3Earth’s Motion is Relative Earth rotates… Earth revolves… Earth precesses… Earth nutates… Earth moves with the solar system… Solar System moves with spiral arm of Milky Way galaxy… Milky Way rotates with other galaxies in the Local Group… Local Group moves within the universe… All this while space is expanding!
4TopicsEarth Cycles Sun’s path and the ecliptic plane Seasons
5Cyclic motions of the Sun and stars in our sky are due to motions of the Earth ROTATION=the spin of the Earth on its axis. It takes one day for the Earth to complete one rotation.REVOLUTION=the movement of the Earth in orbit around the sun. It takes one year for the Earth to complete one revolution.PRECESSION=the slow conical (top-like) motion of the Earth’s axis of rotation. It takes 26,000 years for the Earth to complete one cycle of precession.NUTATION=a small back and forth wobble in the Earth’s axis caused by varying effects of the gravitational forces from the Moon and the Sun.
6The Four Motions of the Earth R = rotationP = precessionN = nutationr = revolutionr
8Not all stars in our sky rise and set, and different latitudes view the movements of the stars differently.The view of the stars movement depends on our latitude. (Note: these views represent east or west orientations)The stars near the poles of the celestial sphere (shown here) move in trails that circle the pole and never set. They are called circumpolar.NORTH POLEEQUATORMIDDLE LATITUDES
9Description of Earth Movement Cyclic motions of the Sun and stars in our sky are due to motions of the EarthROTATION=the spin of the Earth on its axis. It takes one day for the Earth to complete one rotation. The daily motion of the Earth is called diurnal motion.REVOLUTION=the movement of the Earth in orbit around the sun. It takes one year for the Earth to complete one revolution.PRECESSION=the slow conical (top-like) motion of the Earth’s axis of rotation. It takes about 26,000 years for the Earth to complete one cycle of precession.NUTATION=an irregular nodding motion caused by tidal forces that occurs as the Earth precesses.
10Which way to the stars appear to move? The apparent westward motion of the Sun, Moon, and stars across our sky each day is caused by Earth’s rotation.We generalize this motion to make statements such as, “The Sun rises in the east and sets in the west.”The same is true for the Moon, planets and the stars.At middle latitudes, we see the Sun, Moon, and many of the stars first come into view moving upward, rising at some point along the eastern horizon. Then, they appear to arc across the sky, gaining greatest altitude at the local meridian. Finally, they disappear somewhere along the western horizon.
11The apparent westward motion of the Sun, Moon, and stars across our sky each day is caused by Earth’s rotation.At middle latitudes, we see the Sun, Moon, and many of the stars first come into view moving upward, rising at some point along the eastern horizon. Then, they appear to arc across the sky. Finally, they disappear somewhere along the western horizon.We generalize this motion to make statements such as, “The Sun rises in the east and sets in the west.”
12The Sun’s PathEach day we see the Sun’s path across our sky, it is a snapshot of a series of 365 separate paths that occur throughout the year. Each daily “snapshot” of the Sun is just one position of the Sun along the ecliptic plane (as seen from the Earth).
13The seasons we experience are linked to the motion of the Sun along the celestial sphere. The point of the Sun’s path farthest north on the celestial sphere is called the summer solstice (JUN 21), while the point of the ecliptic farthest south is called the winter solstice (DEC 21).The two points on the ecliptic where the Sun crosses the celestial equator are called equinoxes. During the vernal equinox (MAR 21), the Sun is moving north, while during the autumnal equinox (SEPT 21), the Sun is moving south.Remember that the seasonal names of the equinoxes and the solstices refer to seasons in the NORTHERN hemisphere. The seasons occurring in the SOUTHERN hemisphere are exactly opposite.
14The Sun rises at different points along the horizon at different times of the year and also peaks at different heights.
15Different parts of the world experience different times of day as the Earth rotates. TIME ZONES which can be used to calculate the time of day in any given part of the world.
16Earth RevolvesAs the Earth revolves, it changes position relative to Sun. This causes two effects:Zodiac constellations appear to shift along the ecliptic slightly less than 1 degree per day. Eventually, constellations visible at night become invisible during the day.The altitude of the Sun in the sky at local meridian changes daily. This alters the Sun’s path across the sky.
