or an accurate time-keeping device was necessary, and neither was available until the 18th century.

As early as Galileo had devised a way to determine longitude by observation of the passage of the 4 moons of Jupiter. By the early 18th century, while still using the transit of Jupiter's moons, navigators had learned to make approximate calculations of longitude by observing the changing angular distance between the Moon and a prominent star such as Antares.

But numerous careful observations and extensive calculations were required for this method to work and neither of these was really feasible on board ship or in the field. Numerous sea trials and several improved versions of Harrison's original model were required before, in , Harrison had a working chronometer, durable enough for extensive field observation.

It was a version of this chronometer that Lewis and Clark carried with them. All this seems quite simple. Why, then, were the captain's longitudinal observations so prone to error? The answer is also simple: they did not have, as we do, reliable battery-powered watches with quartz movements.

When their chronometer ran down, it had to be re-calibrated on local time, which required them to make observations to determine the sun's zenith or local "noon" and then setting the chronometer by estimating the Expedition's current longitudinal position. Over the course of their journey, the small incremental errors produced by this procedure became larger ones. There were other methods of calculating longitude available to them, using astronomical observation.

But many astronomical readings had to be acquired over the course of a night in order to obtain sufficiently precise data to determine longitude. It was asking a lot for men exhausted by the rigors of their daily trek, to spend three or four hours in the cold and damp of a mountain night taking sightings of the moon and stars, recording observations, and making calculations by firelight. It was only natural that errors would exist in data obtained in this manner. Even the most skilled astronomer or surveyor would have been hard pressed to make highly accurate observations under such circumstances.

L ongitude can be calculated using either time or astronomical observation. If time can be fixed along any meridian of longitude, then longitudinal distance can be determined by comparing time at that meridian with local time, usually based on the point at which the sun reaches its zenith. These times are referred to respectively as being "ahead" or "behind" Greenwich Mean Time GMT , or time along the prime meridian. Measure the height of Polaris applying the equipment above.

You merely have to point the long side of the triangle towards the Northstar. Having done this, put a finger on the seewing thread and move into your house. Standing in clear light, read the number and write it down.

Figure 6: The Jacobian Stick. The Jacobian device had the disadvantage of being a bit clumsy - it is difficult to keep an eye on both a star and the horizon. If you construct this instrument - beware - don't fall while you are aiming it out in the dark! A much more handsome device was presented to Vasco da Gama by native sailors outside India. This device is called a "Kamal".

The Kamal is far more easy and handsome compared to the Jacobian Stick. Notice, you only need a piece of wood - and a tiny rope. Pull the rope - until the wooden piece fills your field of vision from the horizon up to the North Star.