# Tag: centrifugal force

## Why is the Sun so round?

By Phil Plait | September 12, 2012 7:00 am

[NOTE: When I originally wrote this, I made a mistake – I said the Sun was 30 arcseconds across, when it’s actually 30 arcminutes. For some reason, that number got stuck in my brain, and the math I did was based on the incorrect number! I have corrected the math in the text below. Usually I keep the original mistake in an article (striking through the text) along with the correction – that’s my way of admitting mistakes. But given that this is math, I was afraid that might look a bit confusing, so instead I’ll note my brain hiccup here, and keep the math clean by simply fixing it. However, this does change the analogy I used in the text comparing the Sun to a basketball, so in that case I struck through the text and added the correct analogy. I know, it sounds confusing, but it’ll be clear when you read the article. My apologies for this!]

A new study has been published that seems very simple yet has some very interesting repercussions: it shows the Sun is the most spherical natural object ever measured.

Measuring the Sun’s diameter is actually rather difficult. For one thing, observations from the ground have to deal with our atmosphere which warbles and waves above us, distorting images of astronomical objects. To get past that, the researchers used a camera on NASA’s Solar Dynamics Observatory, which orbits high above the Earth. The camera is very stable, and gets past a lot of the problems of measurement uncertainty.

Another problem is that the Sun doesn’t have a solid surface. It’s not like a planet – and even that can be tough to measure. Since the Sun is gaseous, it just kind of fades away with height, so if you try to get too precise you find a lot of wiggle room in the size. In fact, the largest variation the researchers found in the solar diameter was due to intrinsic roughness of the Sun’s limb – in other words, on very small scales the Sun isn’t smooth.

Still, there are ways around that. The point here isn’t necessarily to find the actual size, but the ratio of the diameter of the Sun through the poles (up and down, if you like) to the diameter through the equator. That tells you how spherical the Sun is.

What I would expect is that the Sun is slightly larger through the equator than through the poles, because it spins. That creates a centrifugal force, which is 0 at the poles and maximized at the equator. Most planets are slightly squished due to this, with Saturn – the least dense and fastest spinning planet, with a day just over 10 hours long – having a pole to equator ratio of about 90%. It’s noticeably flattened, even looking through a relatively small telescope.

The Sun spins much more slowly, about once a month. That means the centrifugal force at its equator isn’t much, but it should be enough to measure. So the scientists went and measured it.

And what they found is that the polar and equatorial diameters are almost exactly the same. In fact, they found that the equatorial diameter is 5 milliarcseconds wider than the polar diameter. An arcsecond is a measure of the size of an object on the sky (1° = 60 arcminutes = 3600 arcseconds), and the Sun is about 30 arcminutes (1800 arcseconds) across. In other words the equatorial diameter is only 0.0003% wider than the polar diameter!

The Sun is a 99.9997% perfect sphere. Hmmm.

Put another way, if you shrank the Sun to the size of a basketball, the equatorial diameter would be wider than the polar one by about 0.4 microns – the width of a human hair less than the size of an average bacterium! That’s actually pretty cool.

CATEGORIZED UNDER: Astronomy, Cool stuff

## A hoopy frood

By Phil Plait | February 10, 2012 7:00 am

I caught this video on Geekologie, and it made me laugh. This is a brilliant idea: a woman put a camera on a hula hoop, and then, well, hula’ed:

[WARNING: some folks might feel ill watching this. I will not be blamed if you have to wipe vomit off your keyboard.]

[Note: at the end of the video there are links to other videos like it.]

I found this fascinating. For one thing, the motion is slower than I would’ve expected. I suspect that may be due to an illusion when you watch from the outside as a hula hoop being used; humans are notoriously poor at judging rotating reference frames. After all, people still try to argue with me that centrifugal force isn’t real, when it it quite clearly is.

Even more amazing to me was that I didn’t get ill watching that video. I tend to get a seasick on a kid’s swing or when reading in a car, so the fact I was fine watching this is weird. But I have pretty good 3D spatial reasoning, and have a lot of practice swapping reference frames — trying to figure out when the Moon rises, what configuration planets are in, and how to point a telescope give you a lot of practice there — so maybe that helped. Beats me.

But I wonder what other weird change-of-frames would benefit from using this camera technique? That might make a fun series of videos.

