Celestial Navigation
Student activities
Activity 1: What is latitude and longitude?
Background information
The voyages of European navigators about 500 years ago led to the establishment of
shipping routes and the need for accurate maps and charts. However, it was only
about a century ago that an international convention devised a terrestrial coordinate
system of longitude and latitude lines. There are two main (imaginary) reference lines
that are marked 0 degrees. The equator is one of them and divides the Earth into two
equal halves. The equator marks 0 degrees latitude and the lines of latitude run
parallel to one another, measuring the distance north or south from the equator. The
North Pole is at 90 degrees north latitude and the South Pole is at 90 degrees south
latitude.
Important latitude lines:
The Tropic of Cancer
The Tropic of Capricorn
The Arctic Circle
The Antarctic Circle
23.5 degrees N
23.5 degrees S
66.5 degrees N
66.5 degrees S
The Tropic of Cancer and the Tropic of Capricorn mark the furthest points north and
south of the equator where the Sun’s rays fall vertically. The Arctic Circle and the
Antarctic Circle mark the furthest points north or south of the equator where the Sun
appears above the horizon each day of the year. Inside the Antarctic and Arctic
Circles, the Sun does not appear above the horizon for the winter months and never
drops below the horizon during the summer months.
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Celestial Navigation
Student activities
The other 0 degrees (imaginary) reference line is the line that runs through the North
Pole, the South Pole and Greenwich in England. This line is called the prime
meridian. Greenwich was chosen as the point through which the prime meridian is
drawn because the Royal Observatory at Greenwich played a major part in
determining longitude at sea. Imaginary lines that run from the North Pole to the
South Pole divide the Earth into sections. These imaginary lines, called meridians of
longitude, are measured east or west of the prime meridian.
Using this coordinate system, one can locate any place on Earth with accuracy.
Some examples:
The latitude and longitude for:
a)
Melbourne
b)
Los Angeles, USA
c)
Nairobi, Kenya
d)
Paris
38 degrees S, 145 degrees E
34 degrees N, 118.5 degrees W
1.5 degrees S, 37 degrees E
48 degrees N, 2.2 degrees E
What you need
Spherical object, eg. foam ball, blown up balloon
Texta
Skewer
Earth globe
String
Sticky-tape
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Celestial Navigation
Student activities
What to do
Use an Earth globe to help you do the following activity.
1
Mark the North and South Poles on your spherical object.
2
Mark and label the equator half way around your spherical object as best you
can, either using a texta or sticking a piece of string horizontally around the
diameter.
3
Add lines of latitude 30 degrees S, 30 degrees N, 60 degrees S and
60 degrees N.
(The lines (‘parallels’) of latitude are measured in degrees from the equator. The
number of degrees is determined by the angle formed by drawing imaginary lines
from the equator and from the particular latitude to the centre of the Earth. For
example, the angle for the Poles is a right angle, so the latitude of the Poles is 90
degrees).
4
Mark and label the Tropic of Cancer (23.5 degrees N) and the Tropic of Capricorn
(23.5 degrees S).
5
Mark and label the Arctic Circle (66.5 degrees N) and Antarctic Circle
(66.5 degrees S).
6
Mark and label the prime meridian that passes through the North and South
Poles and Greenwich in England.
7
Mark in lines (‘meridians’) of longitude: 45 degrees E, 90 degrees E, 135 degrees
E, 180 degrees E, 45 degrees W, 90 degrees W, 135 degrees W.
(The number of degrees is determined by the angle formed by drawing imaginary
lines from the prime meridian and from the particular longitude to the centre of the
Earth. For example, the angle for the International Date Line is 180 degrees).
8
Find out the longitude and latitude of three different places you would like to visit
and mark the (approximate) positions using the scale on your model.
For the coordinates of a range of cities around the world, see
http://www.infoplease.com/ipa/A0001769.html
Note that degrees (symbol ) are divided into 60 equal parts called ‘minutes’
(symbol ').
