Observing the Night Sky

• How many starts do you think you can see on a crystal clear night?

Visible stars on a clear night

• Unaided human eye – 6,000 stars• At any one time – 3,000 stars–½ above horizon, ½ below horizon

• Reality – light pollution and other factors limit visible stars to a few hundred– Suburban: 500+– Inner-city: 300+

Constellations

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

mythological figures.

Asterism vs. Constellation

• An asterism is a star pattern that is not a Constellation– Examples: The Big and Little Dipper, the Summer

Triangle, and the Winter Hexagon

Looking at the Sky• Constellations– 88 official constellations

according to the IAU (International Astronomical Union)

• Many origins– Greeks, Southern

European cultures, Middle East, Asia, etc.

• Traced back as far as 5,000 years ago

• Stars define the celestial sphere

• Question: Are the stars of a Constellation close to each other?

The stars of a constellation only appear to be close

to one another.

Usually, this is only a projection effect.

The stars of a constellation may be located at very different distances

from us.

Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of

bright stars in those regions. They serve as a way to break up and organize the sky (celestial sphere).

How can we use the Constellations?

• Think of the constellations as a map of the sky. The more constellations you can recognize the better your ability to navigate the night sky will be.

• Every celestial object can be found within a constellation. Knowing the constellations tells you where to look for objects and phenomena like planets, galaxies, comets, and meteor showers.

Our View of the Sky

• Our view changes because of the motions that occur in the solar system.– Earth is spinning (rotating)– Earth is orbiting (revolving around) the Sun– The Moon is orbiting Earth– The other planets are also orbiting the Sun

Paths of Stars • Earth’s counter-clockwise rotation– Stars rise in the East, set in the

West• But some stars never set!– Polaris stays nearly stationary

in the sky– Stars near Polaris move

slightly, in circular, counterclockwise paths

– Some objects are never visible from the north• The Southern Cross (SH)

• Star positions change from night to night as the Earth revolves about the Sun – rise and set 4 minutes earlier each day.

Risings and Settings (apparent motion)

The spin of the earth causes the stars to appear to rotate about the celestial pole. Some stars are therefore circumpolar and never set, while others dip below the horizon. Which stars are which depend on where you are on earth.

Polaris happens to be very near the North celestial pole.

Apparent Motion

What would the apparent motion of the stars be if…

1. You were standing at the North Pole.2. You were standing at the equator3. You were standing at the South Pole

Circumpolar Stars (from our latitude)

Circumpolar Constellations

Seasonal Stars and Constellation

• All other constellations and stars are seasonal and can only be see at certain times of the year.– Winter: Orion, Taurus, Canis Major– Spring: Leo, Bootes, Virgo– Summer: Cancer, Scorpio, Sagittarius– Fall: Andromeda, Pisces, Gemini

The Yearly Motion

In addition to rotating, the Earth also revolves about the Sun.

As the earth revolves the Sun is projected in front of different constellations at different times of year. The path the Sun takes across heavens is called the ecliptic. The constellations which the Sun passes through are zodiac constellations.

Because the Sun is bright, we can only see some constellations at certain times of year.