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Earth in Space. Chapter 16. Motion of Earth through space. Seven conspicuous motions: Daily rotation at 1,670 km/h (at the equator) Monthly rotation about Earth-Moon center of mass Yearly rotation about the Sun at 106,000 km/h Orbit of solar system about center of Milky Way at 370,000 km/h.

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Earth in space

Earth in Space

Chapter 16

Motion of earth through space
Motion of Earth through space

Seven conspicuous motions:

Daily rotation at 1,670 km/h (at the equator)

Monthly rotation about Earth-Moon center of mass

Yearly rotation about the Sun at 106,000 km/h

Orbit of solar system about center of Milky Way at 370,000 km/h

Orbital motion within the Sun’s local star group at 1,000,000 km/h

Motion of Milky Way galaxy relative to remote galaxies at 580,000 km/h

Minor motions

Changes in shape and size of Earth’s orbit

Changes in the tilt of Earth’s axis

Slowing of Earth’s rotation rate

Earth s motion in space cont
Earth’s motion in space, cont.

Overall picture from space: moving along a helix at fantastic speeds

Viewed from Earth

Ecliptic plane: plane of Earth’s orbit about Sun

Planets move in a narrow band about ecliptic plane (Solar system relatively flat)

Sun moves along the ecliptic across background stars

Shape and size of earth
Shape and size of Earth

Very close to an oblate spheroid


Slightly pear-shaped

Lump in the Pacific

Depression in Indian Ocean

Elevations and depressions of deviations less than 85 m.

Motions of earth
Motions of Earth

Three of Earth’s motions are independent of the motions of the Sun and galaxy

Yearly revolution about the Sun

Daily rotation about its axis

Clockwise wobble of its rotation axis


Earth’s movement around the Sun

Orbit defines plane of the ecliptic

Ecliptic - apparent path of Sun through the heavens as seen from Earth

Elliptical orbit, not circular

2.5 million km closer in January

2.5 million km more distant in July

Earth receives about 6% more solar energy in January

Rotation and earth s axis
Rotation and Earth’s axis

Earth’s rotation axis inclined 23.5º as shown


Determined by orientation of rotation axis

Winter solstice, summer solstice

Spring equinox, autumnal equinox

Celestial equator

Line on celestial sphere above Earth’s equator

Rotation does earth rotate
Rotation: Does Earth rotate?

Evidence from three observations

Other celestial bodies are seen rotating

Jupiter, Sun

Foucault pendulum

Pendulum oscillates in fixed direction

Orientation changes as Earth rotates

Coriolis effect

Deflection of paths as Earth rotates beneath moving objects


Slow wobble of Earth’s rotation axis

Reaction of Earth to gravitation pull on its equatorial bulge by the Moon and Sun

26,000 years for one precession

Changes direction of rotation axis on celestial sphere

Polaris not always the North Star

Position of equinoxes in the zodiac changes

Place and time
Place and time

Earth’s periodic motions provide a basis for determining place and time

Rotation provides an axis of rotation useful in determining locations on the globe

Rotation and revolution determine cycles which can be used for time standards

Identifying place
Identifying place

Position on flat surface

Intersection of two straight lines

Won’t work for Earth’s curved surface

Position on Earth

Intersection of parallels and meridians defined with respect to rotation axis

Parallels - latitude

Meridians - longitude

Special parallels climate related

Measuring time
Measuring time

Time standards depend on measuring intervals between evenly spaced periodic events

Astronomical examples: rotation of Earth on its axis, revolution of Earth around the Sun