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Celestial Navigation
Student activities
Activity 2: Diurnal motion
Background information
The Earth spins once every 24 hours resulting in day and night. This rotation causes
the stars, (including the Sun), to appear to rise in the east and set in the west (diurnal
motion). Stars in most parts of the sky travel in a large arc, then disappear below the
horizon. However, for observers in the Southern Hemisphere, stars in the southern
part of the sky never ‘set’ below the horizon, but trace circles above the South Pole.
Astronomers think of the Earth as being surrounded by an imaginary sphere
(celestial sphere) dotted with stars. The North Pole of the Earth extends out to the
North Pole of the celestial sphere and the South Pole of the Earth extends out to the
South Pole of the celestial sphere. Likewise, the Earth’s equator projects out to the
equator of the celestial sphere. (The Equatorial Coordinate System is based on this
representation of the sky.)
As the Earth turns, the stars seem to move across the sky. If you stood at the South
Pole on Earth, the South Celestial Pole would be directly overhead and the stars
overhead would seem to be rotating clockwise around this point. In Melbourne, the
South Celestial Pole is at an angle of (approximately) 38 degrees above the horizon
– Melbourne’s latitude. From southern Australia, stars like those that make up the
Southern Cross never set but are always visible in our night sky and can be seen
rotating around the South Celestial Pole.
Note that in the same way that Greenwich, England has been chosen by convention
to be the origin of the prime meridian (0 degrees longitude) on Earth, the vernal
equinox (the point at which the Sun crosses the celestial equator during March) has
been chosen to be the starting point for Right Ascension (a star’s equivalent of
longitude).
http://bdaugherty.tripod.com/gcseAstronomy/celestialSphere.html
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Celestial Navigation
Student activities
What you need
•
•
black umbrella
white circular stickers
What you do
1. Open up a black umbrella.
2. On the inside of the umbrella, label or identify the centre or turning point of the
umbrella (above the shaft) which will model the South Celestial Pole.
3. Stick the white circular stickers in the appropriate positions to represent the
Southern Cross and the star Canopus as shown in the diagram below. These
stars are always above the horizon in southern Australia.
4. Hold the umbrella at approximately 38 degrees above the horizontal and slowly
turn it once in a clockwise direction. This models diurnal motion (24 hours) as
viewed from southern Australia.
Adjustments for other places in Australia can be obtained by visiting the CSIRO
website: http://www.csiro.au/helix/scientriffic/pdf/TG36.pdf
5. Hold the umbrella so that the South Celestial Pole is directly above your head.
Turn the umbrella once slowly in a clockwise direction. This models diurnal
motion of the overhead stars at the South Pole for one day (24 hours).
Questions
1.
2.
3.
4.
What is diurnal motion?
Do the stars really move?
Do the stars appear to move during the day-time? Why don’t we see them?
Explain how the stars would appear to move when looking at the night sky from
Melbourne compared to looking up from the South Pole. Use your model to help
you answer this question.
5. How would the stars seem to move if you were standing at the North Pole?
6. How would the stars seem to move if you were standing at the equator?
7. In what ways is the celestial sphere imaginary?
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Celestial Navigation
Student activities
Activity 3: The Local coordinate system
Background information
Celestial coordinates are used to find
the locations of celestial bodies such as
the Sun, Moon, planets and stars just
like the terrestrial coordinates locate
specific places on Earth. One commonly
used coordinate system that
astronomers use is the Local Coordinate
System.
In this system, an object in the sky can
be located relative to an individual
observer standing on the surface of the
Earth. The observer is thought of as
being in the centre of
(i) a disc with the horizon marking the
edges, and
(ii) an imaginary sphere with stars and celestial bodies surrounding the observer.
Half the sky is above the horizon and half is below the horizon. The point directly
overhead is called the zenith. The point directly below the observer is called the
nadir.
The position of an object in the sky is defined by two coordinates: azimuth and
altitude. Azimuth is measured clockwise around the horizon from North in degrees.
