Quiz #1 Review

Thursday, 15 September 2011

I list here the main topics and the main points for each topic. Study also the indicated lecture notes.

Main Topics

• The Scientific Method (Preview section in text)– Notes: “Time” link under Course Outline in syllabus

• The Celestial Sphere (Ch. 1.1)– Notes: “Time” and “Lines in the Sky”

• The Seasons (Ch. 1.2)– Notes: “Seasons”

• The Lunar Cycle (Ch. 1.3)– Notes: “Moon Phases”

• Astronomy Picture of the Day (http://apod.nasa.gov/apod/astropix.html) between– Tuesday 25 January 2011 and – Tuesday 8 February 2011

The Scientific Method

• Scientific method– Hypothesis evolves into a theory after

• Rigorous testing in controlled experiments– Revise hypothesis if experiments do not support original

hypothesis

• Communicating results to peers• More testing by independent researchers/teams• An accepted theory will be able to make accurate

predictions about physical phenomena

The Celestial Sphere• Diurnal motion

– The Sun, stars, moon, and planets rise in the east and set in the west. This apparent daily motion is due to the rotation of the Earth on its north-south axis.

• Orbital motion– The Earth and planets actually orbit the Sun; the Moon orbits the Earth.– By observing what constellations appear on the horizon after the Sun sets in

the evening or before the Sun rises in the morning, it was found that the Sun follows the same path in the sky relative to the background stars year after year.

– The Moon and planets also move in the sky throughout the year on or very near the same path (the planetary and lunar orbits are all nearly in the same plane, within 10 degrees of one another).

– This path is called the ecliptic. The constellations along the ecliptic are the zodiacal constellations.

– The orbital plane of the Earth is also called the ecliptic plane.

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The Seasons

• The tilt of the Earth with respect to the ecliptic plane causes the seasons.

• The hemisphere tilted toward the Sun experiences summer, while the hemisphere tilted away is in winter.

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Summary - Equinoxes

• The two equinoxes occur when the Sun crosses the Celestial Equator– In March, as the Sun is heading North, and – In September, as the Sun is heading South

• Equinox means “equal night”– Length of night is the same as the length of daylight for the

equinoxes• The Sun will rise due East and set due West.• For an observer on the Earth’s equator

– The sun will be at the observer’s zenith (directly overhead) at local noon.

• For an observer on a pole: the Sun circles the observer on the horizon…16 hours from half-to-all or half-to-gone.

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Summary - Solstices

• The two solstices occur when the Sun reaches its extreme North or South positions in the sky.– December Solstice: farthest South (longest night in northern

hemisphere)– June Solstice: farthest North (longest day in northern

hemisphere)

Lunar Cycle

• The Phases of the Moon are the variations in the Moon’s appearance as the Moon orbits the Earth.

• They are due to the changing Sun-Earth-Moon angle through each month.

• A lunar cycle lasts 29.5 days– The cycle is divided into four quarters, each

approximately one week in duration.

Lunar Cycle

The Lunar phases follow the same repeating, predictable cycle starting at New Moon in the figure and preceding counter-clockwise.

Display in slideshow mode to view animation. The line bisecting the moon showsthe portion of the moon that an observer on the Earth can see during the indicated phase of the lunar cycle.

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Phases of the Moon What Can You See

http://kids.niehs.nih.gov/lunar/home.htm

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Lunar Phase ReviewKeep in mind that certain phases are visible only at certain times of the day (or night). Remember the positions for observers on the earth at sunrise, sunset, noon and midnight. As the Earth turns, different phases rise, appear overhead, or set at different times during the cycle. These times are repeatable, however, from one lunar cycle (29.5 days) to another.

sunset

sunrise

noonMidnight

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Lunar Phase Review

Gibbous = more than half-lit

Crescent = less thanhalf-lit

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Lunar Phase Review

Waxing = getting brighter from night to night

Waning = getting less illuminated from night to night

What to Know About Eclipses

• What is the difference between a solar and a lunar eclipse?– What is the orientation of the Sun, Moon, and

Earth for the two eclipse types?– What is the lunar phase for the two eclipse types?

• How often do eclipses occur?– Why don’t we have eclipses every month?

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Solar and Lunar Eclipses• When the shadow

of the Moon strikes the Earth it is called a Solar Eclipse

• When the shadow of the Earth strikes the Moon it is called a Lunar Eclipse

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Solar and Lunar Eclipses

New Moon

Full Moon

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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

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Eclipse Seasons

• Eclipse seasons are roughly 6 months apart– May have two eclipses (1 lunar, 1 solar) 2 weeks apart

during an eclipse season– Eclipses may not be total

• Solar: total, annular, partial• Lunar: total, partial, penumbral