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#### 解释开普勒行星运动定律

Explained In 5 Questions: Kepler's Law of Planetary Motion | Encyclopaedia Britannica

5个问题阐释开普勒行星运动定律
SPEAKER 1: Kepler’s first law of planetary motion states

that all planets move about the Sun in elliptical orbits having

the Sun as one of the foca.

But what does that actually mean?

Well an ellipse is a shape that resembles a kind of squashed circle.

Its foci are two points within the ellipse that describe its shape.

For any point on the ellipse,

the sum of that points distances to the two foci is the same.

The further apart the foci are, the more squashed the ellipse is.

If the foci become so close that they are only one focus,

you just have a circle.

In reality orbits are never perfectly circular.

But we do know that the Sun will always be one

of the foci of the elliptical path of an orbit.

Knowing that the Sun is a focus of the planet’s orbit

can tell us a lot about the shape of that orbit.

Kepler tells us that orbits are ellipses,

which are like circles with some added eccentricity.

But what is eccentricity?

How do you figure it out?

Eccentricity measures how flattened an ellipse is compared to a circle.

We calculate it using this equation.

So what does that mean? Well,

a is the semi-major axis,
a表示半长轴
or half the distance along the long axis of the ellipse.

And b is the semi-minor axis,
b表示短半轴
or half the distance along the short axis of the ellipse.

The equation is a way to compare these

axes to describe how squashed the ellipse is.

An ellipse with zero eccentricity would just be a regular old circle.

As eccentricity increases, the ellipse gets flatter and flatter

until it just looks like a line.

An orbit with an eccentricity greater

than one is no longer an ellipse but a parabola

if e is equal to one a hyperbola it e is greater than one.

For example, the giveaway that Oumuamua, the first interstellar comet,

was not from around here was that its eccentricity was 1.2.

The eccentricity of Earth’s orbit is only 0.0167.

Kepler’s third law states that the squares

of the sidereal periods of revolution

of the planets are directly proportional to the cubes

of their mean distances from the Sun.

Which what does that mean?

Basically it’s saying that how long a planet takes to go around the Sun,

its period,

is related to the mean of its distance from the Sun.

That is the square of the period divided by the cube

of the mean distance is equal to a constant.

For every planet, no matter its period or distance,

that constant is the same number.

Kepler’s second law tells us

that a planet moves more slowly when it’s further from the Sun.

But why should that be? Well,

as a planet orbits

the Sun it may not keep a constant speed

but it does maintain its angular momentum.

Angular momentum is equal to the mass

of the planet times the distance of the planet

to the Sun times the velocity of the planet.

Since the angular momentum doesn’t change, when the distance increases

the velocity has to decrease.

That means when the planet gets further from the Sun it slows down.

Kepler’s second law deals with the speed of planets orbiting the Sun.

So does it tell us

at what point the Earth is moving at top speed?

The second law tells us

that the Earth moves fastest when it’s closest to the Sun,

or at its perihelion.

That happens in early January.

At that point Earth is about 92 million miles from the Sun.

Meanwhile it’s at its slowest in early July,

at its furthest point from the Sun, or aphelion.

That greatest distance is about 95 million miles.

That difference of 3 million miles might sound like a lot

but Earth’s orbit is so vast that it’s actually merely circular.

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