observing the night sky how many starts do you think you can see on a crystal clear night?
TRANSCRIPT
Observing the Night SkyHow 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 stars½ above horizon, ½ below horizon
Reality – light pollution and other factors limit visible stars to a few hundredCountry: 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. ConstellationAn asterism is a star pattern that is not a
ConstellationExamples: The Big and Little Dipper, the Summer Triangle, and the Winter Hexagon
Looking at the SkyConstellations
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 SkyOur view changes because of the motions that
occur in the solar system.Earth is spinning (rotating)Earth is orbiting (revolving around) the SunThe Moon is orbiting EarthThe other planets are also orbiting the Sun
Paths of StarsEarth’s counter-clockwise rotationStars rise in the East, set in
the West
But some stars never set!Polaris stays nearly
stationary in the skyStars near Polaris move
slightly, in circular, counterclockwise paths
Some objects are never visible from the northThe 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.
What would the apparent motion of the stars be if…
1. You were standing at the North Pole.
2. You were standing at the equator
3. 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 MajorSpring: Leo, Bootes, VirgoSummer: Cancer, Scorpio, SagittariusFall: 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.
