AS101 Study Guide - Midterm Guide: Net Force, Northern Hemisphere, Declination

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30 Jan 2013
What is our place in the Cosmos?
The Cosmos
Mostly vacuum (empty space)
Everything we can possibly see is a tiny fraction
Why does it seem like there are so many objects in space
The Sun is Our Star
Massive ball of glowing gas that generates energy through nuclear fusion
100x as wide as the Earth
Source of almost all energy on Earth
Planets are Less Massive than Stars
Non-luminous and spherical
In orbit around a star
“cleared the neighbourhood” of other objects
Over 600 planets confirmed to be orbiting other stars
Thousands of “candidate” exoplanets, observed by the Kepler Space Telescope
Some Planets have Satellites
An object in orbit around a planet
Natural satellites are also known as “moons”
Other Objects in the Solar System
Asteroid: a small, rocky, object orbiting a star
Comet: a small, icy object that orbits the sun
Galaxies: “Cities” of the Cosmos
A large system of stars, dark matter, gas, and dust, all bound together by their combined gravity
Clouds of gas and/or “dust”
Raw materials for new stars from previous generations
Star Clusters
Open clusters: 1000s of stars
Globular Clusters: 100000s of stars
Our Sun probably formed in an open star cluster but has since “moved out
Galaxy Groups and Clusters
A group of galaxies (a few dozen up to thousands) all held together gravitationally
Many clusters form a supercluster which make up the cosmic web
Models in Science
In general, a model is something that represents reality
A scientific model is a hypothesis that describes reality and has withstood observational or
experimental tests
We can also develop conceptual models to help us think about how nature works
Scale Models: miniature or shrunk down versions
o How to the sizes of the planets compare to the distances between them?
o What if we shrunk the solar system by a factor of about 60 billion?
Sun -> radius=12mm
Earth -> radius=0.1mm ; at this scale Earth is 3m away
Jupiter -> radius=1mm ; at this scale Jupiter is 17m away
Neptune -> radius=0.4mm ; at this scale Neptune is 100m away
o The sizes of the planets are tiny compared to the distances between them
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The Astronomical Unit
We can specify distances in the solar system by comparing them to the average Earth-Sun
o 1 AU= 150, 000, 000 km OR 150, 000,000,000 m
A planet twice as far from the Sun would be 2 AU away
Representing Large Numbers
The Earth is on average 150,000,000,000m from the Sun
In Scientific Notation we would write this as a coefficient between one and ten, multiplied by
the appropriate power of ten
Hint: just count the number of places the decimal would move…
o 150, 000, 000, 000 m= 1.5 x 1011 m
Doing Math with Exponents
To multiply two (or more) numbers written in scientific notation, just
o Multiply the coefficients (if there are any)
o Add the exponents
How far away are the Stars?
The nearest star system is Alpha Centauri: 4.1 x 1013 km
Using our scale from our model solar system: 60 billion actual km=1 scale km…puts Alpha
Centauri at 683 km away, in NYC
Light-Travel and Look-Back Time
BUT light has a finite speed: 300,000 km/s
In other words, this star is so far away that we are seeing it as it looked in the past
The further an object appears to be in space, the further we are looking back in time
A light-year, is the distance light travels in one year, about 1013 km (Ten trillion km)
So Alpha Centauri is about 4 light years away
Some Distances in the Universe
The Sun: about 8 light minutes
Alpha Centauri: 4 light-years
Andromeda Galaxy: 2 million light-years
Observable Universe: 13.7 billion light-years
Putting it in Perspective
What if the entire age of the Universe were one calendar year?
o All of recorded history happens on the last day of the year, 30 seconds before midnight
o The Egyptian pyramids were built about 11 seconds ago
o Copernicus and others convinced humanity that the Earth orbits the Sun about 1 second
o You were born about 0.04 seconds ago (assuming your age is 18)
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How the Earth is Moving
Two basic motions:
o Revolves around (“orbits”) the Sun
o Rotates on its axis
Which motion is responsible for rising and setting of the Sun?
o Rotation is responsible for the rising and setting of the Sun
The Rotating Earth and You!
The Sun… rises in the east; and sets in the west
To a moving observer, stationary objects appear to move in the opposite direction as the
observer’s motion
The Earth rotates from west to east (rotates counterclockwise when viewed from the North
Celestial Sphere: A Conceptual Model of the Sky
An imaginary sphere of very large radius surrounding the Earth
Objects appear to be attached to the rotating celestial sphere
Are the stars really moving?
o No the planets are moving making it seem like the stars are moving
We can see exactly one-half of the entire sky at any given time, from horizon up to the zenith
(directly overhead)
The North and South Celestial Poles are directly above the Earth’s North and South poles
The Meridian is the line passing through the North and South points on the horizon, and the
Starry Night Simulation
In the Northern Hemisphere, stars that never appear to set are located near the ___________.
These are called circumpolar stars because they appear to circle the pole
The North Celestial Pole is near the star Polaris
Stars reach their highest points when they cross the ___________
Terrestrial Coordinates
We specify locations on Earth using latitude and longitude
Longitude: in degrees East or West of the Prime Meridian
Latitude: in degrees North of South of the Equator
On Earth, intersecting lines of longitude and latitude form a grid
Imagine this grid projected outward onto the celestial sphere
Celestial Coordinates
We specify these positions in the sky using Right Ascension (RA) and Declination (Dec)
Right Ascension: in hours East of the Vernal Equinox (from 0h to 24h)
Declination: in degrees North (+) or South (-) or the Celestial Equator (from +90 0 to -900)
Latitude Affects What You Can See
Example: you are sitting at the North Pole
Recall: the NCP is directly above the North Pole
o So at the North Pole the NCP is at the Zenith and has an altitude of 900N
o This is the same as the latitude of the observer
Example: you are sitting on the equator
o The altitude of the NCP is equal to your latitude, so it is on the horizon (what about the
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