Download Using the Southern Cross to find south

Student activity
Using the Southern Cross 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.
http://museumvictoria.com.au/scienceworks/education/
57
Student activity
http://museumvictoria.com.au/scienceworks/education/
58
Student activity
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
http://museumvictoria.com.au/scienceworks/education/
59
Student activity
Using the Southern Cross to tell the time
Background information
The position of the stars in the sky slowly changes as the Earth rotates on its axis and
revolves around the Sun. The position of the Southern Cross therefore varies according
to the time of day/night and the time of year.
The following activity explains how to make a Southern Cross finder. The position of the
split pin in the diagrams below represents the South Celestial Pole. This an imaginary
point that is an extension of the South Pole into the sky. The South Celestial Pole is the
point around which all the stars seem to rotate as the Earth turns on its axis and revolves
around the Sun. In southern Australia, the Southern Cross never sets below the horizon
so can always be seen at night.
What you need




Scissors
Southern Cross finder worksheet
Cardboard
Split pin
What to do
1. Paste the worksheet onto thin cardboard.
2. Cut out around the dark outline of the two shapes on the worksheet.
3. Use a split pin to join them through the points marked +.
(a) You can now use the Southern Cross finder to show you the approximate position of
the Southern Cross at any time of year and at any time of night as viewed from
Melbourne.
4. First choose a month and time. Example:
End of October at 8pm.
5. Line up the line dividing October and
November with 8pm as shown in the
diagram. The Southern Cross will be in the
position shown relative to the horizon.
(b) You can also use the Southern Cross finder in
reverse to give you an approximate time.
6. On a clear night, face south to locate the
Southern Cross in the night sky.
7. Hold the Southern Cross finder up in front of
you so that the horizon on the finder
matches the real horizon.
8. Rotate the dial so that the Southern Cross
and the Pointers on the finder approximate
their position in the real night sky.
9. Locate the approximate time of the month
on the dial and read the corresponding time.
http://museumvictoria.com.au/scienceworks/education/
60
Student activity
The Southern Cross finder worksheet
http://museumvictoria.com.au/scienceworks/education/
61
Student activity
Plotting the movement of the Southern Cross
Background information
The Southern Cross, along with the other constellations near the South Celestial Pole,
never disappears below the horizon when viewed from Southern Australia. It never sets
below the horizon, but traces a circle in the sky. Part of this circle can be glimpsed if we
observe the Southern Cross over several hours. (In winter, the Southern Cross is high in
the sky and therefore easier to see).
The following activity raises students’ awareness that the stars do not remain in the same
position in the sky during the night. The activity involves tracing the movement of the
Southern Cross during the night.
What you need