Altitude is measured from the horizon up to the object in degrees. Local coordinates
are different for each individual observer. As the Earth rotates on its axis and makes
its way around the Sun, the objects change position in the sky relative to the
observer. The altitude and azimuth of an object in the sky therefore varies with the
location of the observer on Earth, the time of day and the time of year.
azimuth
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Celestial Navigation
Student activities
What you need
•
Star map for the current month for observers in the Southern Hemisphere at 7pm
or 8pm: http://www.skymaps.com/downloads.html or at
http://museumvictoria.com.au/Planetarium/DiscoveryCentre/Sky-Maps/
What to do
Use the star map and the notes above to estimate the altitude and azimuth of the
bright stars that are currently in the evening sky. Five stars have been suggested
below as a start.
You might like to work in pairs, and then compare your answers with other students.
Star
Altitude
Azimuth
Canopus
Alpha Crux
(in Southern Cross)
Alpha Centauri
Beta Centauri
Achernar
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Celestial Navigation
Student activities
Activity 4: Using the Southern Cross (Crux) to find south
Background information
One of the things that make finding stars and constellations difficult is that the stars
seem to move slowly across the sky during the night, rising in the east and setting in
the west, just like the Sun during the day. From southern Australia, if you watch the
stars for a few hours, they move like this, if you are facing south:
→
↑
↓
South
Some stars travel in a large arc across the sky, then disappear below the horizon.
Other stars never ‘set’ below the horizon but trace a circle in the sky. At the centre of
these circles is a point called the South Celestial Pole. This part of the sky is directly
above the South Pole of the Earth. That is, if you stood at the South Pole, the South
Celestial Pole would be directly overhead.
In the Northern Hemisphere, a star called Polaris or the Pole Star marks the North
Celestial Pole. There is no star near the South Celestial Pole, so instead,
southerners must use the Southern Cross to find South.
Once you are familiar with finding the Southern Cross, it can be used to work out the
direction of south at any time of night and at any time in the year. This is because the
Southern Cross never sets when viewed from the southern part of Australia.
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Celestial Navigation
Student activities
What to do
To find south:
1. Find the Southern Cross.
2. Draw an imaginary line through the long axis of the Southern Cross beginning
with the star that marks the top of the cross. (Note: during summer, the Southern
Cross is low in the sky and therefore upside-down).
3. Extend the line four and a half times the length of the cross.
4. This will bring you to the point in the sky called the South Celestial Pole.
5. From this point, drop a line vertically down to the horizon. This gives you the
direction of true south. Compasses find magnetic north/south, not true
north/south – a compass needle will show magnetic south to be 11˚ west of true
south when measured from Victoria.
Alternative method of finding south:
This method involves using the two bright Pointer stars (Alpha Centauri and Beta
Centauri) that lie near the Southern Cross in the constellation of Centaurus.
•
•
After step 2 above, draw a perpendicular bisector between the two Pointers, that
is, a line starting at the mid-point between the two Pointers and coming out at
right angles. This line should cross the line you drew in step 2.
The intersection of these two lines is close to the South Celestial Pole.
SCP
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Celestial Navigation
Student activities
Activity 5: Finding North
Background information
You can think of the Earth as being one big bar magnet with a very weak magnetic
field. The poles of the big bar magnet do not run exactly along the Earth’s rotational
axis. It is slightly off centre (see diagram below). The difference between magnetic
north/south and true north/south is called the declination, and most good maps
indicate what the declination is for different areas (since it changes slightly depending
on where you are on Earth). Due to the Earth’s very weak magnetic field, a compass
needs to have a lightweight magnet for its needle and a frictionless bearing for it to
be able to rotate freely.
Compasses find magnetic north/south, not true north/south. A compass will measure
magnetic south to be 11 degrees west of true south and magnetic north to be 11
degrees east of true north in Melbourne.
The following methods can be used to find true north.
What you need
•
•
•
•
•
Compass
Watch
Sunrise/sunset times
Chalk
Stick
What to do
Warning: You should never look directly at the Sun because this can seriously
damage your eyes.
Method 1: Finding north using your watch
Find the Sun in the sky. Point the number 12 on your watch to the Sun.
True north lies midway between the number that the hour hand (small hand) points to
and the number 12.
