First you guess. Don't laugh, this is the most important step.
Then you compare the consequences to experience. If it
disagrees with experience, the guess is wrong. In that simple
statement is the key to science. It doesn't matter how beautiful
your guess is or how smart you are or what your name is. If it
disagrees with experience, it's wrong. That's all there is to it. Richard Feynman
Arizona State University AST 111
Introduction to Solar Systems Astronomy
Frank Timmes
[email protected]
cococubed.asu.edu/class_pages/class_planet.shtml
Chapter 2 - Discovering the Universe for Yourself - Learning Goals:
Patterns in the night sky
‣What does the universe look like from Earth?
‣Why do stars rise and set?
‣Why do the constellations we see depend on
latitude and time of year?
What does the Universe look like from Earth?
The human eye can see about 6000 stars at a dark site on a moonless
night, along with a whitish band of light we call the Milky Way.
People across the globe throughout time have organized these stars
into patterns used for story-telling mnemonics, timekeeping, and
navigation.
There are now 88 constellations, which refer to areas on the sky rather
than groups of stars, recognized by the International Astronomy Union.
The stars and constellations appear to lie on a celestial sphere
that surrounds Earth. This is an illusion created by the lack of
depth perception in space, but it is useful for mapping the sky.
The Zodiac is the group of constellations through which the Sun passes
during the year.
Babylonian astronomers developed the 12 constellations of the zodiac. But there actually 13 constellations through which the Sun passes Ophiuchus being the 13th.
The Milky Way circles all around the celestial sphere. It traces our
galaxy’s disk of stars - the galactic plane - as it appears from the
outskirts of the galaxy.
Why do stars rise and set?
From any place on Earth the sky looks like a dome (hemisphere). We can describe any position in the local sky by its altitude and direction.
If you spend a few hours under a starry sky, you’ll notice the universe
seems to be circling around us, with the stars moving gradually across
the sky from a generally eastern direction to a generally western direction.
We can picture the movement of the
sky by imagining the celestial sphere
is rotating around Earth.
Every object on the celestial sphere appears
to make a simple daily circle around Earth.
The motion can look a little more complicated in the local sky because
the horizon cuts the celestial sphere in half.
North celestial pole
Stars
circumpolar
The apparent daily
motions of the stars
depend on your latitude
on Earth.
All stars
circumpolar
Stars
rise & set
Earth's equator
Celestial equator
All stars
rise and set
South celestial pole
Why do the constellations we see depend on latitude and time of year?
The stars we see depend on latitude because the Earth is
spherical and the horizon, the Earth itself, blocks half the sky.
The altitude of the north celestial pole is equal to your latitude.
Different constellations are visible at different times of the year
because of the relative motion between the Earth and the Sun.
The Sun appears to drift eastward about 1º a day along the ecliptic as
Earth orbits the Sun, so we see the Sun against the background of
different zodiac constellations at different times of the year.