Chapter 2- Terrestrial Planet's and Earth's Moon

2.1 Earth Spins on an Axis

Constellations

• ancient cultures grouped the stars into apparent pattersn called constellations

• they built structures to study the sky for time keeping and navigational uses

  • solstice - longest day of the year

    • northern hemisphere - 20-22 of june

Celestial Sphere

• we imagine stars on a sphere

• Earth’s rotation (about it’s axis) and revolution (around the sun) us perceived as motion of the sphere around Earth

• North Celestial Pole (NCP) - projection of Earth’s north pole

• South Celestial Pole (SCP) - projection of Earth’s south pole

• Celestial Equator - projection of Earth’s equator to the sphere

• Ecliptic - path of the Sun, inclined 23.5 degrees to celestial equator

Earth’s Rotation

• when viewed form above the north pole, earth spins (rotates) counterclockwise on its axis

• form earth’s surface, every object seems to move across the sky once a day

• one rotation takes 24 hours

Astronomical Coordinate Systems: Celestial

• angles measured with respect to the celestial equator are used like latitude and longitude on the Earth

• Declination (latitude) - distance of an object north of south of the celestial equator (horizontal line): measured in degrees

• Right Ascension (Longitude) angular distance eastward along the celestial equator from the point where the ecliptic crosses the equator (vertical lines): measured in hours

Astronomical Coordinate Systems: Horizon

• the horizon system

  • unique to each location and time

  • not a global coordinate system

• zenith - point directly above you

• meridian - line form north to south through the zenith, cutting the sky in half

• horizon - the circle 90 degrees away form the zenith

• nadir - point directly below you

• altitude (like latitude) - angle from horizon to object

• azimuth - angle from north measured east along horizon to the object

Star Motion

• what best describes the motions of the stars you see when you are standing on the north pole?

  • circumpolar (making circles around a lone star)

• standing on the equator

  • all stars rise and set

View at the poles

• standing at one of Earth'‘s poles, you would see half the celestial sphere

• a celestial pole is directly overhead

• stars rotate in 24 hours counter around NCP

• no star rises or sets: all circumpolar

View at the Equator

• all stars rise and set

• - the celestial poles are on the northern and souther horizons

• observers can see the whole celestial sphere as it rotates

Between the Poles and The equator

• elsewhere on earth, one pole is above the horizon

• the altitude of that pole is the same as the latitude

• some stars are circumpolar (never rise or set)

• some stars rise and set

• some stars are never visible

Celestial Navigation

• stars are useful for navigation

• latitude is equal to the celestial’s poles altitude

• longitude can be determined the time at which the sun passes overhead (crosses the meridian)

• requires accurate timekeeping devices

Circumpolar Stars

• to determine whether a star is circumpolar, we need to know our latitude adn declination of the state

• a star is cicrumpolar if:

• |omega**declination)| > 90^o - |L|

  • Altitude of polaris is equal to our own latitiude

Practice

Star Coordination

• Deneb - omega = 45^o 16’ 49'“ RA = 20h 41m 26s

• Arcturus - omega=Dec 19^o 10’56”, RA = 14h 15m 40s

1. if you were in London (L=51), would Deneb be circumpolar? yes

2. what is the lowest latitude you could have where you would still see Arcturus as circumpolar?

declination*>90-L

90-declination <L

declination = 19+10’/60 + 56”/3600 = 19.182

L>70.818

3. You measure the altitude of Polaris to be declination=27.5, are either Deneb or Arcturus circumpolar to your location?

declination >90-L

declination >90-27.5

declination > 62.5

• neither are circumpolar because the declination is lower on both

Earth’s Rotation

Why we dont receive our motion

• attributed motion to the celestial sphere

• Copernicus hupothesized that the earth was orbiting the sun in 1515

• in 1605, kepler suggested that the earth orbiting around the sun

• one of the key reasons people did not believe this theory was because they could not feel the motion

Effects of Earths rotation

• objects on Earth’s surface move in a cricle once per day around earth’s rotation axis

• the closer an object is to the poles, the smaller the circle

• earths surface moves faster at the equator than at the latitudes

• the Coriolis effect is the curving of objects as a result of the difference of Earth’s rotation speeds at different latitudes

• projectiles will appear to curve to the right in the Northern Hemisphere and to the left in the Southern

• initial velocity at the equator is faster than the rest

  • velocity at the bottom latitudes are slower and if you threw an object, the object moves slower and curves west since the earth rotates east

Practice

1. If Earth completes one rotation in approximately 24 hours, how many degrees does it rotate per second?

   .004167

   24hrs in seconds

   360 degrees in a circle

   360/86,400 seconds = .004167

2. If the radius of the Earth is 6.376×10^6m, what is the rotational velocity on the Earth’s surface (at the equator)?

   463.7m/s

   t=24h=86,400s

   d=2pir = 4×10^7 m

   v=4..00×10^7/86,400s = 463.7m/s

3. If an observer is at a latitude of 25N, what is the altitude of the North Celestial Pole above the horizon?

   25N

Earth orbits the sun

• the suns motion on the ecliptic reflects earth’s orbit around the sun

• earth revolves aorund the sun once a year

• the zodiac is the group of background constellations that line up with the sun throughout the year

• the constellations that align along the ecliptic

Tilt and Earth’s Axis

• the sun is above hte horizon longer during the summer than winter

• sunlight is more concentrated and more direct in summer

• the southern hemisphere has alternate seasons to the northern hemisphere

QUIZ 2

season - flux/concentration of the sun not the distance

Special Days of the Year

• summer solstice - sun farthest north, longet day of the year

• autumnal equinox - sun on the celestial equator, moving southward

• winter solstice - sun farthest south, shortest day of the year

• vernal equinox - sun on the celestial equator, moving northward

• equinox - equal day, equal nights

Special Latitudes on Earth

• artic/antarctic circles - 66.5 degrees N/S, where the sun is circumpolar on the first day of summer

• tropic of cancer/capricorn - the latitudes where the sun is directly overhead on the equinoxes

Origin of Moon Phases

• the moon shines because of reflected sunlight

• Phases of the moon

  

• There is no dark side of the moon

  • the moon rotates once on its axis for each orbit around earth and so keeps the same face toward earth at all times (tidally locked)

  • synchronous orbit - far side of the moon

  • when the moon rises and sets

    • depending on the time of day

    • full moon sets at sunrise

Eclipses

• lunar - earth is between sun and moon

• solar - moon is between sun and earth

Solar eclipse

• Total eclipse

  • umbrea - small shadow

• partial eclipse

  • penumbra - smaller shadow

• Annular

  • when moon is not completely covering the sun; the moon is further away from the earth at its furthest point of its orbit

  • antumbra - see the annular eclipse if the moon is farther away

Lunar Eclipses

• the moon turns red during a lunar eclipse. Why?

  • red light from the sun is bent through Earth’s atmosphere and other colors of light are scattered or absorbed

If the moon orbits the Earth every month, why don’t we see solar and lunar eclipses every month?

• the moon’s orbit is tilted by 5.2 with respect to Earth’s orbit around the sun

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