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.
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Module 2: Star Gazing
Activity 1:Star Patterns
(a) Constellations & constellation lines
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 theirseasonal activities - for example, Egyptians receiveda yearly warning of the onset of the Nile flood season when Sirius started to appear 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 ...
The Southern Milky Way - Wide angle view towards the Galactic centre
From Centaurus to Carina
The same 3 stars, viewed from Alpha Centauri
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
(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.)
We can describe the angular position of an object in the sky by its altitudeabove the horizon (alt)
and by its angular distance from the northmost point on our horizon, i.e. itsazimuth (az),
both measured in degrees.
The point directly overhead is called the zenith.
We can make a rough estimate of the angular separation of objects in the night skyby holding our hand out at arm’s length:
A finger’s width is roughly 1°
And a fist’s width is roughly 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 Earthorbiting our Sun in an almost circular orbit, against a background of far-distant stars.
(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:
What’s so special about these 13 constellations?
* Do 13 constellations in the zodiac surprise you? Ophiuchusdoes not feature in the pseudo-science of astrology, but astronomers identify it as a zodiacal constellation.
path of the Sun through the sky
The zodiacal constellations are the constellations through which the Sun appears to pass each year.
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
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
Another question may have occurred to you:
when the Sun is “in” Aquarius, for example, Aquariuscan’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.
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.
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.
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!