CONSTELLATIONS

Under the blanket of night, stars appear like a jumble of pinpricks of light. From the most ancient times civilizations have given names and mythologies to conspicuous patterns of bright stars to help make the sky seem more familiar.

It has always been very seductive to think that the stars are positioned in such an order that a relationship between them might be established.

By linking particular patters or groups of stars many ancient civilizations used them for understanding the universe through the creation of mythology.

The patterns of the constellations, then, were invented not discovered.

They are simply distinctive, easy-to-remember patterns of stars as seen from Earth and in scientific terms they provide a system that allows the easy location and identification of individual stars.

Today, we know any relationship to be purely superficial since their positions are only relative to the position from which they are viewed from Earth.

Although the constellations have no scientific significance, astronomers have retained them as convenient visual reference points; it is much easier to speak of a star in Orion than to give its geometrical position in the sky.

Although the constellations have changed over the years defined boundaries were laid down by the International Astronomical Union (IAU).

During the Astronomical Congress of 1928, it was decided to recognize 88 constellations

The origin of the patterns is not known for certain though the ancient Chinese and Egyptians are known to have applied symbolic sky maps. The basic pattern followed today is that of the ancient Greeks. Some names date back to the 7th Century BC.

ORIGINS

Dependence on the sky became an essential part of many early cultures.

Some historians argue that many of the myths associated with the constellations were invented specifically to help farmers construct an accurate understanding of the sky. From ancient times farmers knew that for most crops, you plant in the spring and harvest in the autumn.

Therefore, by ensuring the planting took place at the correct time the risk of a failed harvest was kept to a minimum.

Since different constellations are visible at different times of the year, farmers were able to use them to tell what month it was.

As a result, accurate calendar prediction was an essential skill and was a central aspect of astronomy up to at least the 16th century.

It is convenient to divide the constellations into two groups:

•Those that never set below or rise above the horizon are called 'circumpolar' constellations.

•The rest are divided into 'seasonal' constellations which periodically come into view through the year.

Which constellations are circumpolar and which are seasonal depends on your latitude and will change from place to place around the world.

At the Poles all constellations are circumpolar; at the Equator all are seasonal. From the USA, most of the Southern Constellations, a large group of 32 constellations, are always invisible and can only be viewed by traveling south of the equator.

Circumpolar constellations are…

1.Seasonal2.Only visible in the Southern Hemisphere3.The only “official” constellations4.Visible all year long

Circumpolar constellations are…

1.Seasonal2.Only visible in the Southern Hemisphere3.The only “official” constellations4.Visible all year long

Circumpolar Constellations

Star Trails

It is worth noting here that as a result of precession the constellations have moved over 30º eastwards. This means that they no longer correspond with the Zodiacal signs.

The Earth rotates (white arrows) once a day about its axis of rotation (red); this axis itself rotates slowly (white circle), completing a rotation in approximately 26,000 years

Circumpolar Constellations

• Can be seen all year long• Never fully set below the horizon• Appear to move counter

clockwise around Polaris• Caused by Earth’s Rotation

• All of the above

Examples of Circumpolar Constellations

1. Ursa Major – The Big Bear2. Ursa Minor – The Little Bear3. Cassiopeia – Queen on Her Throne4. Draco- The Dragon5. Cepheus- The King

• # of stars seen as circumpolar depends on the observers latitude

• Further North the observer lives, the more stars will appear circumpolar

• Earth turns west to east

• Sky appears to turn east to west

Ursa MajorThe Great Bear

• Best known constellation• Common name is Big Dipper• Pointer stars- front 2 stars of the

Big Dipper point to Polaris (North Star)

Ursa Minor (the little bear)

• Ursa Minor, also called the Little Dipper, is a circumpolar constellation.

• There are several mythological stories behind these famous constellations. In Greek myth, Zeus was having an affair with the lovely Callisto. When his wife, Hera, found out she changed Callisto into a bear. Zeus put the bear in the sky along with the Little Bear, which is Callisto's son, Arcas.

Ursa Major

3 Stars of Orion’s Belt

Can be used to find 2 other constellations & a star cluster

Canis Major- (Big Dog) follow the line made by the 3 stars of Orion’s belt down to the left.

–Sirius- the brightest star in the nighttime sky is found in Canis Major

Taurus (the Bull)

• Follow the line made by Orion’s belt up & to the right

• Aldebaran- Red star that is the eye of the bull. It is the 13th brightest in the nighttime sky

The most famous of all the constellations are the 12 that make up the Zodiac. All planets can be observed only in these 12 constellations as they pass across the sky during the course of their year. This is because the orbits of all the planets lie within ± 8º of the ecliptic.

The Sun also moves through the Zodiac, though its apparent motion (diurnal motion) is actually caused by the movement of Earth.

The EclipticThe apparent path that the sun (and

planets) appear to move along against the star background.

The Zodiac

Band of 12 constellations along the ecliptic.

Summary Notes Earth rotates on its axis, this makes most constellations appear to rise in the east and set in the west during the night.

Most constellations appear in many different positions in the sky as the Earth revolves around the sun.

There is a group of stars that appear in the sky all night long and all year long. It seems that these stars do not rise and set, but circle the Earth’s north pole each night. These stars are called circumpolar.

Twinkle Twinkle Little Star

"Twinkling Stars" are due to Earth's atmosphere

Final Thought…