THE SKY

(Part 2)

Objectives

• To be able to interpret and apply the term “brightness” to stars (finishing this objective).

• To be able to describe how the sky moves with reference to the Earth’s rotation.

• To be able to predict the seasons and describe what causes them.

• To be able to synthesize information on astronomical cycles to predict Earth’s climate.

Light Intensity (Flux)

• Flux is the light energy from a star that hits one square meter in one second.

• Why did astronomers adopt flux? – unlike brightness (which does not depend on distance), flux does measure distance.– As the distance increases, flux decreases (and vice

versa). It is an inverse relationship (which you do not have to remember).

Flux (Illustrated)

Flux

• Flux was developed because of the subjectivity of “brightness (Apparent Visual Magnitudes).”

• If you like, Table 2-1 in the book shows conversions between magnitude (brightness) and flux (I do not expect you to calculate the conversions between brightness and flux, unless you want to).

Example of Flux V. Magnitude

• Sirius’ light is 24.2 times more intense than light from Polaris.– Thus Polaris is farther away – 425 light years from

Earth – than Sirius, 8.6 light years, from Earth.• That can be converted to magnitude – Sirius is

3.4 magnitudes brighter than Polaris.

The Sky’s Motion

• The Celestial Sphere (the sky) – An imaginary sphere surrounding Earth to which the planets, stars, sun, and moon seem to be attached (conceived by early astronomers)

The Sky’s Motion

• There are a couple of things you should note: – The celestial sphere (which is the sky) appears to

rotate westward around the Earth each day.• This is a result of the Earth’s Eastward rotation.

– Astronomers measure distance across the sky as angles (See page 19 for more on this).

– What you see in the sky depends on your latitude. • Starting at the north pole, the celestial north pole is

directly over your head.

The Sky’s Motion

• As you move south, the celestial pole moves toward the horizon (it moves with you), and you can see farther into the southern sky.• The angular distance from the horizon to the north

celestial pole always equals your latitude. • By the time you reach the equator, the north celestial

pole sinks below your northern horizon (in other words, it disappears and you are in the southern hemisphere (with the south celestial pole, which is expressed in negative degrees). • As you cross the equator and head south, you see the

southern sky with its constellations.