DepartmentAstronomy & Astrophysics
2.1 Patterns in the Sky
What does the earth look like from earth?
On clear and moonless night, assuming you are away from city light, more than 2000
stars can be seen to the naked eye along with Milky Way identified by the whitish band
Patterns of the stars at night have not noticeably changed over the past few thousand
Constellation: a region of the sky with well-defined borders, and the patterns of stars help
to locate the borders.
Official names for constellations visible in the Northern Hemisphere can be traced back
to the ancient civilizations of the Middle East. Southern Hemisphere constellations carry
names that originate from the 17th century European explorers.
The Celestial Sphere:
Stars appear to lie close to one another, but it is an illusion because we lack depth
perception when looking into space. This happened to Greeks and they imagined the stars
and constellations to like on a celestial sphere that surrounds earth.
Earth is at the center of the celestial sphere. Why? Where we are as we look into space
The celestial sphere is used to label four special points and circles.
1.North celestial pole – point directly over the earths North Pole
2.South celestial pole – point directly over earths south pole
3.Celestial Equator – projection of earths equator into space
4.Ecliptic – path sun follows as it appears to circle around the sphere once
each year. (since earth is on a tilt, ecliptic is on a 23.5o )
The Milky Way:
Refer to page 28 for explanation of the Milky Way
The Local Sky:
Sky appears to take the shape of a dome. We can only see half of the celestial sphere
from any particular moment or location, while the other is blocked by the ground.
The half that can be seen refers to the local sky (seen from wherever you may be
Boundary between earth and sky defines horizon. Perpendicular to that is the Zenith.
Meridian, is an imaginary half circle stretching from the horizon due south, through the
zenith, and the horizon due north
Can pin point position of any object by stating the direction and altitude.
Angular Sizes and Distances:
Lack of depth perception means you can’t justify the true size or distance between objects
in the sky, however the angular sizes can be described
Angular size also depends on distance. Ex. The sun is 400 times the size of the moon yet
it has the same angular size because, is still about 400 times farther away like the moon
The angular distance between a pair of objects in the sky is the angle that appers to
To be more precise, each degree is subdivided into 60 arcminutes and each aricminute is
subdivided into 60 arcseconds
Why do Stars Rise and Set?
Sun, moon, plants, and stars move across the sky because the earth rotates, not the other
Stars relatively near the north celestial pole remain perpetually above the horizon. They
don’t rise or set, they instead circle around the north celestial pole counter clockwise.
Stars relatively near the south celestial pole never rise above the horizon at all
All other stars have circle daily but are partly above and below the horizon. Since earth
rotates counter clockwise( west – east), stars appear to rise in the east and set in the west
Why do the constellations we see depends on the latitude and time of year?
Constellations that can be seen change depending on where you are
Variation with Latitude:
Latitude affects the constellation we see because it affects the locations of the horizon
and zenith relative to the sphere.
Sky varies with latitude but longitude. Ex. People in people in Charleston and Sandiego
are about the same attitude so they see the same constellations at night.
The altitude of the celestial pole in your sky is equal to your latitude.
Variation with Time of Year:
Night sky changes throughout the year because the earth is constantly changing its
position because it orbits the sun.
Sun appears to move against the background of the distant star in the constellations. We
do not see the star and the sun at the same time. If it was possible, the sun would be seen
moving eastward along the ecliptic completing a circuit each year.
Constellation along the ecliptic are called the constellations of the zodiac.
Suns location along the ecliptic determines which constellation can be seen at night.
2.2 The Reason for Seasons
What causes the seasons?
The tilt of the earth axis causes sunlight to fall differently on earth at different times of
Figure 2.15 illustrates the key ideas.
Solstices and Equinoxes:
The summer (June) solstice occurs around June 21. Moment where the northern
hemisphere is tipped most directly toward the sun, and vice versa for the winter
The spring (March) equinox occurring around March 21, is the moment when the
Northern hemisphere goes from being tipped slightly away from the sun to being tipped
slightly toward the sun.
The fall (September) equinox occurs around September 22. Moment where northern
hemisphere first start to be tipped away from the sun
Equinoxes will only occur on the two days of the year where the sun rises exactly due
east and sets exactly due west. Also the days where sunlight falls equally on both