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# Is Venus Slow?

11 replies to this topic

### #1 Barlowbill

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Posted 12 January 2019 - 12:32 PM

I read an article about cloud structures found in its atmoshere and it stated that it takes Venus 243 earth-days to make one (1) revolution.  What does that do (or not do) in terms of gravity?  What is earth's spinning speed in miles per hour and what is Venu's speed if it takes 243 "days" to make one spin?  Inquiring minds want to know

### #2 kfiscus

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Posted 12 January 2019 - 12:42 PM

The rotation rate (spinning around an axis) has no effect on the strength of an object's gravity- that's determined by the object's mass.  An object's period of revolution (how long it takes to make one orbit around another object) also has no effect on gravity.

Here is the memory trick I teach my students to keep rotation and revolution from getting mixed up:

D

ROTATION

Y

Y

REVOLUTION

A

R

As a cool side note, Venus' "day" is LONGER than its "year".  My kids assume that the planets are all going to be like Earth with their days shorter than their years.  There is no cause-and-effect relationship between rotation rate and revolution rate.

Edited by kfiscus, 12 January 2019 - 12:53 PM.

### #3 Jim Davis

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Posted 12 January 2019 - 12:48 PM

Earth's surface is moving 1674.4 km/h at the equator. At each pole it is 0 since you are just spinning around.

### #4 Barlowbill

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Posted 12 January 2019 - 01:02 PM

Facinating.  Thanks to both of you.  So, do you know how fast Venus rotates at its equator?

### #5 Jim Davis

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Posted 12 January 2019 - 01:10 PM

Facinating.  Thanks to both of you.  So, do you know how fast Venus rotates at its equator?

6.52 km/h

### #6 Mark9473

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Posted 12 January 2019 - 01:23 PM

There are enormous speeds going on all around us, without us really knowing it.

Here's a really interesting article on the topic: https://astrosociety...71/howfast.html

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### #7 Brett Waller

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Posted 12 January 2019 - 01:32 PM

The gravitational attraction of a body is not affected by it's rate of rotation, as kfiscus mentions. However, the force "felt" by an object on that body is most certainly affected by the rate of rotation.  On Earth, the measured acceleration due to gravity at the poles is 9.807 m/s^2, while at the equator it decreases to 9.780 m/s^2.  For most purposes, this 3% decrease isn't noticeable, but it is easily measured.  The most obvious astronomical implication is the oblate flattening caused by rapid rotation, an effect that is easily visible to anyone on both Jupiter and Saturn.  With respect to that affect on Earth compared to Venus, Earth's equatorial radius is bulged out to 6,378 km while it's polar radius is measurable smaller at 6,357. Venus, on the other hand, shows no measurable difference between it's polar and equatorial radii, both are essentially 6,071 km. So, the takeaway is that Venus is essentially round as it's rotation is insufficient to affect it's figure, while Earth is slightly flattened at the poles due to it rotating much faster.

I am not a meteorologist, so I can't speak to the atmospheric effects, but I suspect anything capable of deforming the solid mass of a planet would have some affect on atmospheric circulation.  AS an example, I would expect the Earth's atmosphere to be affected much more strongly by the Corolis force than would the Venusian atmosphere. Perhaps one of our meteorological colleagues might enlighten us on those issues.

Brett

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### #8 Special Ed

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Posted 12 January 2019 - 01:41 PM

More fun facts to rock your world--Venus is essentially upside down with an axial tilt of 177 degrees and it rotates "backwards", i.e., east to west.

### #9 Barlowbill

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Posted 12 January 2019 - 01:56 PM

That is a "fast" article.  Thank you

### #10 Krzysztof z bagien

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Posted 12 January 2019 - 10:47 PM

Earth's equatorial radius is bulged out to 6,378 km while it's polar radius is measurable smaller at 6,357.

This equatorial bulge can be usefull and have practical application, eg. sun-synchronous orbits wouldn't be possible around Earth without it.

### #11 Magnetic Field

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Posted 13 January 2019 - 04:19 AM

Facinating.  Thanks to both of you.  So, do you know how fast Venus rotates at its equator?

1. It is quite difficult to calculate the rotation speed of planets based on theoretical considerations:

https://physics.stac...eed-of-a-planet

2. As always in physics you cannot do anything without observations and the rotational speed is derived from monitoring surface features (I am quite sure the Great Red Spot was the first tool to derive the rotational speed of Jupiter for example):

https://physics.stac...eed-of-a-planet

3. The rotational speed was first measured by spacecrafts:

https://www.universe...overy-of-venus/

Now here I would have thought someone must have had long time before the space age kicked off an idea about Venus' rotational speed.

### #12 tchandler

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Posted 19 January 2019 - 10:45 AM

Venus is one of the most spherical bodies in the solar system, due to its slow rate of rotation.

The Encyclopedia of the Solar System (3rd ed.) states that one day on Venus lasts 116.7 days but is getting shorter. On Venus, the day is 0.002 days shorter than the estimate made 20 years earlier by Magellan. It is thought that the cause of this change is due to an exchange of angular momentum between Venus's superrotating atmosphere and the solid planet. (NB. A superrotating atmosphere is one that rotates faster than the solid component of the planet.)

The radius of the Earth at the poles is 21 km less than the radius of the Earth measured at the equator (polar flattening). On Venus, the polar flattening is essentially zero.

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