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Chapter 2… The Sky. The Night Sky. People have studied the sky for thousands of years. Early humans organized the night sky by naming groups of stars called constellations .

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the night sky
The Night Sky
  • People have studied the sky for thousands of years. Early humans organized the night sky by naming groups of stars called constellations.
  • Constellations were used to celebrate a culture’s important mythical characters. So different cultures had different constellations.
  • The ancient constellations are limited to the ones seen from the northern latitudes.
  • 48 are still in use today
  • 40 modern constellations were added in 1928
  • A constellation now represents an area of the sky rather than one particular grouping of stars.
  • An asterism is a grouping of stars that forms a pattern but is not a constellation (region of the sky).
  • An asterism may include stars from more than one constellation.
  • It may also be a nickname for a constellation.
  • Examples
    • The Big Dipper (in Ursa major)
    • The Little Dipper (in Ursa minor)
  • Constellation names come from Greek versions that have been translated into Latin.
  • Star names come from ancient Arabic.
  • Johann Bayer (1603) published a sky atlas that named stars by constellation and brightness.
  • A magnitude scale is used to measure the brightness of stars.
  • There were originally 6 classes of stars.
  • 1st class stars were the brightest… 6th class stars the dimmest
  • Now that we can see stars with more advanced telescopes, the magnitude scale includes a wide range of numbers.

This scale is known as apparent visual magnitude because the numbers describe how the stars look to a human observer on Earth.

  • Distance is not factored into the scale.
magnitude and intensity
Magnitude and Intensity
  • Brightness is subjective.
  • Astronomers use the term “intensity” instead.
  • Intensity measures the amount of light energy that hits 1 square meter in 1 second.
  • Modern astronomers combined the ancient magnitude scale (6 classes)with the idea of intensity.
  • Each magnitude increases intensity by a ratio of 2.5
the math
The Math…
  • Stars that differ by 1 magnitude have an intensity ratio of 2.5
  • Stars that differ by 2 magnitudes have an intensity ratio of 2.5 x 2.5 or 6.3
  • The intensity ration equals 2.512 raised to the power of the magnitude difference

IA / IB = (2.512) (mB – mA)

mA – mB = 2.512 Log (IB / IA)

  • Pg. 33 Review Questions #1-4
  • Pg. 34 Problems #1-7
the sky its motion
The Sky & Its Motion
  • Stars are scattered throughout space at different distances from Earth.
  • We use a scientific model known as the celestial sphere when studying the sky.
the celestial sphere
The Celestial Sphere
  • The sky appears to rotate westward around a stationary Earth. The Earth is really rotating east… making day and night.
  • Distances across the sky are measured in angles (degrees, minutes, & seconds).
  • What you see in the night sky depends on where you are on Earth.
the celestial sphere1
The Celestial Sphere
  • Easy to picture the Earth in the center
    • Latitude, longitude are “projected” onto the Celestial Sphere
The North Celestial Pole is directly above Earth’s North Geographic Pole
  • The South Celestial Pole is directly above Earth’s South Geographic Pole
  • The Celestial Equator is directly above Earth’s Equator
Star positions are calculated on the Celestial Sphere
  • “Longitude” is measured as Right Ascension, or the amount of time taken from the Vernal Equinox
  • “Latitude” is measured as Declination, a positive (above) or negative (below) degree from the Celestial Equator (CEq)
  • The Earth’s rotation and the forces acting on it through the Sun and Moon cause the North Pole to “wobble”
  • A “loop” of this wobble takes some 26,000 years or so
  • The “North Star” is actually whichever star is closest to the NCP
  • We live in a time with a moderately bright star close to the NCP
  • Thuban, Vega, etc.
apparent motion of the sun
Apparent Motion of the Sun
  • Rotation
  • Spinning of earth on its axis
  • Causes day and night
  • Takes about 24 hours
  • Revolution
  • Movement of earth around the sun
  • A year
  • Takes about 365 days
what do you know
What Do You Know?
  • Stars in solar system?
  • Noon here, time on opposite side of Earth?
  • How does the Sun appear to move in the sky?
annual motion of the sun
Annual Motion of the Sun
  • If the Sun were less bright, we would be able to see the constellations during the day.
  • We would notice that the Sun appears to move to the East as the year goes on because of Earth’s revolution.
  • This almost circular orbit of Earth is projected onto the Celestial Sphere as the ecliptic.
  • Remember we think of Earth as still as the celestial sphere moves around us.
the ecliptic
The Ecliptic

1. Apparent path of the Sun in its yearly motion around the sky

2. Projection of Earth’s orbit on the sky

3. The plane of Earth’s orbit

what causes the seasons
What Causes the Seasons?
  • Earth is tilted 23.5 degrees on its axis.
  • This tilt causes the amount of solar energy that the two hemispheres receive to vary during the year.
  • Distance from the Sun does NOT cause the seasons.
seasons continued
Seasons Continued…
  • Solstice
  • Summer… longest day
    • Sun’s rays strike Tropic of Cancer
  • Winter… shortest day
    • Sun’s rays strike Tropic of Capricorn
  • Equinox
  • Spring or Vernal
  • Fall or Autumnal
  • Sun’s rays strike equator
other vocabulary
Other Vocabulary
  • Perihelion
    • Earth closest to Sun
    • Around January 4
  • Aphelion
    • Earth farthest from Sun
    • Around July 4
motion of the planets
Motion of the Planets
  • All of the planets follow a counterclockwise circular orbit around the Sun.
  • Most can be seen by the naked eye as they reflect sunlight off their surface… Uranus, Neptune, & Pluto are too faint.
  • They can be seen near the ecliptic because they are in the same plane as Earth.
  • Planet means “wanderer”
motion of planets continued
Motion of Planets Continued…
  • Venus and Mercury are inside Earth’s orbit so they are always near the Sun in the sky.
  • Evening Star – planet that is visible just at sunset in the evening sky (not really a star)
  • Morning Star – planet that is visible just before sunrise (not a star)