Motions of Earth, Moon, and Sun Notes

Motions of Earth, Moon, and Sun

How Scientists Study Motions of Earth, Moon, and Sun

• Common Misconception: Many believe the Moon doesn't rotate.

• The Moon revolves around Earth in approximately 27 \frac{1}{3} days.

• We always see the same features of the full moon.

Demonstration:

• Use a dime as the Moon and a half-dollar as Earth.

• Point the top of Roosevelt's head (Moon) towards the half-dollar (Earth) and revolve the moon around Earth.

  • The dime rotates once as it travels around the half-dollar.

• The same side of the Moon points towards Earth at all times.

• The Moon rotates once per revolution in 27 \frac{1}{2} days.

Vocabulary

• Axis (of rotation): An imaginary line through the center of a body around which it rotates.

• Constellation: A group of stars that form a pattern.

• Coriolis effect: The deflection of moving objects when viewed from a rotating reference frame.

• Eclipse: An obscuring of the light from one celestial body by the passage of another between it and the observer or between it and its source of illumination.

• Foucault pendulum: A freely swinging pendulum whose apparent change in direction demonstrates the rotation of the Earth.

• Geocentric model: An Earth-centered model of the universe.

• Heliocentric model: A Sun-centered model of the universe.

• Local time: The time at a particular location, determined by the position of the sun.

• Phases (of the Moon): The different appearances of the Moon as it orbits Earth.

• Tides: The periodic rise and fall of the sea level under the gravitational pull of the Moon and Sun.

• Time zone: A region that has adopted the same standard time.

Topic Overview

• Cultures have tried to understand the nature and motions of celestial objects.

• Explanations have become part of cultural belief systems.

• Earth and its Moon are sometimes considered a double planet.

• For this book, the Moon is considered a satellite of Earth, and Earth a satellite of the Sun.

Apparent Motions of Celestial Objects

• Apparent motion: A motion that an object appears to make.

• Apparent motions can be real or illusions.

• Stars appear to move across the sky from east to west due to Earth's rotation.

Daily Motion and Stars

• Most celestial objects rise in the east and set in the west.

• About 6000 stars are visible with the naked eye.

• Stars near Polaris appear to move in a complete circle in 24 hours.

• Paths of celestial objects are circular or arcs.

• Motion occurs at a constant rate of approximately 15^\circ per hour or 360^\circ in 24 hours.

• These movements are called daily motion.

Apparent Motions of the Planets

• Planets exhibit daily motion similar to stars.

• Over extended periods, planets change position relative to stars.

• The movement of planets relative to stars is not uniform and appears complicated.

• Similar motion is observed each year in a different region of the sky.

• From a central sun perspective, the complicated motion of the planets is the result of Earth and other planets revolving around the sun in different orbits at different speeds.

• This causes other planets viewed from Earth to sometimes appear to make loops and move back and forth.

Apparent Motions of Earth's Moon

• The Moon follows the daily east-to-west motion of the stars.

• It rises about 50 minutes later each day.

• It shifts eastward each day compared to the background field of stars.

Apparent Sun Motions in Northern Mid-Latitudes

• The Sun seems to move in the sky.

• The Sun's apparent path from sunrise to sunset is an arc.

• The Sun's path changes in position and length with the seasons.

• The greater the length of the Sun's path, the more daylight hours.

• Summer has the longest daylight, winter the shortest, and spring/fall are in-between (approximately 12 hours).

• The position of the Sun at sunrise and sunset varies predictably with the seasons.

• In spring and fall, the Sun rises due east and sets due west.

• In summer, the Sun rises north of east and sets north of west.

• In winter, the Sun rises south of east and sets south of west.

Changes in the Altitude of the Sun at Noon

• The daily altitude of the Sun is lowest at sunrise and increases until noon, then decreases to sunset.

• The Sun reaches its highest position at local solar noon.

• The altitude of the Sun at noon depends on the time of year and latitude.

• Only between latitudes 23 \frac{1}{2}^\circ N and 23 \frac{1}{2}^\circ S can the noon Sun be directly overhead (at 90^\circ).

• The noon Sun is never directly overhead in the continental United States.

Models that Help Explain Apparent Celestial Motions

Geocentric Models

• Early cultures assumed Earth is stationary and celestial objects revolve around it.

• This is the geocentric model (Earth-centered).

• The Moon and Sun were thought to travel at different speeds in their orbits.

• The geocentric model explained motions of stars, Sun, and Moon, but not planets.

• One version proposed planets moved in circles as they revolved around Earth.

