Copyright © 2015, W. W. Norton & Company

Prepared by Lisa M. Will, San Diego City College

Lecture SlidesCHAPTER 2: Patterns in the Sky—Motions of Earth and the Moon

Understanding Our UniverseSECOND EDITIONStacy Palen, Laura Kay, Brad Smith, and George Blumenthal

Patterns in the Sky

What do we observe on the sky due to the: Rotation of Earth? Revolution of Earth? Axial tilt of Earth? Orbit of the Moon?

Rotation of the Earth One day is 24 hours.

• One rotation with respect to stars = 23 hr 56 min = sidereal day

• One rotation with respect to the Sun = 24 hours = solar day

Earth rotates counterclockwise and also orbits the Sun in the counterclockwise direction.

Class Question The Sun rises in the east and sets in the west. Which way is Earth rotating?

A. Toward the westB. Toward the northC. Toward the eastD. Toward the south

Celestial Sphere The celestial sphere is a projection of Earth’s axes and equator into space.

The celestial sphere rotates around the north and south celestial poles each day.

Celestial Sphere: Celestial Equator and Ecliptic

Celestial equator: midway between the two poles.

Ecliptic: apparent path of the Sun, inclined 23.5 degrees to equator.

Celestial Sphere: Earth’s North Pole

At Earth’s North Pole, we would see half of the celestial sphere.

The north celestial pole would be at the zenith, the point directly overhead.

Celestial Sphere: Circumpolar

No star rises or sets =>circumpolar. Observers never see the south celestial pole.

Celestial Sphere: Circumpolar (Cont.)

Celestial Sphere: Circumpolar (Cont.)

Celestial Sphere: Circumpolar (Cont.)

From what location on Earth would all of the starson the celestial sphere be visible?

A. EquatorB. North PoleC. Prime MeridianD. South Pole

Class Question

Celestial Sphere: Hemispheres

At Earth’s equator, all stars rise and set. The celestial poles are on the northern and southern

horizons.

What would the sky look like from a latitudeof 30° N?

A. All stars would be circumpolar.B. All stars would be visible.C. No stars would be visible.D. Some stars would be circumpolar; some

would never be visible.

Class Question

Celestial Sphere: Stars Visibility

Some stars are circumpolar. Some stars are never visible.

Revolution of the Earth

The average distance to the Sun is called the astronomical unit, or AU.

1 AU = 150 million km = 8.3 light-min

Revolution of the Earth: Zodiac

As Earth revolves, the Sun is seen against different constellations—the zodiac.

The constellations are along the ecliptic.

Axial Tilt of the Earth

Earth’s axis is tilted at an angle of 23.5°with respect to the ecliptic plane.

Axial Tilt of the Earth: In Summer

The tilt causes the seasons. In summer: The angle of sunlight is more direct. Energy is more concentrated and days are longer.

Axial Tilt of the Earth: In Summer (Cont.)

Axial Tilt of the Earth: In Summer (Cont.)

Axial Tilt of the Earth: Summer and Autumnal Seasons

Summer solstice: • Sun is at its highest point

above the ecliptic (June 21).• Longest day.

Autumnal equinox:• Sun on the celestial equator

(Sept. 22). • Equal hours of day

and night.

Axial Tilt of the Earth: Winter and Vernal Seasons

Winter solstice:• Sun at its lowest point below

the ecliptic (December 21).• Shortest day.

Vernal equinox:• Sun on the celestial equator

(March 20).• Equal hours of day

and night.

Axial Tilt of the Earth: Winter and Vernal Seasons (Cont.)

Axial Tilt of the Earth: Winter and Vernal Seasons (Cont.)

If the ecliptic were aligned with the celestialequator, what would happen to the seasons?

A. Each season would last longer.B. Nothing. They would be the same as now.C. We would not have seasons at all.D. Cannot tell from the information given.

Class Question

Axial Tilt of the Earth: Changes Over Years Earth’s axial tilt changes

orientation over a period of 26,000 years due to precession.

The location of the poles slowly shifts.

Currently the north celestial pole is near the bright star Polaris.

Orbit of the Moon Synchronous

rotation. The Moon completes

one full rotation in one full revolution around Earth.

Orbit of the Moon: Moon From the Earth The Moon shines

because of reflected sunlight.

Half of the Moon is always illuminated by the Sun.

Phases of the MoonThe phase of the Moon is determined by how much of the bright side we see.

Phases of the Moon: New and Crescent Moon

New Moon: Moon between Earth and the Sun.

Crescent: Less than half of the Moon is lit up from the Earth’s point of view.

Quarter: Moon at right angles with Earth and the Sun.

Gibbous: More than half of the Moon is lit up from Earth’s point of view.

Full Moon: Moon on opposite side of Earth from the Sun.

Phases of the Moon: Quarter, Gibbous, and Full Moon

At what phase of the Moon is a solar eclipsepossible?

A. First QuarterB. FullC. NewD. Third Quarter

Class Question

Eclipses: Solar Eclipse

Solar eclipses happen at new Moon. Only a small portion of Earth can witness each one.

Eclipses: Solar Eclipse (Cont.)

Eclipses: Solar Eclipse (Cont.)

Eclipses: Types of Solar Eclipse Three types of solar

eclipses:• Total: the Moon completely

blocks the Sun.• Partial: only part of Sun is

blocked.• Annular: the Sun appears

as a bright ring surrounding the Moon.

At what phase of the Moon is a lunar eclipsepossible?

A. First QuarterB. FullC. NewD. Third Quarter

Class Question

Eclipses: Lunar Eclipse

Lunar eclipses occur at full Moon. Visible over a wider area of Earth. Longer duration than solar eclipses.

Eclipses: Lunar Eclipse (Cont.)

Eclipses: Lunar Eclipse (Cont.)

Eclipses: Lunar Eclipse (Cont.)

Eclipses: Lunar Eclipse (Cont.)

Do we observe eclipses every month?

A. YesB. No

Class Question

Orbit of the Moon Eclipses do not occur

every month because the Moon’s orbit is tilted about 5 degrees with respect to Earth’s orbit around the Sun (the ecliptic).

If Earth rotated in 12 hours but its orbit were thesame, which of the following would NOT be true?

A. Each season would happen twice per year.B. We would still see only one hemisphere of the

Moon.C. The cycle of lunar phases would last as long

as it does now.D. Observers at the North Pole would only see one

half of the celestial sphere.

Class Question

Chapter Summary The daily patterns of rising and setting => Earth’s

rotation. The annual patterns of the stars and constellations in

the sky => Earth’s revolution. The changes in the length of day and intensity of

sunlight that create the seasons => Axial tilt of the Earth.

The phases of the Moon => Moon’s revolution around the Earth.