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Lecture

# Solar System

Department
Astronomy
Course Code
ASTB03H3
Professor
Eric Shaver

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Astronomy β February 7
β’Size of Solar System
ξCassini & Richer
ξKnowing the distance between two places will give you the distance to the third point (the
planet)
ξOne distance to Mars is known, other distances
ξRadius of Earth's orbit = Astronomical Unit = 150 million km
ξAstronomical Unit is the measure used
ξIf you know how wide a planet looks you can use that information to find out it's distance
ξ
β’Measurement of Speed of Light (1600s)
ξObservations of Jupiter's moons
ξJupiter is constantly eclipsing the moons and vice versa
ξWhen people watch Jupiter over time, the moons are supposed to regularly go behind jupiter
and reappear β but over the course of the year, the eclipses occur earlier than expected and
vice versa
ξWhat Roemer realized was β we are moving in an orbit on earth β as we get further away
from jupiter β the light has to travel further away from the event so it will take longer and
longer to see it?
ξSo changes are a result of light having to travel further to reach Earth as Earth moves in
orbit
ξBased on this time delay he was able to calculate speed of light = 300 000 km/s
ξSo it wasn't intended to find the speed of light
ξ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
β’Reflectors
ξA kind of telescope
ξOne of the first was developed by Isaac Newton (1668)
ξInstead of using a lens to concentrate the light, you can use a concave mirror to concentrate
the light
ξIf you make the mirror bigger, you can gather more light, and the more you can magnify
ξThese were popular towards end of 1600 because they were fairly small telescopes β lighter
weight and high magnification
ξPutting it on a stand to make to easier to use
ξEquatorial mount
ξMounts (stands) that are tilted on an angle
ξRather than having telescope swing two ways, you have it swing one way β which
makes it easier to track things
ξAllows for stability β improves measurements
ξ
β’The Modern Solar System
ξMoons discovered around Saturn (late 1600s)
ξWilliam Herschel discoveres Uranus (1781)
ξHe observed a dot in the sky
ξUranus takes 85 years to go around the sun
ξThis was an accidental discovery

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ξExcellent reflecting telescope maker
ξThe Asteroids
ξ1766 β German mathematician Titius came up with a law: Titius Law
ξLaw: radii of planets' orbits (up to Saturn) described by a RANDOM numerical
sequence
ξPattern that matched up with actual orbits
ξ0,3,6,12 β so double the numbers after 3
ξand then divide each number by 10
ξFinding Asteroids?
ξVon Zach calculates orbital path for hypothetical object at 2.8 AU, initiates search
ξPiazzi also notes movement
ξNotifies Bode in Germany in March but at this point object no longer visible
ξThen Gauss comes in β develops a way of taking 3 observations and finding
observations
β’When you do this you call it an βephemerisβ - tells you when it will become visible
again
ξVon Zach uses ephemeris to re-locate object (1801)
ξPiazzi names object (planet) βCeresβ
ξLocation at 2.77 AU reinforces Titius-Bode Law
ξBut when Herschel looked at Ceres through telescope he noticed it was really small β
only 260 km (now we know its 1000 km across)
ξSo because of it's size β it was called βasteroidβ - which means βstar-likeβ- much smaller
than planet
ξAnother similar object is discovered β Pallas
ξJuno & Vesta were also discovered β all around this 2.8 AU
ξThey are however in a location where there should be a planet
ξThis collection of objects is called βasteroid beltβ - large pieces of rock (10-100 km
diameter) orbiting Sun between 2.2 and 3.3 AU β Ceres is the largest so far
ξLeft over material from formation of solar system β these may be materials that
probably never turned into planets
ξAsteroid belt is there because Mars and Jupiter (gravitational forces) prevent them from
forming into planets β they are pulling on it from both sides as they orbit the sun
ξTwo Asteroids: Ida & Gaspra
β’Ida
β’has its own moon: Dactyl
β’not round so orbit is not stable
β’is similar to a planet
β’Gaspra
ξCelestial Mechanics
ξHow pple describe orbits β orbits are elipses
ξYou can use Newton's laws of motion to predict where something will be β where is it at
a particular time
ξYou also need to know it's speed
ξYou get 6 numbers from 3 observations (Right ascension and declination)
ξTransformed these into another 6 orbital elements to feed it into Newtons laws