1. The Universe on a Large Scale
The nearest galaxy to the Milky Way is 75,000 light years away (controversial)
Toward the constellation Sagittarius
Magellanic Clouds
Two small galaxies that appear to be clouds to the naked eye
Nearest large galaxy is Andromeda Galaxy, M31
A spiral galaxy similar to the Milky Way
2 million light years away
Part of a cluster of 40 galaxies called the Local Group
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2. Farther Away Farther away are quasars
At a distance of 15 million light years we find other small galaxy groups
At 50 million light years there is the Virgo Cluster
Many thousands of galaxies
Our Local Group and the Virgo Cluster are part of supercluster that has a diameter of 60 million light years
Farther away are quasars
Energetic centers of galaxies
Black holes?
Beyond the quasars is the cosmic microwave radiation left over from the Big Bang
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3. The Universe of the Very Small
Most of the universe is empty
1 atom/cm3 in our galaxy
1 atom/m3 between galaxies
Air has about 1019 atoms/cm3
Even atoms are mostly empty
Most of the mass of an atom is in the atomic nucleus
The electrons around the nucleus are at a distance 10,000 times the size of the nucleus
There are 92 naturally occurring elements (atoms)
Evidence has been found for man-made elements up to 118
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4. Composition of the Universe
The universe is roughly 75% hydrogen and 25% helium (by weight)
The remaining 90 elements are relatively rare
Carbon, oxygen, calcium, etc.
Hydrogen and helium come from the Big Bang
The remaining elements were “cooked” in the centers of stars
These elements were ejected when stars used up their hydrogen fuel and exploded
Supernova
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5. The Celestial Sphere
When we look up it seems that the Earth is the center of the universe
Geocentric
The point above your head is called the zenith
Where the sky-dome intersects the ground is the horizon
The sky seems to rotate around us
Celestial sphere
We know that the Earth’s rotation on its axis every 24 hours causes this illusion but the concept of the celestial sphere is still useful
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6. Celestial Poles
The points where the axis of the Earth intersects the celestial sphere are called the north and south celestial poles
The celestial equator lies halfway between the two poles
The circumpolar zone is always above the horizon, day and night
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7. Rising and Setting of the Sun
The sun does more than just rise and set
It changes position with respect to the background stars
Approximately 1 degree per day to the east
The sun moves through the celestial sphere along the ecliptic
The ecliptic is not aligned with the equator of the Earth because the Earth’s axis is tilted 23 degrees with respect to the plane of its orbit around the sun
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8. Tilt of Earth’s Axis
The Earth rotates on an axis tilted 23 degrees from the vertical
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9. Planets and Stars
Not only does the Sun move but the stars, the moon, and the planets move as well
The moon and the planets move with respect to the stars on the celestial sphere
The moon and the planets move on paths close to the ecliptic as well as the sun
The Sun, the moon, and the planets are all found in a 18 degree wide belt in the sky called the zodiac
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10. Constellations The stars form a fixed background
Distinctive patterns of stars have been recognized throughout the ages
These patterns were usually embellished and given names and drawings
These constellations are not necessarily nearby stars but merely appear to be related
Today the term constellation refers to 88 sectors in the sky
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11. Constellations in the Sky
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12. Galactic Equator
The galactic equator is defined by the Milky Way galaxy
The ecliptic and zodiac are not aligned with the Galactic equator nor with the celestial equator
In fact, the Galactic equator is very different from the ecliptic and the celestial equator
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13. Locating Objects on a Skymap
Ecliptic
Locating Objects on a Skymap
Galactic Equator
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14. Locating the Celestial North Pole
The Celestial North Pole is currently located near the star Polaris (aptly named!)
Locate the constellation Ursa Major, the Big Dipper
Locate the outer two stars of the dipper
Those 2 stars point at Polaris
The distance to Polaris is about 5 times the spacing between the two stars
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15. Early Greek and Roman Cosmology
Cosmology is the concept of the basic structure and origin of the cosmos
The Greeks and Romans knew
The Earth was round
The Moon was a sphere
The Sun was farther away than the Moon
The Greeks and Romans thought that the Earth was the center of the universe and all heavenly objects circled the Earth
Geocentric
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16. Measurements of the Earth
The Greeks measured the size of the Earth
Eratosthenes noticed that on the first day of summer, sunlight struck the bottom of a vertical well in Syene at noon
At the same time in Alexandria, the Sun was not directly overhead but made an angle of 7 degrees 360 degrees divided by 7 degrees is about 50
50 times the distance between Syene and Alexandria would be the circumference of the Earth
50 times 5000 stadia is 250,000 stadia
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17. Hipparchus Roman astronomer, 150 BC Compiled detailed star catalog
Invented the system of magnitudes to describe the brightness of stars
1 is the brightest, 5 is the dimmest, with each magnitude about a factor 2 dimmer
In modern astronomy, each magnitude varies by a factor 2.512
By comparing with previous observations, Hipparchus showed that the Earth’s north pole changes
Precession
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18. Ptolemy Roman astronomer around 140 AD
Most important contribution was a geometric model of the solar system capable of predicting the positions of planets
This model lasted more than 1000 years
One puzzling observation that he explained was retrograde motion of the planets
Usually the planets move east light the moon and sun but occasionally stop and move westward for some time
He invented a complicated mathematical model describing the motions of the planets with superposition of circles
Epicycles
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19. Epicycles
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