Chp 2 - Motions of Earth

8/10/2019 Chp 2 - Motions of Earth

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Chapter 2

Patterns in the

SkyMotionsof Earth

2013 W.W. Norton & Company, Inc.

21st CENTURYASTRONOMY

FOURTHEDITIONKay | Palen | Smith | Blumenthal


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, Inc.2013 W.W. Norton & Company, Inc.

!

Our focus: Earths daily rotation.

Earths annual orbit.

The Moons monthly

orbit.

Consequences of therelationship/alignment

of Earth, the Moon,

and the Sun (phasesof the Moon,

eclipses).


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! Constellationsare arbitrary patterns of stars

in the sky.

! Culturally important products of human

imagination.

!

Ancient cultures built structures to study themand other astronomical phenomena.


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!

Just because stars are grouped in the sameconstellation does not mean they are near

each other in space.

! Some stars move in different directions.

! The constellations slowly change shape on

timescales larger than human lives.


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!

As viewed from abovethe North Pole, Earth

rotates (spins)

counterclockwise on

its axis.

! One rotation takes

24 hours.


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!

Celestial sphere:projection of Earths

axes and equator ontospace.

!

A useful fiction.! Points on the sphere

correspond to directions,not distances.


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!

Celestial sphere rotatesaround the northand

southcelestialpoles

once each day.

!

Celestial equator:midway between.

! Ecliptic: path of the

Sun, inclined 23.5degrees to celestial

equator.


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!At Earths north pole, you would see half the

celestial sphere.

! The north celestial pole is directly overhead.

! Stars rotate counterclockwise in 24 hours.

! No star rises or sets: all are circumpolar.


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!At the equator, all stars rise and set.

! The celestial poles are on the northern

and southern horizons.

! Observers can see the whole celestial sphere

as it rotates.


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! At an intermediate latitude, one pole is above the horizon.

!

The angle to the horizon from that pole equals the latitude.

! Some stars are circumpolar.

! Some stars rise and set.

! Some stars are never visible.


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The Size of Earths Orbit

! The average distance of Earth to the Sun is

called the astronomical unit, or AU.

! 1 AU = 150 million km.

! Earth revolves counterclockwise around the

Sun as viewed from above.


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!

The Suns motion on the ecliptic reflects Earthsorbit around the Sun.

! Earth revolves around the Sun once a year.

!As Earth moves, the Sun is seen against

different constellationsthe zodiac.


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Earths Axis

!

Earths axis is not perpendicular to the eclipticplane.

! Instead, it is at an angle of 23.5 degrees.

! This is why there are seasons.


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Earths Axis

! The angle of sunlight is closer toperpendicular in summer.

! Energy is more concentrated.

! The southern hemisphere is opposite the

northern hemisphere.


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! Earths obliquityof 23.5 is the reason for

the seasons.

! If the tilt were larger or smaller, life would

most likely be different due to the difference

in seasonal variation.! The differences in climate over the surface of

Earth have resulted in a broad diversity of

life, specifically adapted to those locations.! The changes in climate over the course of the

year most likely greatly influenced the

behavior of life, including migratory patterns.


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Special Days of the Year

!

Summer solstice: Sunfarthest north.

! Autumnal equinox: Sun

on the equator, moving

southward.

! Winter solstice: Sun

farthest south.

!

Vernal equinox: Sun onthe equator, moving

northward.


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Credit: Robert Polzl Credit: Tunc Tezel


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Precession

!

Currently, the northcelestial pole is near

the bright star

Polaris.

!

Earths axis wobbleswith a period of

26,000 years.

!

Location of the polesslowly shifts.


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!

Earth wobbles like a top, slowly.! Since the axis shifts, the equator shifts.

! Positions of the equinoxes precess as well.


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The Moon

What causes moon phases?

Why do we only see one side of the Moon?

How are tides generated?

How do eclipses work?


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1/4ththe diameter of Earth

1/50 the volume of Earth

1/81stthe mass of Earth

1/6ththe gravity of Earth


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Average distance ~ 235,000 miles (385,000 km)

(Earth, Moon, and their separation are scaled in the above

animation. The yellow line represents the scaled speed of light.)

Moons orbital eccentricity - 0.0549

Perigee 363,000 km, Apogee 405,000 km

42,000 km distance change Moon can change in size by ~10%


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The side shaded red is the side we observe from Earth.


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Phases of the Moon

!

The Moon shinesbecause of reflected

sunlight.

! Half of the Moon

is always bright.

! The phase is

determined by how

much of the brightside we see.


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Phases of the Moon

!

New moon: moonbetween Earth andthe Sun.

! Waxingcrescent.

!

Quarter moon:moon at rightangles with Earthand the Sun.

!

Waxing gibbous.! Full moon: moon

on opposite sideof Earth from Sun.


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Phases of the Moon

!

After a full Moon,the amount ofbrightness beginswaning.

!

In the northernhemisphere, theMoon fills in fromright to left and also

fades out from rightto left.


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Orbital Period of the Moon

!

The Moon takes 27.3days to orbit once with

respect to the stars

(sidereal period).

!

Lunar phases repeatevery 29.5 days (synodic

period).

!

The Moons rotationalperiod equals its sidereal

period.


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! Throughout history, humans have made

calenders based on the seasons (withrespect to the Sun) or the Moons cycle of

phases.

!

The number of calendar days (365) does notfit neatly into months (29.5 days) or years

(365.24 days).

! Humans creatively made calendars to deal

with this.

! Today the Gregorian calendar,which

includes leap years to keep it on track, is

used.


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Solar Eclipses

!

Solar eclipses happen at new Moon.! Moon passes between Earth and the Sun.

! Only a small portion of Earth can witness

each one.


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!

Three types of solareclipses:

Total: The Mooncompletely blocks

the Suns light. Partial: Only part

is blocked.

Annular: The Sun

appears as a brightring surrounding the

Moon.


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! The part of the Moons shadow you are in

determines which type of solar eclipse you

see.

! Umbra: Total or annular.

!

Penumbra: Partial.


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! Lunar eclipses happen at fullMoon.

! Earth is between the Sun and theMoon.

! Visible over a wider area of Earth.

!

Last a lot longer than solareclipses.


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! Eclipses do not occur every month because

the Moons orbit is tilted 5.2 with respect toEarths orbit around the Sun.

! Intersection: line of nodes.


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Chp 2 - Motions of Earth

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