m02a01

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!