Tag Archives: Rotation

Frank Force wins Best Illusion of 2019 award with a simple, but effective shape

What was the greatest illusion of 2019? Was it that you’ll finally ‘get in shape at the gym’? Was it true love? Both have their merits but, according to the Best Illusion of the Year contest, it was actually a moving shape that (frustratingly enough for my brain) seems to rotate in all directions at once, created by game developer and artist Frank Force.

Image credits Frank Force via Youtube.

The contest is a yearly event run by the Neural Correlate Society (NCS), a nonprofit that aims to promote scientific research into the neural correlates of perception and cognition.

Seeing is believing

“How we see the outside world — our perception — is generated indirectly by brain mechanisms, and so all perception is illusory to some extent. The study of illusions is critical to how we understand sensory perception, and many ophthalmic and neurological diseases,” the NCS explains.

The contest is ran “democratically”, according to the NCS, with the first, second, and third places being awarded by an online vote.

Force’s “Dual Axis Illusion” won this year’s first prize, and it’s easy to see why: the more you look at it, the weirder it gets. Force helps inch the illusion along through the use of colored lines that highlight how his creation works.

So, without further ado, here it is in action:

Like all optical illusions, Force’s creation hijacks our brain’s tendency to cut corners when interpreting sensory data — sight is especially well-suited to this task. Our brain’s interpretation of sight is built on a huge amount of individual bits of information that the brain tends to treat as a meaningful whole to simplify the process. Force expertly abuses that process by adding and removing contextual information, such as the colored lines that temporarily appear on the screen, or by anchoring your perception on key elements of the shape — in this case, where the lines overlap.

It’s surprising to see the sheer extent to which these cues shape my perception of what is fundamentally a two-dimensional black line on a white background. Practically speaking, there is no volume to this image and no real rotation happening — but Force can still make me perceive them, and then turn that perception on its head.

And I love it.

Saturn Illustration.

Perturbations in Saturn’s rings reveal how long a day is on the gas giant

Saturn’s days are 10 hours, 33 minutes, and 38 seconds long — and we know this by looking at wave patterns in its rings.

Saturn Illustration.

Illustration showing NASA’s Cassini spacecraft in orbit around Saturn.
Image credits NASA / JPL-Caltech.

New observations from NASA’s Cassini spacecraft allowed researchers at the University of California Santa Cruz to calculate Saturn’s rate of rotation. This measurement — the most precise determination of its rotation rate — was based on observations of wave patterns created within the planet’s rings.

Timekeeping rings

“Particles in the rings feel this oscillation in the gravitational field. At places where this oscillation resonates with ring orbits, energy builds up and gets carried away as a wave,” explained Christopher Mankovich, a graduate student in astronomy and astrophysics at UC Santa Cruz, and lead author of the study.

Just like our own planet, Saturn vibrates in response to perturbations (large-scale movement of matter). Unlike our planet, these perturbations come not from the movement of tectonic plates, but likely from heat-driven convection in the planet’s gassy bulk. Such internal oscillations move about massive quantities of gas, which has a noticeable impact on local densities within Saturn’s atmosphere. Such changes, in turn, cause noticeable changes in the planet’s localized gravitational pull. And, even better, the frequency of oscillation within Saturn carries over to the gravitational effects — in short, they share the same ‘fingerprint’, so these internal events can be linked to their external, gravitational effects.

Saturn rings.

Image of Saturn’s rings taken by NASA’s Cassini spacecraft on Sept. 13, 2017.
Image credits NASA / JPL-Caltech / Space Science Institute.

Naturally, we’d need satellites or other sorts of equipment in orbit across the planet to pick up on such gravitational fluctuations. Which we haven’t really brought over yet. Rather conveniently, however, Saturn has a sprawling ring system surrounding it. They do react to the planet’s gravitational pull, its fluctuations causing certain wave patterns to form inside the rings. Not all patterns seen inside the rings are caused by gravitational effects — but most are.

In effect, this makes the rings act similarly to seismographs, devices that we use to measure earthquakes.

“Some of the features in the rings are due to the oscillations of the planet itself, and we can use those to understand the planet’s internal oscillations and internal structure,” says Jonathan Fortney, professor of astronomy and astrophysics at UC Santa Cruz and paper coauthor.

NASA’s Cassini spacecraft allowed researchers to observe Saturn’s rings in unprecedented detail. Mankovich’s team developed a series of models of the planet’s internal structure and used them to predict the frequency spectrum of Saturn’s internal vibrations. Then they compared their predictions to waves observed by Cassini in Saturn’s C ring.

