Quiz #1 Recap

PSC 1010

20110915

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

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.

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.

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).

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.)

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 Fall Winter Spring Summer

Length of Continuous Sunlight (Day) or Darkness (Night) at the

Poles• 6-months

• http://astro.unl.edu/naap/motion3/animations/sunmotions.html

Seasons by HemisphereDecember 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

Equinoxes

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

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°.

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

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

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.

Lunar Cycle – First WeekWaxing

One week after themoon is new, the moonphase is First Quarter.

Lunar Cycle – Second WeekWaxing

After one more week, the moon is full. Therefore, two weeks elapse between the new moon and the full moon.

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).

Full Moon Rises at Sunset, Full Moon Sets at Sunrise

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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

Full Moon is on the Night Side of the Earth

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.