The previous chapter took you on a cosmic zoom through space and time. That quick preview only sets the stage for the drama to come. Now it is time to return to Earth and look closely at the sky and answer four essential questions:

• How do astronomers refer to stars and compare their brightness?

• How does the sky appear to move as Earth rotates?

• What causes the seasons?

• How can astronomical cycles affect Earth’s climate?

As you study the sky and its motions, you will be learning to think of Earth as a planet rotating on its axis. The next chapter will introduce you to some of the most dramatic cycles in the sky.

Guidepost

Constellations

In ancient times, constellations only referred to the brightest stars that appeared to form groups. We

continue to use many of the same groupings today

Constellations

They were believed to represent great heroes and mythological figures. Their

position in the sky seemed to tell stories that were handed down from generation to generation over thousands of years.

Different cultures grouped stars differently. Example: The Pawnee Indians knew the constellation Scorpius as 2 groupings

-The long tail was the snake

-The two bright stars at the scorpion’s tail were the swimming ducks

The stars in constellations are not physically close to each other in space.

Scorpius Constellation

Constellations

Projection Effect:

The stars of a constellation

may be located at very different distances from

us.

Example: Cassiopeia

Northern Hemisphere Constellations

Typically named after animals and mythological beingsExample-Greeks, Northern Asians, and Native Americans

Constellations (5)Stars are named by a Greek letter () according to their relative brightness within a given constellation +

the possessive form of the name of the constellation:

OrionBetelgeuse

Rigel

Rigel = OrionisBetelgeuse = Orionis

Constellations (6)

Some examples of easily recognizable constellations and their brightest stars

Greek Letter Alphabet

The Magnitude Scale

First introduced by Hipparchus (160 - 127 B.C.):

• Brightest stars: ~1st magnitude

• Faintest stars (unaided eye): 6th magnitude

More quantitative:

• 1st mag. stars appear 100 times brighter than 6 th mag. stars

• 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude => fainter object!)

Betelgeuse

Rigel

Magnitude = 0.41 mag

Magnitude = 0.14 mag

The Magnitude Scale (Example)

For a magnitude difference of 0.41 – 0.14 = 0.27, we find an intensity ratio of

(2.512)0.27 = 1.28.

In other words, Rigel is 1.28 times brighter than

Betelgeuse.

The Magnitude Scale (2)

Sirius (brightest star in the night sky): mv = -1.42Full moon: mv = -12.5

Sun: mv = -26.5

The magnitude scale system can be extended towards negative numbers (very bright) and

numbers greater than 6 (faint objects):