1. Quiz #1 Recap
PSC 1010

2. Scientific Method
Hypothesis: initial idea/observation on physical phenomenon (or, an answer to the question how does something work)
Must be testable
Theory: widely accepted explanation for a physical phenomenon
Hypothesis evolves into a theory after
– Rigorous testing in controlled experiments
– Communicating results to peers (publishing)
– More testing by independent researchers/teams
– An accepted theory will be able to make accurate predictions about physical phenomena

3. Motions of Sun, Moon and Planets
The apparent motion of the Sun, Moon, and planets relative to the background stars (through the Zodiacal constellations) as measured from night-to-night is a consequence of the facts that
The Moon orbits the Earth and
The Earth and planets orbit the Sun and
These orbits lie in approximately the same plane in space (within the band of the Zodiac).
Our line of sight to any of the planets, Sun, and Moon changes relative to the background stars and constellations as the Earth, Moon, and planets orbit the Sun.
Since all orbits are in approximately the same plane, the Sun, Moon and planets all cross in front of the same group of constellations, all on the ecliptic plane: the Zodiac.

4. The Zodiacal Constellations
The path in the sky that the Sun, Moon, and planets follow throughout the year, and year after year is called the ecliptic.
The constellations that the ecliptic passes through are called the Zodiacal Constellations.
As with constellations anywhere in the sky, only those Zodiacal constellations in a direction away from the sun are visible at any given time of year in the night sky.

5. The Zodiacal Constellations
The Zodiac constellations are those constellations that the Sun, Moon, and planets pass through during the course of a year, and repeating year after year (yellow dots on figure indicate the ecliptic path).

6. If the Earth’s axis was not tilted…
Every day would be an equinox
For any single spot on the Earth, sunlight angle of incidence would not change from day-to-day
There would be no seasons
Figure shows sunlight angle of incidence on an equinox for an observer at latitude 38 (Washington, D.C.)

7. North/South Hemisphere Seasons
Seasons in the Southern Hemisphere are opposite those in the Northern Hemisphere. Therefore, on the day of Quiz #1, 15 Sep. 2011, the season in South Africa was winter (soon to be spring)
North 
Spring
Summer
Fall
Winter
South 

8. Length of Continuous Sunlight (Day) or Darkness (Night) at the Poles
6-months

9. Seasons by Hemisphere December Solstice Example
Northern Hemisphere
1st day of winter
Compare the amount of light vs. dark at the observer’s latitude on the globe at right: Longest night time of the year
Sun lowest in the sky throughout the day: winter
Southern Hemisphere
1st day of summer
Longest day time of the year
Sun highest in the sky throughout the day

10. Equinoxes
The Equinoxes days with equal daylight and darkness occur in March and September

11. The Seasons 1 We experience seasons on the Earth
because the Earth’s rotation axis (the North-South line through the Earth) is tilted (by 23.5°) with respect to the direction of its motion as it orbits the Sun, or in other words
because the Earth’s rotation axis is tilted with respect to the plane of the Earth’s orbit by 23.5°.

12. The Seasons 2 and the consequences of this tilt are:
day time and night time are not of equal length in time for every day throughout the year
when days are longer (and the Sun is high in the sky throughout the day, and the sunlight is more concentrated on the ground), we have summer
when nights are longer (and the Sun is low in the sky throughout the day, and the sunlight is less concentrated on the ground ), we have winter

13. The Seasons 3 additional consequences of this tilt are:
the sun rises and sets at different positions on the horizon from day to day throughout the year
when the Sun rises at the northern most point along the eastern horizon, that day is the June solstice
when the Sun rises at the southern most point along the eastern horizon, that day is the December solstice

14. The Seasons 4 additional consequences of this tilt are:
The seasons are not the same on the Northern and Southern hemispheres of the Earth
Northern Hemisphere: spring, summer, fall, winter
Southern Hemisphere: fall, winter, spring, summer
Therefore, when summer ends in the U.S. (and fall begins in the U.S.), spring will start in South Africa.

15. Lunar Cycle – First Week Waxing
One week after the
moon is new, the moon
phase is First Quarter.

16. Lunar Cycle – Second Week Waxing
After one more week, the
moon is full. Therefore,
two weeks elapse between
the new moon and the
full moon.

17. Solar and Lunar Eclipses
Solar Eclipse: Lunar phase is new
(compare with Lunar Cycle: First Week slide).
Lunar Eclipse: Lunar phase
is full
(compare with Lunar Cycle:
2nd Week slide).

18. Full Moon Rises at Sunset, Full Moon Sets at Sunrise

19. The Moon’s Orbital Plane is Tipped
Eclipses do not occur every month because the Moon’s orbital plane around the Earth is not aligned with the Earth’s orbital plane around the
Since the Moon is far and small it’s easy for its shadow to miss the Earth if the two aren’t closely aligned with the Sun.
An alignment happens where the orbital planes cross each other
Earth
Moon
The Earth-Moon system to scale
It isn’t easy for eclipses to occur between the Earth and Moon!

20. Full Moon is on the Night Side of the Earth

21. Can see any lunar phase at some time during the day except full
However, the different phases are visible only certain times during the lunar cycle. Those times are the same from one lunar cycle to the next.