others gaseous nebulae
a)
Efforts of William and John Herschel, Charles Messier, J.L.E. Dreyer made this catalog possible
142)
Cepheid Variable – is a star that belongs to one of two classes of yellow supergiant pulsating stars
a)
Most important ling to galaxian distances
b)
Nearest is the Clouds of Magellan are only about 150,000 to 200,000 light years away from the sun
143)
Light years – the distances that separate galaxies are measured in hundreds to millions of Light Years
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a)
Less than 20 known galaxies within 2.5 million light years (LY) away
b)
Many thousands within 50 million light years (LY) away
144)
Johannes Kepler – astronomer and mathematician
a)
Studied the theory of planetary motion
b)
Developed three laws of planetary motion
c)
Most important contribution
in Commentaries on the Motions of Mars
c.i) Kepler's First Law states that each planet moves about the sun in an orbit that is an Ellipse with the
sun at one focus of the ellipse.
(c.i.1)
You don't need to really understand much of the geometry of ellipsesyou just need to
know that an ellipse contains two points called foci (singular: focus).
(c.i.2)
According to Kepler's first law, the path that the planet follows will be an ellipse (not a
circle as assumed by Copernicus!), and the sun will be located at one of the foci of that elliptical
orbit:
c.ii)
Kepler’s Second Law states that the straight line joining a planet and the sun sweeps out equal
areas in space in equal intervals of Time.
(c.ii.1)
This one can be difficult to understand without an illustration; take a look at the graphic
below. The point of this law is that when a planet is closer to the sun, it moves more quickly.
(c.ii.2)
Because planetary orbits are elliptical, the distance between a given planet and the sun
varies.
(c.ii.3)
When a planet is nearest to the sun (this point is called perihelion) it is at its fastest speed;
when it's at the furthest point of its orbit (aphelion), it is slowest.
c.iii)
Kepler's Third Law states that the squares of the sidereal periods of the planets are in direct
Proportion to the cubes of the semimajor axes of their orbits.
(c.iii.1)
This law sounds a lot more complicated than it is. A "sidereal period" is the amount of
time it takes for a planet to do one complete orbit around the sun.
(c.iii.2)
The "semimajor" axis of an ellipse is almost like the "radius" of an ellipse.
(c.iii.3)
The point of this law is that the period of time it takes a planet to orbit the sun increases
rapidly with the radius of its orbit; i.e. Mercury takes 88 days while Pluto takes 248 years.
(c.iii.4)
To arrive at his third law, it was not necessary for Kepler to know the actual distances of
the planets from the sun, but only the distances in units of the Earth's distance, the astronomical
unit.
145)
Astronomical Unit (AU) – is approximately equal to the distance from the Earth to the sun.
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 Winter '12
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