Chapter 3 - Cycles of the Sky
Making observations is crucial to our study of the universe. It would be
convenient if Earth were a stable observation platform from which we could view of the
planets and the stars, but this is not the case. Why? Because we are riding on planet
Earth, and the Earth undergoes several complex motions that complicate our views of
the universe. Any deep understanding of the universe had to wait until early observers
figured out the motion of our Earth through space, so this is where we will begin.
People have always used observations of the stars as the basis for time
measurements. The apparent Sun cycles determined day length, the phases of the
Moon established the months, and the seasonal variations in climate as well as changes
in the background star patterns in the sky defined the years. To early civilization,
understanding the seasons often meant the difference between life and death, because
crops have to be planted and harvested in the proper seasons. In ancient societies, priests
and astronomers were very powerful people!
One other offshoot of the annual motion of the Earth around the Sun is the
changing position of the Sun, as seen from Earth against the fixed background of stars.
This means that the Sun can be located in a different “house” during each month of the
year leading to the development of the ideas of astrology, the non-scientific belief that
the Earth’s position in orbit can influence people’s personal lives in mystical ways.
The signs of the zodiac are simply the 12 principle constellations along the ecliptic (the
plane in which the Earth orbits the Sun). They are not believed by scientists to have
any influence over people’s lives.
The seasons are due to Earth’s orbit and tilt. Seasons affect crop growing
seasons, animal migration, and all sorts of natural events on Earth. This is because of
annual changes in the weather and hours of daylight for various locations on Earth.
Seasons are NOT caused by the Earth’s distance to the Sun. The effects of the Sun
that produce Spring, Summer, Fall and Winter, are explained on pages 26 and 27 of the
textbook. Make sure that you go over these.
The motion of the Moon around the Earth is why we have months (moonths).
One month is the time it takes for the moon to orbit around the Earth. During each month
the lighted surface of the Moon (as seen from Earth) cycles from dark to fully lit and
back again. You need to know the phases of the moon. When the moon is getting more
lit, it is waxing. When it is getting less lit, it is waning.
No moon visible - new moon
Less than half visible - crescent moon
More than half visible - gibbous moon
Totally visible - full moon
So if the moon is more than half visible, and getting more lit each night, it’s waxing
gibbous.
Eclipses occur when the Moon, Sun, and Earth are perfectly aligned. When the
Moon blocks our view of the Sun, it’s a solar eclipse. When the Earth blocks sunlight
from hitting the Moon, it’s a lunar eclipse. You should know the difference between
total and partial eclipses (pages 32-35 in the textbook). Note that solar eclipses can only
occur during the time of the new moon phase, and lunar eclipses can only be seen during
the full moon phase.
You should understand why a new moon is only visible during the day, and a full
moon is only visible at night. (Hint: a full moon means the Earth is between the Sun and
the Moon, since we’re seeing the lit up half of the moon. So you’ll only see the Moon if
you’re facing away from the Sun. This means it’s only visible at night!) You should also
understand why eclipses do NOT happen every month. It is because the Moon’s orbit
is not exactly on the ecliptic; it is slightly tilted.
The Earth’s climate is not exactly the same each year. Because of slight
variations in the orbital distance of Earth from the Sun, along with other subtle factors,
the Earth can go through hotter periods and cooler periods on a ~100,000 year cycle.
The Milankovich hypothesis gives us some insight into this cyclic process.
Chapter 3 summary and practice problems are on pages 42 and 43.
Homework
Define the following terms:
rotation
revolution
solar eclipse
zodiac
lunar eclipse
sidereal period
synodic period
ecliptic
Answer the following problems on page 42:
3, 4, 6, 8, 10, 12