Astronomy – Seasons
Earth’s Size and Shape
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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:
o Earth makes a circular shadow on the moon during a lunar eclipse.
o The topmast of a ship is always the last part to disappear and the first part to appear over the horizon.
Earth’s Motions
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Rotation – spinning on its axis in a counterclockwise rotation. One rotation is 23 hours and 56 minutes.
o 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.
o 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.
Revolution – movement around the sun counterclockwise. Takes Earth 365.25 days.
o 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.
o Evidence of Earth’s Revolution
 Parallax shift of nearby stars.
 Seasonal change in constellations.
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
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Seasons in the Northern Hemisphere are more extreme than
seasons in the Southern Hemisphere. Why?
Summer solstice
o June 21st
o Summer begins
o Longest day of the year
o Sun reaches its highest point in the sky
o Earth is at is Perihelion.
Winter Solstice
o December 21st
o Winter begins
o Shortest day of the year
o Sun is at its lowest point in the sky
o Earth is at its Aphelion.
Spring Equinox
o Around March 21st
o Spring begins
o Sun is directly overhead at equator
o All latitudes have 12 hours of daylight and nighttime
Autumnal Equinox
o Around September 21st
o Fall Begins
o The sun is directly overhead at equator.
o All latitudes have 12 hours of daylight and nighttime
Time
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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 Sideral month is one moon orbit from new moon to new moon – 27 1/3 days
A Sideral year is one complete Earth orbit relative to the stars.
Time zones = Earth is divided up into 24 times zones of about every 15° longitude.
o Times zones have been changed slightly over time to accommodate social, economic, and political
reasons.
o Times zones were established in 1883
o 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