• The geocentric model could not explain some Earth motions and pendulum behavior.

• The geocentric model was largely accepted up to the 16th century in European cultures.

Heliocentric Models

• Some ancient cultures and most modern societies use a heliocentric model (Sun-centered).

• Earth rotates on an axis and revolves around the Sun.

• The Moon revolves around Earth as Earth revolves around the Sun.

• The daily motion of stars is explained by Earth's rotation.

• The motions of the Moon are explained by Earth's rotation, Earth's revolution around the Sun, and the Moon's revolution around Earth.

• The apparent motions of the Sun are explained by Earth's rotation on a tilted axis and Earth's revolution around the Sun.

• Early heliocentric models used circular orbits, which did not accurately predict planetary motions.

• Adding elliptical orbits and varying orbital speeds improved predictions.

• The heliocentric model explained the behavior of a freely swinging pendulum.

Actual Earth Motions

• Earth moves constantly.

• Earth moves with the Milky Way Galaxy as the universe expands.

• It moves around the center of the Milky Way Galaxy with our solar system in a 225-million-year period.

• Earth rotates and revolves around the Sun.

Rotation of Earth

• Spinning of Earth on its axis (imaginary line from North Pole to South Pole).

• The axis is tilted 23^\circ from a line perpendicular to the plane of its orbit.

• As Earth orbits the Sun, Earth's axis remains tilted at 23^\circ.

• The north end of the axis points towards Polaris.

• Earth rotates 360^\circ from west to east in 24 hours, at an angular rate of 15^\circ per hour.

• Looking down over the North Pole, the direction of Earth's rotation is west to east (counterclockwise).

Evidence of Earth's Rotation

• Scientists have searched for evidence that Earth rotates.

• Earth's movement is not apparent because people don't feel smooth motions the same way that they are aware of abrupt motions. also There is nothing near Earth with which to compare its motion.

• Only recently have people and satellites been able to observe the rotation of Earth.

The Foucault Pendulum

• A freely swinging Foucault pendulum's path appears to change in a predictable way.

• This is evidence of Earth's rotation because the pendulum would continue to swing in the original path if Earth did not rotate due to inertia.

• The pendulum swings in a fixed direction in space, while Earth rotates under it.

The Coriolis Effect

• The tendency of all particles of matter moving at Earth's surface to be deflected, or curve away, from a straight-line path.

• Deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

• This occurs because Earth is rotating, and therefore Earth's surface is moving with respect to the path of the particles.

• Rockets, ocean currents, large storms, and winds are deflected with respect to Earth's surface.

• The Coriolis effect is evidence for Earth's rotation.

Evidence of Earth's Revolution Around the Sun

• Earth revolves around the Sun in a slightly eccentric elliptical orbit once a year.

• Earth revolves in a counterclockwise direction (as viewed from Polaris) at approximately 1 degree per day (360^\circ in 365 days).

• Seasonal changes associated with changes of the Sun's path is evidence of revolution.

• If Earth did not revolve around the Sun, the same part of Earth would tilt toward the Sun all the time.

• Having different constellations associated with each of the four seasons is further evidence of Earth's revolution.

• At night you can see different constellations at different times of the year.

• During the year the apparent diameter of the Sun changes in a cyclic fashion.

• Earth's changing distance from the Sun as Earth revolves in its slightly elliptical orbit causes the size of the Sun to appear to change.

• The Sun appears largest when Earth is closest to the Sun around January 3.

• The Sun appears smallest when it is farthest from Earth on about July 4.

• As Earth revolves around the sun, Earth is traveling toward a given star for about half the year, and the other half of the year it is moving away from a given star. These movements toward or away from a star result in small yearly cyclic changes in blue and red shifting of starlight.

Earth and Moon Motions and Time

• Cultures have used Earth and Moon motions to determine time.

• The concept of the month is based on the length of one cycle of our moon's phases.

• The day is based on one rotation of Earth as measured by apparent motions of the sun.

• The year is based on one revolution of Earth around the sun as measured by changes in the apparent motions of the sun or other stars.

• Specific time within a day is more difficult to determine.

Local Time

• Time based on the rotation of Earth as reflected in motions of the sun is local time.

• All places on the same north-south line, or meridian of longitude, have the same local time.

• Locations east or west of any meridian have different local times, due to Earth's rotation.
  

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Local Solar Time

• For any location, the sun reaches its highest point in the sky once each day at solar noon.

• The time it takes for Earth to rotate from solar noon to solar noon on two successive days at any fixed location on Earth is the solar day.