17Variations in the Earth’s rotation Earth averages one rotation every 24 hours, which means that some days are slightly longer or shorter than the average.Earth’s rotational speed throughout geologic time has slowed due to it’s gravitational (tidal) interaction with the Moon.
18Variations in The Earth’s Revolution The Earth’s orbital speed varies because its orbital path is not a perfect circle; it is elliptical. The Earth orbits fastest when it is furthest away from the Sun (at aphelion) and slowest when it is closest (at perihelion).
20Changing Angle of Sunlight Reason for the Seasons2Changing Angle of SunlightThe hemisphere tilted toward the Sun receives sunlight at higher angles than the hemisphere tilted away from the Sun.Higher insolation angle means greater intensity of radiant energy (the sun shines more directly on a smaller area).
21Why does the equator receive more energy from the Sun throughout the year than Fort Thomas? Since the angle of the Sun at the equator is higher above the horizon than at Fort Thomas, more solar radiation is received at the Equator due to the higher insolation angle.The angle of the Sun above the horizon at Fort Thomas varies from 74 to 27 degrees during the year.The angle of the Sun above the horizon at the equator is never lower than 66.5 degrees during the year.
22Identify the Seasons and the Equinox or Solstice, and comment about the number of daylight vs. night hoursSeason _________Equinox or Solstice_________Season _________Equinox or SolsticeSeason _________Equinox or SolsticeSeason ________Equinox or Solstice
23Seasons are caused because the Earth’s axis is tilted and as the Earth revolves around the Sun, different parts of the Earth receive more direct sunlight (summer), while other parts of the Earth receive sunlight which is more spread out (winter).
24Why do seasons change? 2 Time and Seasons Factors that affect season change are tilt of axis, revolution and rotation.The reason that the seasons change on Earth is because the tilt of the Earth at 23.5° as it revolves around the Sun.This causes two effects:1) sunlight strikes the Earth’s surface at different angles during different times of the revolution year.2) the number of hours of daylight each rotational day varies because the Sun is up for different numbers of hours per day.
25The EclipticIt is the flat plane of the orbit of the Earth around the Sun. (Earth orbits at an angle) The apparent path of the Sun across the sky. (Sun takes a slightly different path across the sky each day) The angle between the ecliptic plane and the celestial equator is 23.5o.
26The Earth also revolves around the Sun, which changes our view of the stars. From an outside view, we see the Earth revolve around the Sun. We define the plane of the Earth’s orbit as the ecliptic plane.From our perspective, the Sun appears to move through the stars along a special path called the ecliptic.
29SeasonsTilted axis and rotation of Earth combined with revolution around Sun causes the seasons.Tilted axis (23.5 degrees), rotation and revolution cause the northern hemisphere to receive more direct sunlight for more hours than the southern hemisphere during the summer and less direct sunlight for less hours during the winter.So, as one hemisphere is warmer, the other is colder. This condition reverses every 6 months. In other words, this condition is seasonal.The fall and spring represent 3-month transitions from summer to winter and from winter to summer.
31Precession of the Equinoxes Sunrise during the vernal equinox will occur in a different zodiac constellation every years or so. This is due to the precession of the equinoxes.
32This precession causes the position of the North Celestial Pole to slowly change over time. Today, the North Celestial Pole is near the star Polaris, which we call the “North Star.” However, in 3000 BC, Thuban was close to the North Celestial Pole and in 14,000AD, Vega will be in this location.
33Gravitational forces of the Sun and the Moon pulling on the Earth as it rotates causes the Earth to undergo a top-like motion called precession. Over a period of 26,000 years, the Earth’s rotation axis slowly moves in a circular motion.North Star isn’t always Polaris!
35Earth’s Orbit Around the Sun The Earth revolves around the Sun, which changes our view of the stars. Seasons are caused because the Earth’s axis is tilted and as the Earth revolves around the Sun. As the Earth revolves, different parts of the Earth receive sunlight at different angles. The more direct the sunlight, the more energy delivered. The angle of the Sun’s path across the sky determines the daylight hours. Daylight hours + the angle of the sun affect the amount of light energy received by the Earth.
36One common misconception is that the Moon is only visible at night One common misconception is that the Moon is only visible at night. However, the time of day in which the Moon is in our sky varies depending on its phase. This picture clearly displays the Moon, visible during the day.