Directions

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Lesson 5 Directions
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Compass
compass is an instrument containing a freely
suspended magnetic element which displays the
direction of the horizontal component of the
Earth's magnetic field at the point of
observation. The magnetic compass is an old
Chinese invention.
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The first thing you need to learn, are the
directions. North, South, East and West. Look at
the figure and learn how they are. North is the
most important.
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Is a compass all we need when navigating?
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Map of the world
Latitude
Longitude
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Battle ships
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3
Latitude
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3
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2
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Longitude
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How to find North without a compass
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If you have an analogue wrist watch, you can use
the time to find north. Hold your watch up in
front of you, and let the short hand, red on the
figure, that indicates hours point at the sun.
While holding it like this, cut the angle between
the red arrow and 12 o'clock in two, (noonwards
if the time is before 6am or after 6pm), that way
is south. (The reason you need to cut it in two,
is because the clock takes two rotations while
the sun takes one around the earth, it is of
course the other way around, but never mind.)
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First of all, there will be fewer branches to the
north. This is usually easiest to see if you look
up along the trunk of the tree. The north face
of the tree would be more humid than the south
face, which is something most species of lichen
(or moss) likes, and consequently, there will be
more of it on the north face. On the image above,
you can also see that ants likes to build their
nests on the south side of the tree.
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LESSON NOTES
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The principle of satellite navigation is very
simple your position can be determined if you
can measure your distance from each of three
objects whose positions (i.e., coordinates in a
well-defined reference frame) are known to you.
In order to implement a global navigation system
based on this principle, GPS has fielded a
constellation of 24 satellites in medium-earth
orbits with a 12-hour orbital period. These
satellites are the objects at known locations
from which a GPS receiver measures ranges.
Actually, the satellites are moving in space at a
speed of about 4 kilometres (about 2.5 miles) per
second, but the position of each at any instant
can be estimated from its broadcast message with
an error no worse than a few meters. The distance
between the user and a satellite is measured in
terms of the time it takes a radio signal to
travel from the satellite to the user. Precise
measurement of transit time is accomplished by
transmitting signals with precision in accordance
with nearly perfectly synchronized clocks carried
aboard the satellites.
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