UNIT 3: EARTH’S MOTIONS

After Unit 3 you should be able to:

o Differentiate between rotation and revolution of the Eartho Apply the rates of rotation and revolution to basic problemso Recall the evidence for rotation and revolutiono Understand the causes of daily and seasonal observations as viewed

from Eartho Recognize the appropriate arcs of the Sun for the different seasonso Recall the dates for the solstices and equinoxeso Calculate eccentricityo Describe the shape of an orbit using eccentricityo Understand how the orbital velocity of an object is impacted by

gravityo Understand how stars in the night sky arc around Polaris

Unit 3 vocabulary you should be able to use and understand:

o Real motiono Apparent motiono Axiso Tilto Rotationo Rate of Rotationo Polariso Foucault’s Pendulumo Coriolis Effecto Northern hemisphereo Southern hemisphereo Arco Celestial sphereo Constellationo Equatoro Revolutiono Rate of Revolutiono Ellipseo Orbit

o Seasonal constellations

o Tropic of Cancero Tropic of Capricorno Summer Solsticeo Winter Solsticeo Autumnal Equinoxo Vernal Equinoxo Insolationo Zenitho Heliocentric modelo Eccentricityo Focio Major Axiso Gravityo Velocityo Orbital velocityo Period of Rotationo Period of Revolution

Although you cannot feel it, our Earth is in constant motion. As observers on the surface of Earth, we bear witness to apparent motions. There are also real motions.

Apparent motions are perceived motions to the viewer, whether they are accurate or not

Real motions are motions that are actually taking place

There are two real motions of the Earth in space that provide apparent motions that we observe:

Rotation of the Earth on its axis

Revolution of the Earth around the Sun

The Earth is tilted relative to the Sun

Earth’s axis is an imaginary line running through the North and South Poles

The axis is tilted toward or away from the Sun 23.5 degrees depending on the season

Rotation of Earth

Rotation = spin The Earth rotates on its axis 1 rotation (360 degrees) every 24 hours 15 degrees/hour

Rotational Direction

The Earth rotates from west to east This is a counterclockwise rotation looking

down at the North pole The axis remains pointed directly at the distant

star, Polaris

What evidence is there for rotation of the Earth? There are two pieces of evidence that suggest rotation is taking place:

Changing path of Foucault’s Pendulum Coriolis Effect

Changing path of Foucault’s Pendulum

Path of the pendulum changes due to the rotational force of the Earth acting upon it

The Coriolis Effect

The Coriolis Effect is the tendency of objects moving over the Earth such as ocean currents or air to be deflected (curve away) from a straight line path.

The deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

This deflection occurs because Earth’s surface is rotating with respect to the objects.

The real motion known as rotation leads to apparent motions that we observe on the Earth.

Stars appear to circle Polaris (the North Star) from East to West

The Sun appears to arc across the sky from East to West

Making Observations in the Sky

When we look up, the stars appear to be fixed upon a dome around Earth

This apparent dome is called the celestial sphere

A constellation is a man-made association of stars

Looking upward we see the stars fixed on the celestial sphere

Earth’s eastward rotation makes the stars between the equator and north celestial pole appear to move westward

Rise in the east and set in the west

Stars rising in the east

Stars moving east to west

Northern part of the sky around Polaris

Earth’s counterclockwise rotation makes the stars appear to revolve counterclockwise around the north celestial pole (Polaris) at 15o per hour

The complete circular path can be seen for stars in the northern portion of the sky around Polaris

REMEMBER

The rotation of the Earth is responsible for daily (or nightly) observations/changes

The observed motions in the sky show objects (sun, stars) arcing at a rate of 15o per hour

The Other Real Motion: Revolution

Earth revolves around the Sun in a slightly eccentric elliptical path once a year

Ellipse = oval

It takes Earth 365.25 days to revolve (orbit) around the Sun

The rate is approximately 1o per day 360o / 365.25 days = 1o per day

What evidence is there to indicate that revolution actually takes place?

Change in seasonal constellations

Change in Seasonal Constellations

Because of the Earth’s change in position around the Sun, some constellations are visible only during certain times of the year

The constellations that are visible are on the dark side of the Earth (pointing away from the Sun)

Apparent Motions Associated with Revolution

Alteration of the length of the Sun’s arc

Seasonal Arcs of the Sun

The noon perpendicular rays of the Sun travel southward from the Tropic of Cancer (23.5o N) on June 21.

Passes the equator on September 23rd and hits the Tropic of Capricorn on December 21st.

Longest arc June 21st (longest day)

Shortest arc December 21st (shortest day)

Zenith Position of the Sun

Because of the Earth’s spherical shape, on any particular date, there is just 1 place where insolation (incoming solar radiation) is at an angle of 90o. All other places are less than 90o.

Does the 90o ever reach New York State?

An observer in New York State will never see the Sun directly overhead because we are north of 23.5o North

Consequences of Tilt and Revolution of the Earth

The combination of these elements results in the change in seasons. Contrary to popular belief, we are not closer to the Sun in the summer.

When the northern hemisphere is tilted towards the Sun, its rays hit us more directly and we experience summer

The opposite is true for winter

Important Dates

Winter Solstice shortest day December 21st

Summer Solstice longest day June 21st

Vernal Equinox equal day and night in Spring March 20th

Autumnal Equinox equal day and night in Fall September 23rd

Describing Orbits

The shape of an orbit can be described using the equation for eccentricity.

Each of the planets revolve around the Sun (heliocentric model)

Gravitational attraction causes the planets to travel in nearly circular paths

The Sun is considered one of the foci in an elliptical path of a planet

Eccentricity

If eccentricity is equal to 0, then the path is a circle

The path becomes more elliptical moving away from zero and towards 1

How is eccentricity calculated?

Measure the distance between foci (to the nearest tenth of a centimeter

Measure the length of the major axis (to the nearest tenth of a centimeter)

Divide the distance between foci by the length of the major axis (to the nearest thousandth)

Orbital Velocities

The speed (velocity) that an object orbits another is dependent on gravitational attraction

Gravitational attraction is greatest when the two objects are closer together, producing the fastest velocity

Apparent Motion of Sun due to Rotation and Revolution