Earth in Space

1. Earth in Space Chapter 16

2. Motion of Earth through space Seven conspicuous motions: Daily rotation at 1,670 km/h (at the equator) Monthly rotation about Earth-Moon center of mass Yearly rotation about the Sun at 106,000 km/h Orbit of solar system about center of Milky Way at 370,000 km/h Orbital motion within the Sun’s local star group at 1,000,000 km/h Motion of Milky Way galaxy relative to remote galaxies at 580,000 km/h Minor motions Changes in shape and size of Earth’s orbit Changes in the tilt of Earth’s axis Slowing of Earth’s rotation rate

3. Earth’s motion in space, cont. Overall picture from space: moving along a helix at fantastic speeds Viewed from Earth Ecliptic plane: plane of Earth’s orbit about Sun Planets move in a narrow band about ecliptic plane (Solar system relatively flat) Sun moves along the ecliptic across background stars

4. Shape and size of Earth Very close to an oblate spheroid Deviations Slightly pear-shaped Lump in the Pacific Depression in Indian Ocean Elevations and depressions of deviations less than 85 m.

5. Motions of Earth Three of Earth’s motions are independent of the motions of the Sun and galaxy Yearly revolution about the Sun Daily rotation about its axis Clockwise wobble of its rotation axis

6. Revolution Earth’s movement around the Sun Orbit defines plane of the ecliptic Ecliptic - apparent path of Sun through the heavens as seen from Earth Elliptical orbit, not circular 2.5 million km closer in January 2.5 million km more distant in July Earth receives about 6% more solar energy in January

7. Rotation and Earth’s axis Earth’s rotation axis inclined 23.5º as shown Seasons Determined by orientation of rotation axis Winter solstice, summer solstice Spring equinox, autumnal equinox Celestial equator Line on celestial sphere above Earth’s equator

10. Rotation: Does Earth rotate? Evidence from three observations Other celestial bodies are seen rotating Jupiter, Sun Foucault pendulum Pendulum oscillates in fixed direction Orientation changes as Earth rotates Coriolis effect Deflection of paths as Earth rotates beneath moving objects

11. Precession Slow wobble of Earth’s rotation axis Reaction of Earth to gravitation pull on its equatorial bulge by the Moon and Sun 26,000 years for one precession Changes direction of rotation axis on celestial sphere Polaris not always the North Star Position of equinoxes in the zodiac changes

13. Place and time Earth’s periodic motions provide a basis for determining place and time Rotation provides an axis of rotation useful in determining locations on the globe Rotation and revolution determine cycles which can be used for time standards

14. Identifying place Position on flat surface Intersection of two straight lines Won’t work for Earth’s curved surface Position on Earth Intersection of parallels and meridians defined with respect to rotation axis Parallels - latitude Meridians - longitude Special parallels climate related

15. Measuring time Time standards depend on measuring intervals between evenly spaced periodic events Astronomical examples: rotation of Earth on its axis, revolution of Earth around the Sun Basis for day, month, season and year Different ways to measure day, month and year

16. Daily time Earth-Sun motion Viewed from above North Pole Earth revolves counterclockwise about Sun and rotates in the same direction On Earth Sun rises in the east and sets in the west

17. Sun’s motion across the sky “Noon” Time at which the Sun is at the celestial meridian “Apparent local noon” at a particular longitude Apparent local solar time Given by position of Sun in the sky Can be measured with gnomon or sundial Not time shown on clocks Mean solar time Time averaged from apparent solar time Averaged over two effects Earth’s orbital speed changes throughout its elliptical orbit The equator is inclined to the ecliptic Mean solar day 24 hour average from mean solar time

18. A day Length of time required for Earth to rotate once on its axis Sidereal day Interval between two consecutive crossings of the celestial meridian by a particular star Little variation because of Earth’s constant rotational rate Apparent solar day Interval between two consecutive crossings of the celestial meridian by the Sun Revolution about Sun changes Earth-Sun orientation each day About 4 minutes longer than sidereal day 23 hours, 56 minutes and 4 seconds long

19. Standard time zones 360º of longitude divided into 24 15º zones Adjusted for local consistency Daylight saving time clocks set ahead in spring and back in fall for extra hours of sunlight during summer evenings International date line The 180º meridian Designated to correlate days with 24 hour time zones

20. Yearly time Tropical year Interval between two consecutive spring equinoxes 365.24220 mean solar days Sidereal year Interval between two consecutive alignments of the Earth and Sun relative to the stars About 20 minutes longer than a tropical year Adjustments to keep calendar year current with the seasons: Leap year, leap seconds, …

21. Monthly time Sidereal month Orbital time measured with respect to the stars 27 1/3 days Synodic month Time interval between two consecutive phases 29 1/2 days Includes effects of Earth revolving around the Sun

22. The Moon 380,000 km distant Characterized by craters, lunar highlands and Maria (ancient lava flows) No atmosphere Subject of 12 Apollo missions

23. Composition and features of the Moon 3 m of fine gray dust on the surface Accumulated from micrometeorite impacts Glass beads formed from melted material Rocks are basalts Formed from molten lava Light colored highland rock formed 4 billion years ago Dark colored Maria rocks range from 3.1 to 3.8 billion years old Internal structure 65-130 km of outer rock (thickest on far side) 900 km partly molten iron core

24. History of the Moon Stage 1 - origin stage Formed from material ejected from a collision of a large object with Earth Stage 2 - molten surface stage Molten surface 100 km deep 200 million years after formation Heating from solar system debris impacts Stage 3 - molten interior stage Accumulated heat from radioactive decay Began 3.8 million years ago; ended about 3.1 million years ago Stage 4 - cold and quiet stage 3.1 million years ago to present Surface scarred by micrometeorites and meteorites

25. The Earth-Moon system Moon/Earth mass ratio highest in solar system Diameter: 1/4 that of Earth Mass 1/81 that of Earth Large enough to affect Earth’s orbit Earth and Moon rotate about common center of mass

27. Phases of the Moon Result from changing relative positions of Earth, Sun and Moon New moon, first quarter, full moon, last quarter The same side always points toward Earth

28. Eclipses of the Sun and Moon Moon’s orbit inclined 5º from that of Earth Proper alignment of Earth, Moon and Sun needed Conical shadows have two parts Umbra: inner cone, complete shadow Penumbra: outer cone, partial shadow Solar eclipses Where tip of umbra touches Earth Annular eclipse: when umbra doesn’t reach Earth Lunar eclipses Moon engulfed by Earth’s umbra

33. Tides Result from different gravitational pulls on front and back of Earth Three factors Earth, Moon and Sun positions Elliptical orbit of Moon Spring tides when aligned; neap tides when Moon and Sun at 90º Greatest pull at perigee; less effect at apogee 48,000 km difference Size, shape and depth of water basin Ranges from 1/3 m in Gulf of Mexico to 15 m in Bay of Fundy