A4 piece of paper (or graph paper)
Clip board or a hard covered book to lean on.
Pencil
What to do
1. On a clear night, go outside about half an hour after sunset (~6.30pm in winter).
2. Find the Southern Cross and face south. (The activity ‘Using the Southern Cross to find
south’ is helpful here.)
3. Along the bottom of your A4 paper, make a rough sketch of the buildings, trees etc.
you can see on the horizon.
4. Draw the Southern Cross on the sheet of paper, taking care to draw it in the exact
position that you see it.
5. Go back inside. After an hour look at the Southern Cross again. Draw it again on the
same diagram, noting how it has moved.
6. Repeat step 5 two or three times if possible. When you finish you will have a record of
how the Southern Cross moves in the sky over the period of a few hours.
7. On your diagram draw lines between the end stars of each Cross (the stars alpha and
gamma) and extend these lines until they all meet. They should meet at a point. This
point is called the South Celestial Pole and it is the point in the sky around which all of
the stars seem to rotate.
Optional Questions:
1. In which direction does the Southern Cross seem to be moving? (Clockwise or
anticlockwise?) Explain why.
(Clockwise or in a westerly direction, since the movement is due to the Earth’s rotation
from west to east.)
2. Where would the South Celestial Pole be in the sky if you were at the South Pole?
(The South Celestial Pole is directly overhead at the South Pole.)
3. Where would this point be in the sky if you were at the equator?
(The South Celestial Pole would be on the Southern horizon if you were at the
Equator.)
http://museumvictoria.com.au/scienceworks/education/
62
Student activity
Plotting the movement of the Southern Cross
http://museumvictoria.com.au/scienceworks/education/
63
Student activity
Taking a look at astrology
Background information
Astrology versus astronomy
There was a time when astronomy and astrology were the same. The ancient
astronomers believed that the Sun, Moon and planets were symbols of the Gods. Hence
the planets have names from Greek and Roman mythology. Originally, these bodies
were believed to influence the fortunes of kings and nations. The ancient Chinese
astronomers had possibly the most detailed and accurate records of the sky, because
Chinese emperors believed that the heavens sent signs and good omens for their
dynasty.
The Greeks introduced the notion that the planets influenced each and every person, not
just the nobility. This notion was formalised by Ptolemy in 110BC. Many centuries later,
the famous astronomer, Tycho Brahe (1546-1601) made his living by drawing
horoscopes for the wealthy while he pursued his research measuring the movements of
the stars and planets. Today, however, astrology and astronomy are quite distinct.
Dictionaries define astrology as:
‘A study which assumes, and professes to interpret, the influence of the heavenly bodies
on human daily life’.
In contrast, astronomy is defined as:
‘The science of the celestial bodies, their motions, positions, distances, magnitudes, and
other related information’.
Signs of the zodiac
In Ptolemy’s time, over 2000 years ago, the Sun, Moon and planets as viewed from
Earth moved through 12 constellations. These constellations are known as the zodiac
constellations. According to astrologers, whichever of these 12 constellations lay behind
the Sun when you were born is your star sign. Most astrologers today still base their star
signs and charts on those established by Ptolemy, and use them to somehow make
predications about things like who to marry or when to buy a lottery ticket.
In the 2000 years since the time of Ptolemy, the path of the Sun across the sky has
changed due to the precession of the Earth. The Earth is like a spinning top that slowly
wobbles on its axis. This causes the position of the Celestial Poles to shift and alters the
apparent path of the Sun across the sky. The Sun now passes through the 12 zodiac
constellations at quite different times than was the case 2000 years ago, and now
spends more than two weeks in a 13th constellation – Ophiuchus (the Serpent Holder).
Astrological signs assume that the Sun takes an equal amount of time to move through
each zodiac constellation. This is not the case. The time the Sun spends in each
constellation varies considerably, as the following table demonstrates. Compare your
astrological and astronomical star signs. Are they the same?
http://museumvictoria.com.au/scienceworks/education/
64
Student activity
Discuss:
Do you think that the position of the Sun when you were born can determine your future?
If so, how do you think that the Sun does this?
Is astrology a science or an entertainment?
Constellation
Astrological Zodiac
Astronomical Zodiac
Aries
21 March – 20 April
19 April – 13 May (25 days)
Taurus
21 April – 21 May
14 May – 19 June (37 days)
Gemini
22 May – 21 June
20 June – 20 July (31 days)
Cancer
22 June – 22 July
21 July – 9 August (20 days)
Leo
23 July – 23 August
10 Aug – 15 Sept (37 days)
Virgo
24 Aug – 23 Sept
16 Sept – 30 Oct (45 days)
Libra
24 Sept – 23 Oct
31 Oct – 22 Nov (23 days)
Scorpio
24 Oct – 22 Nov
23 Nov – 29 Nov (7 days)
Ophiuchus
Not recognised as a sign of the zodiac by
astrologers
30 Nov – 17 Dec (18 days)
Sagittarius
23 Nov – 21 Dec
18 Dec – 18 Jan (32 days)
Capricorn
22 Dec – 20 Jan
19 Jan – 15 Feb (28 days)
Aquarius
21 Jan – 19 Feb
16 Feb – 11 March (24 days)
Pisces
20 Feb – 20 March
12 March – 18 April (38 days)
http://museumvictoria.com.au/scienceworks/education/
65
Student activity
What you need

A daily horoscope from a newspaper, magazine or website (one that the students
have not seen and a couple of days old)

Scissors

Pen

Paper
What to do
1. Cut out the twelve horoscopes from a publication.
2. Make a master copy for yourself.
3. Cut off the dates and zodiac designations.
4. Copy the set of predictions for each student making sure there is room to write above
each horoscope.
5. Distribute a copy of the horoscopes to each student.
6. Students should read through the astrologer’s predictions, choosing the prediction
that most accurately describes their past days.
7. Have them write their birth date and zodiac sign above their chosen prediction.
8. Collect all the horoscopes and mix them up.
Redistribute them so that each student gets someone else’s paper.
9. Write the true dates/zodiac signs the astrologer specified for each prediction onto a
chalk/white board.
10. Have the students total how many ‘hits’ and ‘misses’ there were.
Pool class results and discuss.
Optional
 Collect several horoscope columns for the same day or week and compare the
predictions and statements of different astrologers for the same star sign.

See if everyone’s personality can be grouped into just 12 categories.

Ask students to find another person who has the same star sign. Compare
similarities and differences.
http://museumvictoria.com.au/scienceworks/education/
66