An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.

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From the course by University of Rochester

Confronting The Big Questions: Highlights of Modern Astronomy

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An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.

From the lesson

Are we alone in the Universe?

Planets and Life in The Universe - Exoplanets searches, exoplanet census, astrobiology

- Adam FrankProfessor

Physics and Astronomy

Welcome back, everyone.

Â Well today's lecture is just a tiny bit of maths.

Â And what we're really looking for, or geometry, at least.

Â And what we just want to describe is

Â a object that is extremely important for astronomy.

Â And it's what's called the Ellipse.

Â Now, ellipses are, you know?

Â They're like circles in the sense

Â that they're geometrical objects, or mathematical objects.

Â But they play an essential role in astronomy.

Â So in the mid 1600s, Johannes Kepler figured out, that all planets

Â orbit on ellipses, they're motion can be described by ellipses.

Â So that's why they're so important to astronomy.

Â So we just want to sort of think a little bit about what ellipse is.

Â So an ellipse kind of looks like a squashed circle where a circle

Â just has one radius, there are sort of, two radiuses to describe an ellipse.

Â One is called the semi-major axis, and that is the distance from the center.

Â Out to the long, axis,

Â and then the semi-minor axis is the

Â distance from the uh,center to the short axis.

Â And, but of course that's actually not, doesn't completely describe the ellipses.

Â But they are important for the ellipses are what are called the foci, and the foci

Â are these two points over here, in the

Â circle the foci are actually at the center.

Â But why are the focii important, because the sun is at the center

Â of one of the focii for all of the orbits in the solar system.

Â In fact, anything which is orbiting anything else the, the central

Â object always will be at the focus of the elliptical orbit.

Â So you know all orbits or ellipses.

Â And the thing your orbiting is always going to be at one of the foci.

Â Now there's this thing called the eccentricity which

Â describes sort of how squashed the circle is.

Â So mathematically the, the eccentricity is defined as the distance from the center.

Â To the focus, divided by

Â the center to the semi major axis.

Â so what that means is, is that for a circle, since the focus is actually at the

Â center, the eccentricity is zero, and for a really

Â really sco, squashed circle, it's really a squashed ellipse.

Â Which would be sort of like the orbit of

Â a comet then they had eccentricities close to one.

Â Okay.

Â So remember the star's always at the focus and here's the interesting thing, as

Â the star, so if this is the sun and this is the planet moving

Â around on ecce, eccentric orbit.

Â That was a very eccentric orbit by the way, I just described, most

Â of the planetary orbits in the solar system are pretty close to circles.

Â But the important thing to remember is that, as you get close

Â to the star or the focus of your orbit, you speed up.

Â So a typical orbit looks something like this.

Â Zoom.

Â You zoom around, slow down then zoom back in.

Â So that's why comets as they come in start moving very

Â quickly and then as they go out tend to spend, you know,

Â a very long time out at the

Â edges of their orbit.

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