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Course: Cosmology and astronomy > Unit 3
Lesson 3: Earth's rotation and tilt- Seasons aren't dictated by closeness to sun
- Season simulator
- How Earth's tilt causes seasons
- Are southern hemisphere seasons more severe?
- Milankovitch cycles precession and obliquity
- Precession causing perihelion to happen later
- What causes precession and other orbital changes
- Apsidal precession (perihelion precession) and Milankovitch cycles
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What causes precession and other orbital changes
What causes precession and other orbital changes. Created by Sal Khan.
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so,will the earth's gravity be lower in some places than others. If this does not make sense just say it.(26 votes)
Yup, the strength of gravity changes from about 9.78 m/s^2 at the equator to about 9.83 m/s^2 at the poles, mostly because of the spin of the earth and the resulting centrifugal force. Gravity also changes depending on how high up you are: the further you get from the center of mass the lower the gravity. There are also other variations resulting from the fact that the earth has different density depending on where you are. Check out http://en.wikipedia.org/wiki/Gravity_of_Earth if you want to know more.(41 votes)
What about the other planets in the solar system? Are they more or less spherical shaped than Earth?(6 votes)- It depends greatly on the spinning speed of the planets and their composition. If they spin fast and are made of soft material (such as gas), they will bulge out a lot. Here's a list of the flattening ratios of some planets:
Earth: 0.3%
Mars: 0.7%
Jupiter: 7.1%
Saturn: 9.8%
Uranus: 2.3%
Neptune: 1.7%
Venus and Mercury have negligible flattening ratios.(16 votes)
why does the earth rotate,and what will happen if it stopped(3 votes)
The Earth rotates due to angular momentum, an object's tendency to rotate. Let's hope the Earth never stops rotating, since it would be catastrophic. Earth rotates at a whopping 1600 kilometers an hour near the Equator. If the Earth suddenly stopped, people, plants, and animals would be sent flying at 1600 kilometers an hour. Near the poles the Earth rotates slower (hence the Coriolis effect), but you'd still be going pretty fast. If the Earth stopped gradually, we'd probably still end up being destroyed. Once Earth stops we'd have six months days and six month nights. Where it once took us twenty-four hours to complete a day-night cycle it would now take the Earth a year. One side of Earth would freeze for six months without sunlight, while the opposite side of Earth would be subject to sunlight for six months. When Earth rotates it creates a bulge of water at the equator. Without rotation this water would go to the poles and flood landmasses. In short, it wouldn't be good.
To learn more, watch this hypothetical (and apocalyptic) documentary by National Geographic.
https://www.youtube.com/watch?v=5btuj7fFIAM(5 votes)
Is changing the position of continents also affects the change in axis of the earth due to change in Earth's centre of mass? Can we consider it to be a cause for obliquity and precission also?(3 votes)- Keep in mind that the Earth's crust is comparatively very thin. Yes, mountain ranges might have a bit more gravity than deep ocean (stone is heavier than water) but the effect on the Earth's center of mass will be slim compared to the currents of magma in the mantle.(5 votes)
- At1:15he says the reason the earth is fatter than tall, is because of other gravitational forces. But, could another factor be the speed of our rotation @ ~600 mph?(3 votes)
- (He didn't say that, but...)
Yes, definitely (900 mph at the equator)(5 votes)
Does Earth's magnetic field affect it's shape? Is that why the Earth is fatter than it is tall? Or is there some other factor solely to it, or are there multiple factors?
Also, How do we know that Earth's orbit around the sun is elliptical and not that the orbit is circular, but the Sun is not offset form the center of the orbit?(2 votes)
The Earth is an "oblate spheroid", which is fancy language for saying that it's equatorial diameter is greater than it's polar diameter, by about 21 km. This is due to the fact that the Earth rotates around it's axis once every 24 hours. That motion causes the equator to bulge outwards slightly.
https://www.scientificamerican.com/article/earth-is-not-round/
We know that the Earth's orbit is elliptical because we can measure the apparent motion of the Sun against the background stars, and can see that the Sun's apparent motion is not regular. If we orbited the Sun in a perfect circle, the Sun would move {apparently) with precisely the same speed at all parts of our orbit. Since the Sun appears to move slightly faster at some times and slightly slower at other times in our annual orbit, we know that the Earth's orbit is elliptical.(4 votes)
HOW IS OUR PLANET FATTER THEN OUR HIEGHT? IM CONFUSED(1 vote)- When the Earth spins it does it. To experience it, have a ball in your hand. Extend the ball out with your hands so that it is in front of you. Now start spinning around. Spin a couple of times around and let go of the ball (don't do it in the house!). You will see that the ball has gone a distance away from you. And the Moon's gravity has an effect on it to, and that is why all the other planets have it to! Hope I helped!Bye!(3 votes)
- A question regarding general idea about precession: Is the sideways movement of an axis in a horizontal plane about the vertical, such as that of a table fan also a precession?(2 votes)
- Are you talking about an oscillating fan? It moves back and forth because of a motor, not because it's precessing.(3 votes)
What makes the earth spin?(2 votes)
When our solar system formed out of a gas cloud, called a nebula, there was lots of dust and gas coming together due to the force of gravity. The dust and gas was already moving around in a circle. As it all clumped together to form the Sun and planets, these new objects started to spin – and then spin faster.(3 votes)
- This is not related to the discussion of earth's cycles and obliquity. But, why is the moon moving away from the earth at a few centimeters a year?(2 votes)
- Jay is almost right. The moon does drag the earth with the tides it causes. The result of this, though, is not to speed up the moon, but to slow it down. But angular momentum has to be conserved. How do you conserve angular momentum if the angular speed is slowing down? You have to move further away from the center of the circle. It's like when a figure skater wants to slow down a spin, she can spread her arms out wide.(3 votes)
1:15Video transcript
I've talked a lot
about precession and the changes in the
inclination of Earth's orbit, but I haven't told you
why they are happening. And I'm not going to go
into the physics of it. It's a little bit beyond this
discussion right over here. But they're really a
byproduct of the Earth and sun's interactions with
Earth and with the fact that Earth is not
a perfect sphere. So if I draw Earth-- so this
is my little drawing of Earth, and let me put the poles over
here, North Pole and South Pole, it actually turns
out that Earth is fatter than it is taller. So if you were to
measure Earth's diameter along the equator,
it is 43 kilometers, which is approximately 27
miles longer than if you were to measure its
diameter from pole to pole. So longer than the
pole to pole diameter. And the fact that Earth has this
equatorial bulge, that it's not a perfect sphere--
and once again, I'm not going to go
into the math here-- it's the interactions
between that, I guess you could call it, that
one asymmetry of the Earth, it's that interaction
between that and the pull of gravity
between the Earth and the sun and the moon that
causes these long term cycles, this axial
precession and other less noticeable changes
in Earth's orbit. And as we'll see
in the next video, these aren't the only types
of changes in orbits we have. We also have changes
in the actual ellipse that Earth's orbit has
actually rotates over time. But that's due more
to interactions with Earth's orbit,
and the orbit of other planets in
our solar system. And once again, it's
one of those things that happened over thousands
and thousands of years. So all of these changes,
they're because Earth isn't completely symmetrical,
more fat than it is tall, those interactions
between the gravity of the sun and the moon. And Earth's orbit, as a whole,
changes because of interactions with other objects inside
of our solar system.