UNIT 3: EARTH S MOTIONS
After Unit 3 you should be able to: o Differentiate between rotation and revolution of the Earth o Apply the rates of rotation and revolution to basic problems o Recall the evidence for rotation and revolution o Understand the causes of daily and seasonal observations as viewed from Earth o Recognize the appropriate arcs of the Sun for the different seasons o Recall the dates for the solstices and equinoxes o Calculate eccentricity o Describe the shape of an orbit using eccentricity o Understand how the orbital velocity of an object is impacted by gravity o Understand how stars in the night sky arc around Polaris
Unit 3 vocabulary you should be able to use and understand: o Real motion o Apparent motion o Axis o Tilt o Rotation o Rate of Rotation o Polaris o Foucault s Pendulum o Coriolis Effect o Northern hemisphere o Southern hemisphere o Arc o Celestial sphere o Constellation o Equator o Revolution o Rate of Revolution o Ellipse o Orbit o Seasonal constellations o Tropic of Cancer o Tropic of Capricorn o Summer Solstice o Winter Solstice o Autumnal Equinox o Vernal Equinox o Insolation o Zenith o Heliocentric model o Eccentricity o Foci o Major Axis o Gravity o Velocity o Orbital velocity o Period of Rotation o 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 15 o 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 15 o 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 1 o per day 360 o / 365.25 days = 1 o 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 (23.5 o N) on June 21. Passes the equator on September 23 rd and hits the Tropic of Capricorn on December 21 st. Longest arc June 21 st (longest day) Shortest arc December 21 st (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 90 o. All other places are less than 90 o. Does the 90 o ever reach New York State? An observer in New York State will never see the Sun directly overhead because we are north of 23.5 o 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 21 st Summer Solstice longest day June 21 st Vernal Equinox equal day and night in Spring March 20 th Autumnal Equinox equal day and night in Fall September 23 rd
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