Basis for day, month, season and year

Different ways to measure day, month and year

Daily time
Daily time

Earth-Sun motion

Viewed from above North Pole

Earth revolves counterclockwise about Sun and rotates in the same direction

On Earth

Sun rises in the east and sets in the west

Sun s motion across the sky
Sun’s motion across the sky


Time at which the Sun is at the celestial meridian

“Apparent local noon” at a particular longitude

Apparent local solar time

Given by position of Sun in the sky

Can be measured with gnomon or sundial

Not time shown on clocks

Mean solar time

Time averaged from apparent solar time

Averaged over two effects

Earth’s orbital speed changes throughout its elliptical orbit

The equator is inclined to the ecliptic

Mean solar day

24 hour average from mean solar time

A day
A day

Length of time required for Earth to rotate once on its axis

Sidereal day

Interval between two consecutive crossings of the celestial meridian by a particular star

Little variation because of Earth’s constant rotational rate

Apparent solar day

Interval between two consecutive crossings of the celestial meridian by the Sun

Revolution about Sun changes Earth-Sun orientation each day

About 4 minutes longer than sidereal day

23 hours, 56 minutes and 4 seconds long

Standard time zones
Standard time zones

360º of longitude divided into 24 15º zones

Adjusted for local consistency

Daylight saving time

clocks set ahead in spring and back in fall for extra hours of sunlight during summer evenings

International date line

The 180º meridian

Designated to correlate days with 24 hour time zones

Yearly time
Yearly time

Tropical year

Interval between two consecutive spring equinoxes

365.24220 mean solar days

Sidereal year

Interval between two consecutive alignments of the Earth and Sun relative to the stars

About 20 minutes longer than a tropical year

Adjustments to keep calendar year current with the seasons: Leap year, leap seconds, …

Monthly time
Monthly time

Sidereal month

Orbital time measured with respect to the stars

27 1/3 days

Synodic month

Time interval between two consecutive phases

29 1/2 days

Includes effects of Earth revolving around the Sun

The moon
The Moon

380,000 km distant

Characterized by craters, lunar highlands and Maria (ancient lava flows)

No atmosphere

Subject of 12 Apollo missions

Composition and features of the moon
Composition and features of the Moon

3 m of fine gray dust on the surface

Accumulated from micrometeorite impacts

Glass beads formed from melted material

Rocks are basalts

Formed from molten lava

Light colored highland rock formed 4 billion years ago

Dark colored Maria rocks range from 3.1 to 3.8 billion years old

Internal structure

65-130 km of outer rock (thickest on far side)

900 km partly molten iron core

History of the moon
History of the Moon

Stage 1 - origin stage

Formed from material ejected from a collision of a large object with Earth

Stage 2 - molten surface stage

Molten surface 100 km deep

200 million years after formation

Heating from solar system debris impacts

Stage 3 - molten interior stage

Accumulated heat from radioactive decay

Began 3.8 million years ago; ended about 3.1 million years ago

Stage 4 - cold and quiet stage

3.1 million years ago to present

Surface scarred by micrometeorites and meteorites

The earth moon system
The Earth-Moon system

Moon/Earth mass ratio highest in solar system

Diameter: 1/4 that of Earth


1/81 that of Earth

Large enough to affect Earth’s orbit

Earth and Moon rotate about common center of mass

Phases of the moon
Phases of the Moon

Result from changing relative positions of Earth, Sun and Moon

New moon, first quarter, full moon, last quarter

The same side always points toward Earth

Eclipses of the sun and moon
Eclipses of the Sun and Moon

Moon’s orbit inclined 5º from that of Earth

Proper alignment of Earth, Moon and Sun needed

Conical shadows have two parts

Umbra: inner cone, complete shadow

Penumbra: outer cone, partial shadow

Solar eclipses

Where tip of umbra touches Earth

Annular eclipse: when umbra doesn’t reach Earth

Lunar eclipses

Moon engulfed by Earth’s umbra


Result from different gravitational pulls on front and back of Earth

Three factors

Earth, Moon and Sun positions

Elliptical orbit of Moon

Spring tides when aligned; neap tides when Moon and Sun at 90º

Greatest pull at perigee; less effect at apogee

48,000 km difference

Size, shape and depth of water basin

Ranges from 1/3 m in Gulf of Mexico to 15 m in Bay of Fundy