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Module 2: Star Gazing
Activity 1:
Star Patterns
Summary:
In this Activity, we will investigate
(a) constellations & constellation lines,
(b) the zodiac, and
(c) the ecliptic.
(a) Constellations & constellation lines
The night sky has been well studied for thousands of
years, and has been used over the ages to act as clock,
calendar and compass.
It’s human nature to identify with patterns in nature animal shapes in cave formations, castles in clouds,
and mythological creatures in star patterns.
Labelling patterns in the night sky had practical
importance too: it helped our ancestors learn to orient
themselves in space and time.
They navigated by the stars, and used them to time their
seasonal activities - for example, Egyptians received
a yearly warning of the onset of the Nile flood season
when Sirius started to appear in the night sky.
Cultures around the world identified patterns
in the night sky. . .
Small Magellanic Cloud
For example, some
Australian aboriginal
communities identified the
Large and Small Magellanic
*
Clouds ...
* these are actually small
galaxies neighbouring the
Milky Way
Large Magellanic Cloud
… as an old
couple, keeping
watch over the
members of
their tribe ...
… and the Milky Way
as a river carrying the
dead to their final
resting place.
The Southern Milky Way - Wide angle
view towards the Galactic centre
Constellations
Modern
astronomers still
use star groupings,
called
constellations, to
identify regions of
the sky.
From Centaurus to Carina
Constellations are groupings of stars which appear
close together on the sky.
For example,
3 stars in a constellation, viewed from Earth
In fact, they may not be close together at all - some may be
relatively close to Earth, while others stars in the same
constellation may be much further away (but still bright
enough to be seen).
sightlines to Earth
Earth
The same 3 stars, viewed from
Alpha Centauri
(not to scale!)
Our modern system of 88 constellations is based
partly on constellations first labelled in Mesopotamia,
Babylon, Egypt and Greece, and partly on
constellations added to fill in the southern sky (plus
regions of the northern sky previously neglected
because they contain no bright stars.)
Constellation Lines:
Constellation lines are the the lines that connect stars
to make patterns in the sky that people have used to
find their way around the sky over centuries - for
example,
Leo
Cancer
We can describe the angular position of an object in the
sky by its altitude above the horizon (alt)
and by its angular distance from the northmost point
on our horizon, i.e. its azimuth (az),
both measured in degrees.
For
example:
alt
az
horizon
north west
north
The point directly overhead is called the zenith.
north east
We can make a rough estimate of the
angular separation of objects in the night sky
by holding our hand out at arm’s length:
A finger’s width is roughly 1°
1°
And a fist’s width is roughly 10°
10°
We will not look in this Activity at the tremendous
intellectual journey astronomers and philosophers
took in moving from an Earth-centred (or geocentric)
Universe to a Sun-centred (i.e. heliocentric) Universe.
… and from there, to a Universe with no centre at all!
We will start with our modern understanding of the Earth
orbiting our Sun in an almost circular orbit, against a
background of far-distant stars.
The Sun
The Earth
(b) The Zodiac
Most people think of the zodiac when constellations are
mentioned. There are only 13*constellations in the zodiac,
out of a total of 88:
Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini,
Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus &
Sagittarius.
What’s so special about these 13 constellations?
* Do 13 constellations in the zodiac surprise you? Ophiuchus
does not feature in the pseudo-science of astrology, but
astronomers identify it as a zodiacal constellation.
The zodiacal constellations are the constellations
through which the Sun appears to pass each year.
East
Gemini
Cancer
West
Aries
Leo
Ophiuchus
Pisces
Virgo
Taurus
Libra
Zodiac
band
Aquarius
Capricornus
Sagittarius
Scorpius
path of the
Sun through
the sky
Many great thinkers over the centuries have worked
to piece together a clear picture of how the apparent
movements of the Sun, constellations and planets in
the sky relate to the Earth’s orbit around the Sun.
The following animations might help relate the zodiacal
constellations to the way the Earth orbits the Sun each year.
sightline from Earth
zodiacal
constellations
Earth’s orbit
around the Sun
Viewed from Earth, the Sun is “in” Taurus: May 13 - June 21
The Sun is “in” Gemini: June 21 - July 20
The Sun is “in” Cancer: July 20 - August 11
The Sun is “in” Leo: August 11 - September 18
The Sun is “in” Virgo: September 18 - November 1
The Sun is “in” Libra: November 1 - November 22
The Sun is “in” Scorpius: November 22 - December 1
The Sun is “in” Ophiuchus: December 1 - December 19
The Sun is “in” Sagittarius: December 19 - January 19
The Sun is “in” Capricorn: January 19 - February 18
The Sun is “in” Aquarius: February 18 - March 13
The Sun is “in” Pisces: March 13 - April 20
The Sun is “in” Aries: April 20 - May 13
You may have noticed that the dates corresponding to
each zodiacal constellation are not the same as the
dates commonly quoted for “star signs”. In the next
Activity, we will investigate why this is so.
Another question may have occurred to you:
when the Sun is “in” Aquarius, for example, Aquarius
can’t be seen because it is up at the same time as the
Sun - that is, during the day.
So why would ancient peoples label times after
constellations they can’t see at the time?
Think about it, then click here to see if you agree with
our answer.
(c) The Ecliptic
The apparent path of the Sun across the sky is called
the ecliptic.
East
Cancer
Gemini
West
Aries
Leo
Ophiuchus
Pisces
Virgo
Taurus
Libra
Aquarius
Capricornus
Sagittarius
Scorpius
From our heliocentric (Sun-centred) point of view, this
apparent motion reflects the Earth’s orbit around the Sun.
The plane of the ecliptic is an imaginary planar surface in
space containing the Earth’s orbit and the Sun:
The Earth takes one year to
make a complete orbit around the Sun.
Since all the planets are in nearly the same plane, they all
appear to travel quite close to the ecliptic plane (in the
zodiac band actually).
Ecliptic
Zodiac
band
Mercury
Mars
Venus
Earth
In the next Activity we will investigate the origin of
the seasons on Earth, the Earth’s precession, and
measuring angles and positions on the sky.
Image Credits
AAO © David Malin: From Centaurus to Carina (reproduced with permission)
http://www.aao.gov.au/local/www/dfm/image/cen_crux_car.jpg
AAO © David Malin : image reference UKS 17
The Small Magellanic Cloud (reproduced with permission)
http:// www.aao.gov.au/local/www/dfm/uks017.html
AAO © David Malin : image reference UKS 14
The Large Magellanic Cloud (reproduced with permission)
http://www.aao.gov.au/local/www/dfm/uks014.html
AAO © David Malin : image reference AAT 28
Wide angle view towards the Galactic centre (reproduced with permission)
http://www.aao.gov.au/local/www/dfm/aat028.html
Now return to the Module home page, and
read more about the star patterns in the
Textbook Readings.
Hit the Esc key (escape)
to return to the Module 2 Home Page
Let’s take Aquarius as an
example. The Sun is “in”
Aquarius from February 18 March 13.
We will not see Aquarius in this period, but ...
in the month before the Sun is “in” Aquarius, Aquarius
sets just after the Sun does;
and in the month after the Sun is “in” Aquarius,
Aquarius rises just before the Sun does.
So to people who watched the sky,*
a constellation would be very noticeable
as it moved from being visible just after
sunset, to being visible approximately
one month later, just before sunrise.
* without bright city lighting and distractions
like televisions & online courses!
Click here to return to the Activity!