# Chapter 25 Earth’s Motions.

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Chapter 25 Earth’s Motions

Rotation The spin of the Earth on its axis (rate of 15º/hr)
Earth is tilted at an angle of 23.5º from perpendicular (90º) . In other words, the tilt of the Earth’s axis is 23.5º off of the 90º it should be if it had a perpendicular tilt. The Earth’s axis is always tilted in the same direction throughout its orbit (parallelism)

Remember, the ecliptic is the plane of the path that the Earth takes around the Sun

Evidence of Movement Foucault Pendulum- Changes its direction of swing over time Coriolis Effect- The deflection of air and water to their right in the northern hemisphere Sorry, Bart Simpson was incorrect. Water DOES deflect counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, HOWEVER… in toilets, the direction is caused by the force of the pump and direction it pumps the water in, not the CORIOLIS force! In fact, YOU can determine the direction that water goes down the tub drain if you turn your hand in the water around the drain and spin the water either CW or CCW as it is draining out of the tub!!! Try it!!!!!

Object moves to ITS RIGHT Object moves to ITS LEFT

Solar Time The measurement of time based on the apparent movement of the sun through the sky Solar Noon is the time of day when the sun is at its highest altitude above the horizon. (Wherever the Sun “Peaks” in the sky) Problem - solar noon does not occur at the same time for everyone (solar noon differs by approx. 1 minute for every 12 miles east/west of a specific location)

To make up for this “problem”, time zones were established (each 15º longitude wide – 1 hour of rotation) Time zones begin at the Prime Meridian Every 15 degrees EAST is +1 hour, every 15 degrees WEST is –1 hour Clock time is the average solar time

Revolution Movement of the Earth around the sun in its elliptical orbit One complete revolution is days (1 year) In conjunction with AXIAL TILT, this causes a change in the seasons If there was no tilt of the axis = no seasonal changes Evidence of movement: Change in visible constellations throughout the year

Solstices and Equinoxes
SOLSTICE – extreme differences in daylight vs. darkness. The following values are applicable to the NORTHERN HEMISPHERE ONLY. REVERSE THESE VALUES FOR THE SOUTHERN HEMISPHERE SUMMER 6/21/05 – 16 hours daylight/8 hrs darkness. The Sun is directly above the Tropic of Cancer. (23.5º N) WINTER 12/21/05– 8 hours daylight/16 hrs darkness. The Sun is directly above the Tropic of Capricorn. (23.5º S) EQUINOX – equal distances from the sun (equal daylight and darkness) Autumn (Autumnal) 9/23/05, Spring (Vernal) (Verde/Vern means green) 3/20/05 – 12 hours daylight and 12 hours of darkness FOR THE ENTIRE EARTH – NORTHERN AND SOUTHERN HEMISPHERES. The Sun is directly above the Equator.

Nighttime on the Artic Circle during the Summer. The sun never sets
Nighttime on the Artic Circle during the Summer. The sun never sets! It only gets as low as the horizon and as high as it looks about an hour before sunset!!!

The Sun’s Apparent Path Across the Sky
Because of the tilt of the Earth and the Revolution of the Earth around the Sun, the Sun appears to rise and set each day in a slightly different place on the horizon. Also, Solar Noon (the highest point the Sun reaches in the sky) also varies slightly each day. These are predictable apparent motions and are easiest to pick out on the Solstices and Equinoxes.

Sun’s Apparent Path from a Viewpoint in NYS
December 21 June 21 March 21 & September 21

Solar Noon at The Equator

Causes of Seasons Earth’s revolution around the sun
Tilt of the Earth on its axis Parallelism of the axis More daylight hours = more insolation = warmer time of year Less daylight hours = less insolation = cooler time of year Sun’s rays are more direct (perpendicular) to the Earth’s Surface in the Northern Hemisphere during our summer

Causes of Seasons #6 explains why even though the Sun shines for 24 hours a day for nearly 6 months a year at the North and South Poles, the temperature never gets above freezing!!!! If the Sun’s rays don’t hit the Earth at a perpendicular or near perpendicular angle, they aren’t all that powerful and warming!!!!

Direct solar rays Less direct solar rays