Astr100 ch02 Aug30 2

Chapter 2Discovering the Universe for Yourself: 

Chapter 2 Discovering the Universe for Yourself

What does the universe look like from Earth?: 

What does the universe look like from Earth? With the naked eye, we can see more than 2,000 stars as well as the Milky Way.

Constellations: 

Constellations A constellation is a region of the sky. 88 constellations fill the entire sky.

Thought QuestionThe brightest stars in a constellation…: 

Thought Question The brightest stars in a constellation… All belong to the same star cluster. All lie at about the same distance from Earth. May actually be quite far away from each other.

The brightest stars in a constellation…: 

The brightest stars in a constellation… All belong to the same star cluster. All lie at about the same distance from Earth. May actually be quite far away from each other.

The Celestial Sphere: 

The Celestial Sphere Stars at different distances all appear to lie on the celestial sphere. Ecliptic is Sun’s apparent path through the celestial sphere.

The Celestial Sphere: 

The Celestial Sphere The 88 official constellations cover the celestial sphere.

The Milky Way: 

The Milky Way A band of light making a circle around the celestial sphere. What is it? Our view into the plane of our galaxy.

The Milky Way: 

The Milky Way

The Local Sky: 

The Local Sky An object’s altitude (above horizon) and direction (along horizon) specifies its location in your local sky

The Local Sky: 

The Local Sky Zenith: The point directly overhead Horizon: All points 90° away from zenith Meridian: Line passing through zenith and connecting N and S points on horizon

Slide12: 

We measure the sky using angles. One degree =60 arc minutes One arc minute =60 arc seconds

Thought Question The angular size of your finger at arm’s length is about 1°. How many arcseconds is this?: 

Thought Question The angular size of your finger at arm’s length is about 1°. How many arcseconds is this? 60 arcseconds 600 arcseconds 60  60 = 3,600 arcseconds

The angular size of your finger at arm’s length is about 1°. How many arcseconds is this?: 

The angular size of your finger at arm’s length is about 1°. How many arcseconds is this? 60 arcseconds 600 arcseconds 60  60 = 3,600 arcseconds

Why do stars rise and set?: 

Why do stars rise and set? Earth rotates east to west, so stars appear to circle from west to east.

Our view from Earth:: 

Our view from Earth: Stars near the north celestial pole are circumpolar and never set. We cannot see stars near the south celestial pole. All other stars (and Sun, Moon, planets) rise in east and set in west. Celestial Equator Your Horizon A circumpolar star never sets This star never rises

Thought Question What is the arrow pointing to?A. the zenithB. the north celestial poleC. the celestial equator: 

Thought Question What is the arrow pointing to? A. the zenith B. the north celestial pole C. the celestial equator

Thought Question What is the arrow pointing to?A. the zenithB. the north celestial poleC. the celestial equator: 

Thought Question What is the arrow pointing to? A. the zenith B. the north celestial pole C. the celestial equator

Why do the constellations we see depend on latitude and time of year?: 

Why do the constellations we see depend on latitude and time of year? They depend on latitude because your position on Earth determines which constellations remain below the horizon. They depend on time of year because Earth’s orbit changes the apparent location of the Sun among the stars.

Review: Coordinates on the Earth: 

Review: Coordinates on the Earth Latitude: position north or south of equator Longitude: position east or west of prime meridian (runs through Greenwich, England)

The sky varies with latitude but not longitude. : 

The sky varies with latitude but not longitude.

Altitude of the celestial pole = your latitude : 

Altitude of the celestial pole = your latitude

Thought Question The North Star (Polaris) is 50° above your horizon, due north. Where are you?: 

Thought Question The North Star (Polaris) is 50° above your horizon, due north. Where are you? You are on the equator. You are at the North Pole. You are at latitude 50°N. You are at longitude 50°E. You are at latitude 50°N and longitude 50°E.

Thought Question The North Star (Polaris) is 50° above your horizon, due north. Where are you?: 

Thought Question The North Star (Polaris) is 50° above your horizon, due north. Where are you? You are on the equator. You are at the North Pole. You are at latitude 50°N. You are at longitude 50°E. You are at latitude 50°N and longitude 50°E.

The sky varies as Earth orbits the Sun: 

The sky varies as Earth orbits the Sun As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. At midnight, the stars on our meridian are opposite the Sun in the sky.

Thought Question: 

Thought Question TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter.

Thought Question: 

TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter. Hint: When it is summer in the U.S., it is winter in Australia. Thought Question

Thought Question: 

TRUE OR FALSE! Earth is closer to the Sun in summer and farther from the Sun in winter. • Seasons are opposite in the N and S hemispheres, so distance cannot be the reason. The real reason for seasons involves Earth’s axis tilt. Thought Question

What causes the seasons?: 

What causes the seasons? Seasons depend on how Earth’s axis affects the directness of sunlight

Axis tilt changes directness of sunlight during the year. : 

Axis tilt changes directness of sunlight during the year.

Sun’s altitude also changes with seasons: 

Sun’s altitude also changes with seasons Sun’s position at noon in summer: higher altitude means more direct sunlight. Sun’s position at noon in winter: lower altitude means less direct sunlight.

