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Chapter 2: Problem 16

Polaris was used for navigation by seafarers such as Columbus as they sailed from Europe to North America. When Magellan sailed the South Seas, he could not use Polaris for navigation. Explain why.

Short Answer

Expert verified
Magellan could not use Polaris for navigation because it is only visible in the Northern Hemisphere, while he was sailing in the Southern Hemisphere.

Step by step solution

01

Consider the Position of Polaris

Polaris, also known as the North Star, is positioned nearly directly above the North Pole of the Earth. It is located in the northern sky and has been used for navigation because its position remains relatively fixed in the sky.
02

Understand Visibility from the Southern Hemisphere

Because Polaris is almost directly above the North Pole, it is only visible in the Northern Hemisphere. As one moves southward, Polaris appears lower in the sky until it eventually is not visible at all once crossing the equator.
03

Connect to Magellan's Voyage

When explorers like Magellan sailed the 'South Seas,' they were in the Southern Hemisphere. In this region, Polaris would not be visible and therefore could not be used for navigation.
04

Summarize the Reason

Since Magellan operated in the Southern Hemisphere, the North Star, Polaris, was not visible and he had to rely on other celestial bodies or navigational instruments for his journey.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Celestial Navigation
Celestial navigation is the practice of using the positions of celestial bodies to determine one's position on Earth. Historically, sailors and explorers relied on the stars, sun, and moon for navigation. This method is incredibly valuable, especially when sailing across open oceans without landmarks.

The key tools for celestial navigation include:
  • Astrolabe: Measures the altitude of a star above the horizon
  • Sextant: A more advanced tool for finding the angle between a celestial body and the horizon
  • Nautical Almanac: Contains information on the positions of celestial objects at specific times
Using these tools, navigators could calculate their latitude and longitude and set their course accurately. However, understanding which stars are visible in different parts of the world is crucial.
Northern Hemisphere
In the Northern Hemisphere, Polaris, also known as the North Star, plays a vital role in navigation. It is located almost directly above the North Pole and remains relatively stationary in the sky. Because of its fixed position, it serves as a reliable point of reference for determining direction.

Key facts about Polaris for Northern Hemisphere navigation include:
  • Polaris is part of the constellation Ursa Minor (Little Dipper).
  • Its altitude above the horizon is roughly equal to the observer's latitude.
  • It remains visible throughout the year in the Northern Hemisphere.
Polaris's visibility decreases as one moves southward. By the time an observer reaches the equator, Polaris dips below the horizon and is no longer visible. Sailors like Columbus used Polaris to navigate from Europe to North America because they remained in the Northern Hemisphere.
Southern Hemisphere
In the Southern Hemisphere, Polaris is not visible. This poses a challenge for navigators who have to rely on other celestial bodies. Sailors like Magellan, who explored areas in the Southern Hemisphere, could not use Polaris for navigation.

Instead, they relied on other methods, including:
  • The Southern Cross: A constellation that helps determine the south celestial pole.
  • Other Southern Hemisphere stars and constellations.
  • Navigational instruments such as the astrolabe and later the sextant.
These tools and stars helped navigators find their way in the Southern Seas. Understanding which celestial bodies are visible in different hemispheres is essential for successful celestial navigation. Magellan's inability to use Polaris highlights the need for alternative navigation techniques when venturing into different parts of the world.

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Most popular questions from this chapter

We always see the same side of the Moon because a. the Moon does not rotate on its axis. b. the Moon rotates on its axis once for each revolution around Earth c. when the other side of the Moon is facing Earth, it is unlit. d. when the other side of the Moon is facing Earth, it is on the opposite side of Earth.

Constellations are groups of stars that a. are close to each other in space. b. are bound to each other by gravity. c. are close to each other in Earth's sky. d. all have the same composition.

Polaris, the North Star, is unique because a. it is the brightest star in the night sky. b. it is the only star in the sky that doesn't move throughout the night. c. it is always located at the zenith, for any observer. d. it has a longer path above the horizon than any other star.

Go to the U.S. Naval Observatory website (USNO "Data Services," at http://aa.usno.navy.mil/data). Look up the times for sunrise and sunset for your location for the current week. (You can change the dates one at a time or bring up a table for the entire month.) How are the times changing from one day to the next? Bring up the "Duration of Days/Darkness Table for One Year" page for your location. Are the days getting longer or shorter? When do the shortest and the longest days occur? Look up a location in the opposite hemisphere (Northern or Southern). When are the days shortest and longest?

There is an angle between the ecliptic and the celestial equator because a. Earth's axis is tilted with respect to its orbit. b. Earth's orbit is tilted with respect to the orbits of other planets. c. the Sun follows a rising and falling path through space. d. the Sun's orbit is tilted with respect to Earth's.
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