Earth’s Movement and Seasons

Earth’s Size and Shape Earth is shaped like a ball or sphere, only slightly flattened at the poles and bulging at the equator. Evidence that the Earth is round: Earth makes a circular shadow on the moon during a lunar eclipse. The topmast of a ship is always the last part to disappear and the first part to appear over the horizon.

Earth’s Motion Rotation – spinning on its axis in a counterclockwise rotation. One rotation is 23 hours and 56 minutes. Axis of Rotation – an imaginary line running through the Earth from the North Pole to the South Pole. The axis is tilted 23.5 degrees. Evidence of Rotation: A pendulum will change directions Coriolis Effect = apparent deflection of wind and/or water on Earth’s surface. Northern Hemisphere = deflection to the right. Southern Hemisphere = deflection to the left.

Earth’s Motion Revolution – movement around the sun counterclockwise. Takes Earth 365.25 days. Every four years we have a Leap Year which has 366 days instead of 365. Origin of modern calendar = Ancient Egyptians, edited by Julius Caesar who added leap years. Evidence of Earth’s Revolution Parallax shift of nearby stars. Seasonal change in constellations.

Earth’s Motion

Earth’s Motion Parallelism – direction that Earth’s axis point does not change as Earth orbits the Sun. Earth’s position during its orbit Aphelion – when the Earth is furthest away from the sun (June 21) – 94.5 million miles away Perihelion – when the Earth is closest to the sun (December 21) – 91.4 million miles away

Seasons, Daylight, and Time Axial Tilt + Earth’s Revolution Parallelism of Earth’s axis = The reason for our seasons!! (and changes in daylight!)

Seasons and Daylight Summer solstice Winter Solstice June 21st Summer begins Longest day of the year Sun reaches its highest point in the sky Earth is at is Aphelion. Winter Solstice December 21st Winter begins Shortest day of the year Sun is at its lowest point in the sky Earth is at its Perihelion.

Seasons and Daylight Spring Equinox Autumnal Equinox Around March 21st Spring begins Sun is directly overhead at equator All latitudes have 12 hours of daylight and nighttime Autumnal Equinox Around September 21st Fall Begins The sun is directly overhead at equator. All latitudes have 12 hours of daylight and nighttime

Seasons and Daylight

Time Our month comes from the moon’s 29 ½ day cycle called synodic or lunar month. Sun and moon meet in the sky with every new moon. A synodic month is not the moon’s true orbit. A Sidereal month is one moon orbit from new moon to new moon – 27 1/3 days A Sidereal year is one complete Earth orbit relative to the stars.

Time Time zones = Earth is divided up into 24 times zones of about every 15° longitude. Times zones have been changed slightly over time to accommodate social, economic, and political reasons. Times zones were established in 1883 Times zones are set s the entire area is set to the same time and the next time zone west is an hour behind. AM = ante meridiem – before the middle of the day PM = post meridiem – after the middle of the day