Astronomy Lecture 03/04

With the naked eye, about how many stars can be seen?

2,000 (or more)

When you look up the at stars, you look through a layer of air how many km deep?

100

What shape it the Milky Way Galaxy?

Disk

What is the Milky Way?

A pinwheel of stars, gas, and dust with a rich center containing lots of stars and activity (our solar system is in that disk)

What is a Constellation?

A region of the sky

How many constellations are there?

88

What is astrisms?

Pattern or grouping of stars

Do some patterns of stars span several constellations?

Yes

Do patterns of stars have physical significance?

No! Stars that appear closer together may lie at very different distances

These 2 stars have about the same luminosity- which one appears brighter, Alpha Centauri or the Sun?

The Sun

What does the apparent brightness of a light source depend on?

Luminosity, and the distance from us

The brightest stars in a constellation

May actually be quite far away from each other

What is a Celestial Sphere?

An imaginary sphere where the observer is the center and where all celestial objects are considered to lie

What is the Celestial Sphere useful for?

For describing the location (but not distance) of the stars in the sky.

Also useful for describing the sky

What is Ecliptic?

Sun’s apparent path through the celestial sphere

What is an Angular size/Angular distance?

Measure to describe the apparent size of an object in the night sky (measured in degrees)

Why do stars rise and set?

Earth rotates west to east, so stars appear to circle from east to west

What do you see when you stand at the poles?

You will see nothing rising or setting

What will you see if you are standing at the equator?

You will see everything rises and sets 90 degrees to the horizon

As the Earth rotates

The sky appears to us to rotate in the opposite direction

Daily Motion of Objects in the Sky

Stars rise and set every night

Earth rotates from West to East so stars appear to circle from East to West

What latitude does Winnipeg sit on?

50 degrees North

Which object moves the least over 24 hours?

The North Star

What is Latitude?

Position North or South of equator

What is Longitude?

Position East or West of prime meridian (runs through Greenwich, England)

The night sky varies with latitude but not longitude

The night sky depends on latitude because your position on Earth determines which constellations remain below the horizon

Does the sky vary on latitude or longitude?

Latitude, NOT longitude

Altitude of the celestial pole

= your latitude

The North star is 50 degrees above your horizon, due North. Where are you located?

You are at latitude of 50 degrees North

Why don’t we see the same constellations throughout the year?

depends on whether you stay home: constellation vary with latitude

depends no time of year: constellations vary as Earth orbits the Sun

Yearly motion of constellations

Constellation we see depend on time of year because Earth’s orbit changes the apparent location of the Sun among the stars

Annual Motion

As the Earth orbits the Sun, the Sun appears to move eastward with respect to the stars

The sun circles the celestial sphere once every year

Ecliptic

The apparent path of the Sun through the sky

Equinox

Where the ecliptic intersects the celestial equator

Solstice

Where the ecliptic is farthest from the celestial equator

Zodiac

The constellations which lie along the ecliptic constellations