Telescopes and Horoscopes

Chapter 1

Our Place in Space

• Are we unique?• Are we alone?• How did we begin?• The universe is the totality of all matter,

energy, time and space.• Astronomy is the study of this universe.• How big is the universe? How do we

measure such vast distances?

The Obvious View

• Watching our sky….. what can we observe?• Stars – 3000/6000 can be observed with

naked eye• Constellations (88) map our sky

• Not proximal – only ‘look’ to be close together

• Historical significance / Navigation (Polaris)• Astrologers – pseudoscientists?

                                                 

                                

Motion of the Sun and Stars

• 24-hour solar day – how long it takes Earth to align back up with the Sun (set/rise/noon)

• Earth has 2 main motions:• Rotation – about axis

• Revolution – about Sun in an orbit

• These cause the sidereal day (“according to the stars”). It is 23 hours 56 minutes long.

• 0.986o additional angle to rotate through

Solar Day vs. Sidereal Day

Path of Sun (same time of day) throughout the year

                                                 

                                

What causes the seasons?

Is it how far we are from the Sun?

Earth: What causes the Seasons?

Seasonal Changes

• Ecliptic – path the Sun follows on the celestial sphere over one year

• The ecliptic and the celestial equator are not the same because Earth is tilted 23.5 degrees

• Summer solstice – June 21 (around or on)• Winter solstice – December 21

• Autumnal equinox – September 21• Vernal equinox – March 21

• Tropical year = 365.242 solar days

Summer Solstice

Winter Solstice

Sun Clocks

The Motion of the Moon

• Our Moon has a name... meet Luna!

• 8 Phases – can you name them all?

• Sidereal month – 27.3 days

• Synodic month – 29.5 day (1 phase cycle)

• Causes eclipses – both lunar and solar

The Eight Phases of the Moon

                                                 

                                

What’s wrong with this picture?

Sun/Moon – Time of Day

Eclipses

• The Sun and Moon are in opposite directions

• There are two main types of eclipses

• Solar and lunar• Eclipses can be partial

or total

Lunar Eclipses

• Earth is in the way and blocks the sun’s light from

the moon.• Most cases are a partial

eclipse• Lasts about 100 minutes

total• Can sometimes appear red

Solar Eclipses

• The Moon blocks out the Sun

• They have the same diameter as seen

from Earth

• Can be total or partial

Solar Eclipses

Why doesn’t it happen every month?

Effects of an Solar Eclipse

• Dark shadow on a section of Earth• Umbra -- complete darkness

• Penumbra -- diffracted darkness

Summer/Winter Constellations

• Constellations change as Earth orbits the Sun• Summer triangle – Vega, Deneb and Altair

• Why am I a Leo…I can’t see it on my birthday?• Zodiac – Sun passes through 12 ‘animal’

constellations throughout the year• Time it takes for all 12 constellations to pass (from one point on Earth) is the sidereal year –

which is 365.256 days

Ecliptic and Zodiac

Long Term Changes

• The difference between the sidereal and tropical years is only 20 minutes!

• Why? The tropical year occurs 20 minutes sooner than the sidereal year because the Earth

wobbles like a top as it spins about its axis.• This additional movement is called precession.

• If you don’t want summer to occur in the month we call February – we have to use the

tropical year to keep time/calendars.

Measurements in Space

Distances to the Stars

• Measuring distance in space could be accomplished by

using miles and kilometers.• Talk about huge numbers!!!• The Sun is (take your pick):

– 93 million miles away– 150 million kilometers away– 1 Astronomical Unit (AU)

– 8 light-minutes away

• The Moon is the distance of 1 light-second from Earth.

“Tools” for Determining Distance

The Light-Year

• The distance that light travels in 1 year

• Light travels 300,000 km/sec

• Therefore the Moon is one light-second from Earth

• One light-year is almost 10 trillion kilometers!

• The l.y. is a rather small distance in the

astronomical scale.

• 1 Parsec = 3.26 light-years

Meet your neighbors…

Parallax• This is what enables you to hit

a baseball with a bat• Your two eyes and brain work

together to interpret distances to things

• Deer doesn’t have it; neither do children under 9

• θ is the parallax angle, but what is THAT?

• This angle allows us to use a little geometry to determine distances to objects – almost

anywhere• Index finger = 3o

• Parallax (in space)

1 2 34 5 6

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January

Cepheid Variable Stars

• Determining the brightness of a star is a science in itself

• Many methods• But using a special kind of

star’s brightness variance helps us find distance

• Longer cycles are brighter• Galaxies that contain

Cepheid stars help us determine how far away the

entire galaxy is.

Can you tell which is the Cepheid variable star?