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Astronomy (100)
Lecture

Solar System


Department
Astronomy
Course Code
ASTB03H3
Professor
Eric Shaver

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Astronomy โ€“ February 7
โžขSize of Solar System
๎€†Cassini & Richer
๎€†Knowing the distance between two places will give you the distance to the third point (the
planet)
๎€†One distance to Mars is known, other distances
๎€†Radius of Earth's orbit = Astronomical Unit = 150 million km
๎€†Astronomical Unit is the measure used
๎€†If you know how wide a planet looks you can use that information to find out it's distance
๎€†
โžขMeasurement of Speed of Light (1600s)
๎€†Observations of Jupiter's moons
๎€†Jupiter is constantly eclipsing the moons and vice versa
๎€†When people watch Jupiter over time, the moons are supposed to regularly go behind jupiter
and reappear โ€“ but over the course of the year, the eclipses occur earlier than expected and
vice versa
๎€†What Roemer realized was โ€“ we are moving in an orbit on earth โ€“ as we get further away
from jupiter โ€“ the light has to travel further away from the event so it will take longer and
longer to see it?
๎€†So changes are a result of light having to travel further to reach Earth as Earth moves in
orbit
๎€†Based on this time delay he was able to calculate speed of light = 300 000 km/s
๎€†So it wasn't intended to find the speed of light
๎€†When you look at the moon, you look at how it was a second ago, when you look at the sun
you look at how it was 8 minutes ago
โžขReflectors
๎€†A kind of telescope
๎€†One of the first was developed by Isaac Newton (1668)
๎€†Instead of using a lens to concentrate the light, you can use a concave mirror to concentrate
the light
๎€†If you make the mirror bigger, you can gather more light, and the more you can magnify
๎€†These were popular towards end of 1600 because they were fairly small telescopes โ€“ lighter
weight and high magnification
๎€†Putting it on a stand to make to easier to use
๎€†Equatorial mount
๎€„Mounts (stands) that are tilted on an angle
๎€„Rather than having telescope swing two ways, you have it swing one way โ€“ which
makes it easier to track things
๎€„Allows for stability โ€“ improves measurements
๎€†
โžขThe Modern Solar System
๎€†Moons discovered around Saturn (late 1600s)
๎€†William Herschel discoveres Uranus (1781)
๎€„He observed a dot in the sky
๎€„Uranus takes 85 years to go around the sun
๎€„This was an accidental discovery
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๎€†Excellent reflecting telescope maker
๎€†The Asteroids
๎€„1766 โ€“ German mathematician Titius came up with a law: Titius Law
๎€„Law: radii of planets' orbits (up to Saturn) described by a RANDOM numerical
sequence
๎€„Pattern that matched up with actual orbits
๎€„0,3,6,12 โ€“ so double the numbers after 3
๎€„add 4 to each number
๎€„and then divide each number by 10
๎€†Finding Asteroids?
๎€„Von Zach calculates orbital path for hypothetical object at 2.8 AU, initiates search
๎€„Piazzi also notes movement
๎€„Notifies Bode in Germany in March but at this point object no longer visible
๎€„Then Gauss comes in โ€“ develops a way of taking 3 observations and finding
observations
โ€ขWhen you do this you call it an โ€œephemerisโ€ - tells you when it will become visible
again
๎€„Von Zach uses ephemeris to re-locate object (1801)
๎€„Piazzi names object (planet) โ€œCeresโ€
๎€„Location at 2.77 AU reinforces Titius-Bode Law
๎€„But when Herschel looked at Ceres through telescope he noticed it was really small โ€“
only 260 km (now we know its 1000 km across)
๎€„So because of it's size โ€“ it was called โ€œasteroidโ€ - which means โ€œstar-likeโ€- much smaller
than planet
๎€„Another similar object is discovered โ€“ Pallas
๎€„Juno & Vesta were also discovered โ€“ all around this 2.8 AU
๎€„They are however in a location where there should be a planet
๎€„This collection of objects is called โ€œasteroid beltโ€ - large pieces of rock (10-100 km
diameter) orbiting Sun between 2.2 and 3.3 AU โ€“ Ceres is the largest so far
๎€„Left over material from formation of solar system โ€“ these may be materials that
probably never turned into planets
๎€„Asteroid belt is there because Mars and Jupiter (gravitational forces) prevent them from
forming into planets โ€“ they are pulling on it from both sides as they orbit the sun
๎€„Two Asteroids: Ida & Gaspra
โ€ขIda
โ€ขhas its own moon: Dactyl
โ€ขnot round so orbit is not stable
โ€ขis similar to a planet
โ€ขGaspra
๎€†Celestial Mechanics
๎€„How pple describe orbits โ€“ orbits are elipses
๎€„You can use Newton's laws of motion to predict where something will be โ€“ where is it at
a particular time
๎€„You also need to know it's speed
๎€„You get 6 numbers from 3 observations (Right ascension and declination)
๎€„Transformed these into another 6 orbital elements to feed it into Newtons laws
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