• This type of local time, based on the motions of the sun in the sky, is solar time, apparent solar time, or sundial time, because a sundial is used to measure it.

• If Earth moved around the sun in a circular orbit at a constant speed, the apparent solar day would have a constant length.

• As a result, the length of the apparent solar day also varies during the year. Days of varying length make it very difficult to measure time using clocks and watches. Therefore, for convenience in timekeeping, a solar day of average length-the mean solar day—has been established.

• mean solar time, which is based on the mean solar day.

• It has been divided into exactly 24 hours. On most days of the year, the solar day is either slightly more or slightly less than 24 hours in length.

Time Zone or Standard Time

• Time zones have been created to standardize time for regions.

• There could be a different local time for each meter or kilometer where you live.

• To solve this problem, 24 separate time zones in 15-degree-wide east-to-west bands have been established.

• All parts of Earth within each of these 15-degree longitude bands maintain the same time.

• Since Earth rotates 360 degrees in a 24-hour period, each 15-degree band has a difference of one hour in time.

• Each time zone keeps the local mean solar time of the meridian that is in the middle of that particular time zone.

Actual Motions of Earth's Moon

• The revolution of the Moon around Earth as Earth revolves around the Sun results in many common observable events, including phases of the moon, tides, and eclipses.

• The moon revolves around Earth in an elliptical orbit that is tilted about 5^\circ from Earth's orbit and that has a period of 27 \frac{1}{3} days.

• Besides revolving around Earth in 27 \frac{1}{3} days, the moon also rotates on its axis in 27 \frac{1}{3} days.

• Thus as the moon revolves once, it rotates once. This is the reason why the same "face" or side of the moon always points towards Earth.

Moon Phases

• Half of the Moon is always receiving light from the Sun at any given time, except during lunar eclipses.

• Since the Moon revolves around Earth, an observer on Earth sees varying amounts of this lighted half as the Moon moves through its orbit.

• The varying amounts of the lighted Moon as seen from Earth are known as the Moon's phases.

• Because the revolution of the moon around the centre of the earth-moon system is cyclic, the phases of the moon are also cyclic.

• The period from one full moon to the next is 29 days, whereas the moon's period of revolution and rotation is just 27 \frac{1}{3} days.

Tides

• The gravitation between Earth, the Moon, and the Sun results in a cyclic rise and fall of ocean waters on Earth called tides.

• The tidal effect is caused primarily by the Moon, not the Sun, even though the moon is much smaller than the sun, it is about 400 times closer to Earth.

• There is a bulge of water—a high tide directily under the moon's positon.

• solid Earth is closer to the Earth Moon centre of gravitation than the ocean water. This causes Earth to be pulled away from the water, leaving the high tide.

• At right angles to the positions of high tide, the gravitational pull of the moon is least, and the levels of ocean water are low, resulting in a low tide.

• The timing of high and low tides is not only affected by the rotating Earth, it is also influenced by the Moon revolving around Earth.

• Thus, the ideal time between two high tides or two low tides would be about 12 hours and 25 minutes.

• When Earth, the Moon, and the Sun are in a straight line, the Sun enhances the tidal effect, and high tides are higher and low tides are lower. This large tidal range happens during new and full moon phases.

• When the moon is in one of the quarter phases, the sun is pulling at right angles to the moon. As a result, high tides are then lower, and low tides are higher, producing a smaller tidal range.

Eclipses

• When a celestial object partly or completely comes into the shadow of another celestial object, there is an event called an eclipse.

Lunar Eclipse

• When the Moon revolves into the shadow of Earth at the full moon phase, a lunar eclipse, or eclipse of the moon, can occur.

• A lunar eclipse doesn't happen during most full moons because the Moon's orbit of Earth is tilted about 5 degrees compared to Earth's orbit.

• If Earth's shadow (umbra) covers only part of the moon, there is a partial eclipse. The curved shadow of Earth on the moon during a partial lunar eclipse is evidence for Earth's spherical shape.

• When there is a total lunar eclipse, the moon is completely covered by Earth's shadow and all people on the dark half of Earth can view the eclipse for up to about 100 minutes.

• The complicated cycle of lunar eclipses results in an average of two total lunar eclipses a year.

Solar Eclipse

• Under rare ideal conditions at the new moon phase, the Moon can just barely block out the Sun, casting a shadow on Earth that causes a solar eclipse.

• A total eclipse of the Sun can only be observed at any one location on Earth for up to 7\frac{1}{2} minutes once every 200 years or so.