Related posts:

When I say centrifugal, I mean centrifugal!
Space station gives physics a boost
You spin me right round baby right round
xkcd, wherein Randall Munroe and I agree

CATEGORIZED UNDER: Cool stuff, Science

## Space station gives physics a boost

By Phil Plait | November 10, 2011 6:29 am

This is one of the coolest videos I’ve seen in a while: during a routine reboost of the International Space Station to a higher orbit, the astronauts on board show that the station tries to leave them behind!

What a fantastic example of Newtons’s First law: an object in motion tends to stay in motion unless acted upon by an outside force. As the ISS circles the Earth, all the forces on it are balanced. You can think of it this way: the force of gravity pulling it toward the Earth is balanced by the centrifugal force (or the centripetal acceleration, which is equivalent*) outward. Because there are no leftover forces on the ISS, it feels like it’s in free fall, what some people call weightlessness. No force means no acceleration which means no weight.

However, that’s not always the case. Even a few hundred kilometers up, there’s air. It’s thin, but over time it robs energy from the ISS, dropping it lower in its orbit. This is called drag, and it’s a very tiny force (too small to feel on board the ISS), but it adds up over time. To prevent the station from falling too far and burning up, every now and again low thrust rockets are used to push it up into a higher orbit.

But that applies a force that is not balanced! Read More

CATEGORIZED UNDER: Cool stuff, NASA, Science

## Spinning white dwarf bombs may lurk the galaxy

By Phil Plait | September 20, 2011 6:30 am

Some new research just released asks a question near and dear to me: are there thousands of spinning white dwarfs in our galaxy, just waiting to explode as they gradually slow their rotation?

The answer is very probably yes. Let me be clear, as I always must be when covering topics like this: we’re not in any real danger from these things. Space is vast, and supernovae are few. If these things were that volatile we wouldn’t be here to talk about them in the first place.

But it’s still a very cool scientific question, and actually a fairly simple concept. Here’s how it works.

Imagine a binary system of two stars like the Sun, orbiting each other. One star nears the end of its life, swells up into a red giant, and blows off its outer layers. After a few millions years, all that’s left is its core: a dense, hot ball called a white dwarf. The size of the Earth but with the mass of a star, white dwarfs are pretty weird. They have incredibly strong gravity, which wants to crush them down even further, but they are supported by the electric repulsion of electrons, which is a pretty mighty force. It’s an uneasy truce.

It’s made even uneasier by the other star. It too eventually swells up, and can start to dump matter onto the dwarf (like in the picture above). If enough mass piles up, the immense gravity of the dwarf can induce nuclear fusion. Sometimes the material explodes, flaring in brightness, and we get a nova. Other times, if enough matter piles up — making the total mass of the white dwarf a bit more than 1.4 times that of the Sun — the ignition of fusion can cause a runaway reaction in the star, disrupting it entirely. The white dwarf tears itself apart, and you get one of the biggest and most violent explosions in the Universe: a supernova.

But there’s a hitch. Read More

CATEGORIZED UNDER: Astronomy, DeathfromtheSkies!

## You spin me right round baby right round

By Phil Plait | October 4, 2010 7:00 am

New Scientist is reporting that scientists have created the fastest spinning object ever: a fleck of graphene spun up to an incredible rate of a million rotations per second!

Normally, while very cool, that’s not the sort of thing I’d write about here. But I had an idle moment, and wondered about what that rotation really meant. I did a little math, and came up with some astonishing numbers.

First off, graphene is a flat sheet made up of carbon atoms; each atom connects to the others in a hexagon pattern, and the sheet is only one atom thick! This sheet is incredibly strong, and scientists are excited by it because if it can be produced on a large scale it would have tremendous use.

In this case, the scientists created tiny flakes of it only a micron (one-millionth of a meter) across; that’s about 1/50th the width of a human hair. They suspended the flakes in a chamber using electric fields, then spun them up using a beam of light.

I started picturing what that must be like, these tiny whirling motes of carbon, and realized that the forces on a spinning flake must be huge. And by huge, I mean monstrous. When you spin, you feel a force called the centripetal force. It’s what you feel when you’re on a merry-go-round, or a car making a turn (it’s the same thing as centrifugal force, just seen in a different way). The magnitude of this force, how strong it is, depends on how fast you’re moving, and how big a circle you’re making.

I decided to calculate the size of the force on a flake. Read More

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