Summary: The Real Reason for Seasons: 

Summary: The Real Reason for Seasons AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.

Why doesn’t distance matter?: 

Why doesn’t distance matter? Variation of Earth-Sun distance is small — about 3%; this small variation is overwhelmed by the effects of axis tilt.

How do we mark the progression of the seasons? : 

How do we mark the progression of the seasons? We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinox

We can recognize solstices and equinoxes by Sun’s path across sky:: 

We can recognize solstices and equinoxes by Sun’s path across sky: Summer solstice: Highest path, rise and set at most extreme north of due east. Winter solstice: Lowest path, rise and set at most extreme south of due east. Equinoxes: Sun rises precisely due east and sets precisely due west.

Seasonal changes are more extreme at high latitudes: 

Seasonal changes are more extreme at high latitudes

How does the orientation of Earth’s axis change with time?: 

How does the orientation of Earth’s axis change with time? Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years. Polaris won’t always be the North Star. Positions of equinoxes shift around orbit; e.g., spring equinox, once in Aries, is now in Pisces! Earth’s axis precesses like the axis of a spinning top

What have we learned?: 

What have we learned? What causes the seasons? The tilt of the Earth’s axis causes sunlight to hit different parts of the Earth more directly during the summer and less directly during the winter We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon

What have we learned?: 

What have we learned? How do we mark the progression of the seasons? The summer and winter solstices are when the Northern Hemisphere gets its most and least direct sunlight, respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight. How does the orientation of Earth’s axis change with time? The tilt remains about 23.5 degrees (so the season pattern is not affected), but Earth has a 26,000 year precession cycle that slowly and subtly changes the orientation of the Earth’s axis

2.3 The Moon, Our Constant Companion: 

2.3 The Moon, Our Constant Companion Why do we see phases of the Moon? What causes eclipses? Our goals for learning:

Why do we see phases of the Moon?: 

Why do we see phases of the Moon? Lunar phases are a consequence of the Moon’s 27.3-day orbit around Earth

Phases of Moon: 

Phases of Moon Half of Moon is illuminated by Sun and half is dark We see a changing combination of the bright and dark faces as Moon orbits

Phases of the Moon: 

Phases of the Moon

Moon Rise/Set by Phase: 

Moon Rise/Set by Phase

Slide45: 

Phases of the Moon: 29.5-day cycle new crescent first quarter gibbous full gibbous last quarter crescent waxing Moon visible in afternoon/evening. Gets “fuller” and rises later each day. waning Moon visible in late night/morning. Gets “less” and sets later each day. } }

We see only one side of Moon: 

We see only one side of Moon Synchronous rotation: the Moon rotates exactly once with each orbit That is why only one side is visible from Earth

What causes eclipses?: 

What causes eclipses? The Earth and Moon cast shadows. When either passes through the other’s shadow, we have an eclipse.

Lunar Eclipse: 

Lunar Eclipse

When can eclipses occur?: 

When can eclipses occur? Lunar eclipses can occur only at full moon. Lunar eclipses can be penumbral, partial, or total.

Solar Eclipse: 

Solar Eclipse

When can eclipses occur?: 

When can eclipses occur? Solar eclipses can occur only at new moon. Solar eclipses can be partial, total, or annular.

Slide52: 

Why don’t we have an eclipse at every new and full moon? The Moon’s orbit is tilted 5° to ecliptic plane… So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon.

Summary: Two conditions must be met to have an eclipse:: 

Summary: Two conditions must be met to have an eclipse: It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse). AND 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes).

Predicting Eclipses: 

Predicting Eclipses Eclipses recur with the 18 yr, 11 1/3 day saros cycle, but type (e.g., partial, total) and location may vary.

Planets Known in Ancient Times: 

Planets Known in Ancient Times Mercury difficult to see; always close to Sun in sky Venus very bright when visible; morning or evening “star” Mars noticeably red Jupiter very bright Saturn moderately bright

What was once so mysterious about planetary motion in our sky?: 

What was once so mysterious about planetary motion in our sky? Planets usually move slightly eastward from night to night relative to the stars. But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion

We see apparent retrograde motion when we pass by a planet in its orbit.: 

We see apparent retrograde motion when we pass by a planet in its orbit.

Why did the ancient Greeks reject the real explanation for planetary motion?: 

Why did the ancient Greeks reject the real explanation for planetary motion? Their inability to observe stellar parallax was a major factor.

The Greeks knew that the lack of observable parallax could mean one of two things:: 

The Greeks knew that the lack of observable parallax could mean one of two things: Stars are so far away that stellar parallax is too small to notice with the naked eye Earth does not orbit Sun; it is the center of the universe With rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.

What have we learned?: 

What have we learned? What was so mysterious about planetary motion in our sky? Like the Sun and Moon, planets usually drift eastward relative to the stars from night to night; but sometimes, for a few weeks or few months, a planet turns westward in its apparent retrograde motion Why did the ancient Greeks reject the real explanation for planetary motion? Most Greeks concluded that Earth must be stationary, because they thought the stars could not be so far away as to make parallax undetectable