Example
If it is 2:50pm and the 12 on your watch is pointing towards the Sun, true north will be
midway between the number 12 and the number 3.
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Celestial Navigation
Student activities
Method 2: Finding north using local noon time
When the Sun is at its highest point in the sky, the Sun will be on the north/south line.
True north will be in the direction of the Sun. You can find this time by subtracting the
sunrise time from the sunset time. The Sun will be at its highest point half way
between these times because it takes the Sun exactly the same amount of time to
rise and reach its highest point as it takes it to descend and set.
Example
If sunrise is at 7:16am and sunset is at 5:16pm, the Sun will be on the north/south
line or at its highest point at 12:16pm. In this case, North will be in the direction of the
Sun at 12:16pm.
Method 3: Finding north using the shortest shadow
Take a piece of chalk into the school yard late in the morning on a sunny day. Using
the shadow cast by the vertical pole of a net ball ring or a stick in the ground in the
school yard, mark the length of the shadow every 5-10 minutes for about 2 hours.
The shortest shadow will indicate when the Sun was at its highest point. North will be
in the opposite direction to the shadow cast by the pole or stick. (See diagram
below).
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Celestial Navigation
Student activities
Activity 6: Measuring your latitude using hand spans
Background information
Latitude is the angular distance of a location north or south of the equator. The
latitude of Melbourne is about 38 degrees south. The latitude of a location can be
determined using the stars, by measuring the height of one of the celestial poles
above the horizon. In the Southern Hemisphere, we use the South Celestial Pole. If
you are unfamiliar with using the Southern Cross to find the South Celestial Pole,
refer to the relevant activity in this education kit.
In the Northern Hemisphere, the North Celestial Pole is used to measure latitude.
The North Celestial Pole is marked by the star Polaris. The angle measured between
the horizon and Polaris in degrees at a location in the Northern Hemisphere is equal
to the latitude of that location.
You can make approximate angular measurements using your fingers and hands at
arm’s length. This helps you to measure your latitude, and to find the positions of
particular stars and constellations.
Measuring large angles
Outstretch one arm so that it is parallel with the horizon. This are will act as your
baseline. Hold your other arm straight up beside your head. Both arms are now at
right angles. A 45 degree angle can be made by moving your upright arm to half the
90 degree angle. With practice you can even divide the 90 degree angle into thirds
and measure a 30 degree angle.
Measuring smaller angles
One finger width at arms length covers about two degrees of sky and can be used to
measure small distances between stars.
Half a hand-span (at arms length) is the distance across the knuckles of one hand. It
covers about 10 degrees across the sky.
A whole hand-span is an outstretched open hand. It measures 20 degrees across the
sky.
Australian latitudes
Melbourne
Sydney
Brisbane
Adelaide
Perth
Hobart
Darwin
Canberra
~ 38 degrees
~ 34 degrees
~ 27 degrees
~ 35 degrees
~ 32 degrees
~ 43 degrees
~ 12 degrees
~ 35 degrees
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Celestial Navigation
Student activities
What you need
•
Star maps or planisphere
A star map for the current month for observers in the Southern Hemisphere at
7pm or 8pm is available at http://www.skymaps.com/downloads.html or at
http://museumvictoria.com.au/Planetarium/DiscoveryCentre/Sky-Maps/
What to do
Ask your student to:
1. On a clear night, find the Southern Cross and use it to find the approximate
position of the South Celestial Pole. (An activity in this kit shows you how to
do this).
2. Measure your latitude by measuring the distance between the horizon and the
South Celestial Pole using hand-spans and finger width measurements as
accurately as you can.
3. Discuss how accurate the measurements are. Compare measurements with
other students. Who had the closest value to the tabulated value? Find out
why.
Optional
4. Use a star map to help you find particular stars and constellations in the
current night sky.
5. Research the azimuth and altitude of particular stars or constellations.
See ‘The Local Coordinate System’ activity in this kit, and then use your hand
or finger measurements to find stars and constellations in the real night sky.
Question
What would your latitude be if you were standing at:
• the South Pole
• the North Pole
• the Equator?
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