Download Intro to Astronomy Notes

Intro to Astronomy
Looking at Our Space Neighborhood
Astronomy: The Original Science
• Ancient cultures used the movement of stars,
planets and the moon to mark time
• Astronomy: the study of the universe
• This led to the development of the first
calendar
– Year: Earth orbits the sun one time (365.25 days)
– Month: about one orbit of the moon around Earth
(29.25 days)
– Day: one rotation of Earth on its axis
Who’s Who of Early Astronomy
• Ptolemy – 140 A.D.
– Greek astronomer
– Thought Earth was at the center of the
universe and other planets & the sun
revolved around it
– Incorrect, but predicted motion of planets
better than anyone else had
• Copernicus – 1543 A.D.
– Polish astronomer
– Said the Sun was at the center of the
universe and all planets orbit the sun
– Very controversial! Not quickly accepted
Who’s Who of Early Astronomy
• Tycho Brahe – late 1500’s
– Danish astronomer
– Favored Ptolemy’s Earth-centered theory
– Used tools to make detailed astronomical
observations that helped future astronomers
• Johannes Kepler – early 1600’s
– Assistant to Tycho Brahe
– Used Brahe’s data to show that all planets revolve around
the sun in elliptical orbits
– Gave three laws of planetary motion that are still used
today
Who’s Who of Early Astronomy
• Galileo Galilei – 1609
– One of the first to use a telescope to see space
– Discovered craters and mountains on the
Moon and four of Jupiter’s moons
– Showed that planets are not stars, but more
similar to Earth
• Sir Isaac Newton – 1687
– Showed that all objects in the universe attract
each other by gravitational force (depends on
mass & distance)
– Explained why all planets orbit largest object –
the Sun
– Newton’s Cannon – why object orbit Earth
Modern Astronomy
• Edwin Hubble – 1924
– Astronomers thought our galaxy (Milky Way)
included every object in space
– Edwin proved the existence of other galaxies
• New technology continues to improve our
understanding of space
• New galaxies, planets and moons are
continually being discovered
Earth’s Motion
Early Astronomers’ First Focus
How Does Earth’s Motion Affect Time?
• Axis – the imaginary line that goes
from the North to South Pole
• Rotation – Earth’s spin on its axis (think of a top)
– Earth’s rotation causes day and night
– One rotation takes about 24 hours – 1 day!
How Does Earth’s Motion Affect Time?
• Revolution – Earth’s
movement around the Sun
– One revolution is one year
– Earth’s path around the Sun
is called its orbit
– Earth’s orbit is an oval
(ellipse), not a circle
• 365.25 days (rotations) per year (revolution)
⁻ That’s why we have leap year every 4 years!
How Does Earth’s Motion Affect
Seasons?
• 4 Seasons: summer, winter, spring, fall
• Seasons in Northern Hemisphere happen at
different times than Southern Hemisphere
• Earth has seasons because its axis is tilted 23.5°
as it moves around the sun
The Moon
Our Closest Neighbor in the
Neighborhood
Structure & Origin of the Moon
• 3,476 km in diameter (about distance across
the US) – ¼ the diameter of Earth
• 1/8 the mass of Earth (less dense than Earth)
• Collision Theory – suggests
that an object about the size
of Mars hit Earth and the
scattered material formed the
moon 4.5 billion years ago
Features of the Moon
• Galileo made the first detailed
observations of moon’s features in
1609 with a telescope
• Craters – pits that cover the surface,
caused by impacts of meteoroids
• Highlands – mountainous structures
that cast large shadows
• Marias –flat, dark areas that Galileo
thought were oceans (maria = seas
in Latin)
– Low, dry areas that were flooded with
molten rock billions of years ago
Our Closest Neighbor
• 384,400 km from Earth (about 30 Earth’s lined
up)
• Revolves around Earth as Earth revolves
around the Sun
• The positions of the moon, Earth and Sun
cause the phases of the moon, eclipses and
tides on Earth
Motions of the Moon
• Has its own axis – tilted 5°
• Rotation (day): 27.3 Earth days
• Revolution (year): 27.3 Earth days
– So one year and one day on the
moon are the same length!
• Earth & moon have synchronous
rotation
– Near side of the moon always faces
Earth
– You never see the far side of the
moon from Earth
Phases of the Moon
• The moon does not produce light – it reflects light
from the sun
• The shapes of the moon you see are called phases
• Half of the moon is always lit, but sometimes Earth
gets in the way
• The phase of the moon you see depends on how
much of the sunlit side of the moon faces Earth
Cycle of Moon Phases
• Start: New Moon
– The lit side faces away from Earth so no moon is seen
• Phases in Order:
1. New Moon
2. Waxing Crescent
3. First Quarter
4. Waxing Gibbous
5. Full Moon
6. Waning Gibbous
7. Third Quarter
8. Waning Crescent
…then back to New Moon!
• One complete cycle takes about 28 days (1 month)
Eclipses
• Eclipse: when moon’s shadow hits Earth or
Earth’s shadow hits the moon
• Two Types: (named for what is being blocked)
– Solar
– Lunar
Solar Eclipses
• When a new moon passes between Earth and the Sun,
blocking light from reaching Earth
• Total Solar Eclipses – where light is completely blocked by the
moon
– Only people in the darkest part of moon’s shadow (umbra) experience
this
• Partial Solar Eclipse – where part of the sun is visible; more
common
– People in the lighter part of moon’s shadow (penumbra) experience
this
Lunar Eclipses
• Occurs at a full moon when Earth is directly between the
moon and the sun, blocking light from reaching the moon
• Earth’s shadow also has an umbra (darkness) and penumbra
(partial darkness)
• Total Lunar Eclipse - where light is completely blocked by the
Earth
– Can be seen anywhere on Earth where the moon is visible
• Partial Lunar Eclipse - where the Sun, Earth and moon are not
perfectly in line
– moon passes partly into Earth’s umbra
Tides
• Tide – rise and fall of water every 12.5 hours
• Caused by gravity pulling the moon and Earth
together
• Occur because of differences in how much the
moon pulls on different parts of Earth (and the
water on it)
High & Low Tide
• Water on Earth closer to the
moon is pulled, forming high
tide
• Water on the other side of
the planet has much weaker
pull and resists due to
inertia, forming another high
tide
• Low tides occur between the
two high tides
Spring & Neap Tides
• Spring Tide: Occur when the
moon, sun and Earth are in line
– Gravity of sun and moon
combine to produce greatest
difference in high/low tide
• Neap Tide: when the earth and
sun are at right angles to earth
and the moon
– Produces a tide with the least
difference between high/low tide
– It’s like a tug of war between the
sun and the moon!