One of the main results of this study is an estimation of Saturn’s speed of rotation — which has been notoriously difficult to accurately pin down. Saturn is basically a huge clump of gas and, as such, its surface doesn’t have any fixed, distinctive features we could track as it rotates. The planet is also unusual in that its magnetic poles are nearly perfectly aligned to its axis of rotation — so we can’t track those either. On Earth, for example, the magnetic poles aren’t aligned with this axis.

Mankovich’s team determined that a day on Saturn lasts for 10 hours, 33 minutes, and 38 seconds — several minutes shorter than previous estimates (which were based on radiometry readings from the Voyager and Cassini spacecraft).

“We now have the length of Saturn’s day, when we thought we wouldn’t be able to find it,” said Cassini Project Scientist Linda Spilker.

“They used the rings to peer into Saturn’s interior, and out popped this long-sought, fundamental quality of the planet. And it’s a really solid result. The rings held the answer.”

The paper “Measurement and implications of Saturn’s gravity field and ring mass” has been published in the journal Science.

Sagittarius A*

Researchers find black hole that spins almost as fast as (we think) they can spin

New research led by members from the University of Southampton has identified a black hole spinning around its axis near its maximum possible speed.

Sagittarius A*

A simulated image of supermassive black hole Sagittarius A*, showing against a background of radiation and bright matter swept into the event horizon. The image was generated with data recorded by the Event Horizon Telescope.
Image credits National Radio Astronomy Observatory,

The study involved an international team of astronomers. Starting from observations taken with state-of-the-art sensors, the researchers found evidence that 4U 1630-472, a stellar-mass black hole in our galaxy, is rotating really, really fast — around 92% to 95% of a black hole’s theoretical maximum rotational speed.

Material keeps falling into this black hole as its spinning, being subjected do immense gravitational stress and temperatures. The environment is so violent that this matter shines brightly in X-rays, the team reports, which they used to establish that 4U 1630-472 is rotating and calculate its speed.

So fast it’s glowing

If a black hole is rotating rapidly enough, it should — according to the general theory of relativity — distort space-time around it differently than a non-rotating black hole, the team explains. Such distortions would leave a measurable trace on the radiation emitted by the matter it’s absorbing.

Therefore, researchers can look at a black hole’s emission spectra to determine the rate it’s spinning at.

“Detecting signatures that allow us to measure spin is extremely difficult,” says lead author Dr. Mayukh Pahari from the University of Southampton. “The signature is embedded in the spectral information which is very specific to the rate at which matter falls into the black hole.”

“The spectra, however, are often very complex mostly due to the radiation from the environment around the black hole.”

Dr. Pahari says the team was “lucky” to obtain a spectral reading directly from the matter falling into the black hole, sans the background noise. Armed with that data, it was “simple enough to measure the distortion caused by the rotating black hole,” he says.

The findings from this study are significant, as this is one of only a handful of times we’ve managed to accurately measure a black hole’s spin rate. Only five other black holes have shown high spin rates, the team adds. Astronomical black holes can be fully characterized by mass and spin rate. Therefore, measuring these two properties is key to understanding some extreme aspects of the universe and the fundamental physics related to them.

The paper “AstroSat and Chandra View of the High Soft State of 4U 1630–47 (4U 1630–472): Evidence of the Disk Wind and a Rapidly Spinning Black Hole” has been published in The Astrophysical Journal.

Sunrise

The Earth is spinning slower, making the days longer and longer

Days are getting longer as the Earth’s rotation suffers tiny alterations over time. But you’re probably not going to notice anything anytime soon — a day gets one extra minute every 6.7 million years, a new study estimates.

Sunrise

Image via Pexels.

Each “day” is the amount of time it takes for the planet to do a full rotation around its own axis. So any shift in the speed of Earth’s rotation will have an inverse and proportional effect on the length of a day — higher speeds shorten the day, slower rotation means longer days.

And the latter case seems to be true. A British team estimates that the average day has gained 1.8 milliseconds each century over the past 2700 years. The speed they calculated is “significantly less” than previous estimates which settled on a rate of 2.3 ms per century — which would translate to one minute every 5.2 million years. Still, retired Royal Greenwich Observatory astronomer and lead co-author Leslie Morrison admits that it remains “a very slow process.”

“These estimates are approximate, because the geophysical forces operating on the Earth’s rotation will not necessarily be constant over such a long period of time,” he added.

“Intervening Ice Ages etcetera will disrupt these simple extrapolations.”

The previous figure of 2.3 ms was estimated from calculations of the Moon’s effect on “Earth-braking” — its gravitational force pulls on the Earth’s water and land, effectively pulling against the force of rotation.

But Morrison and his team also factored in gravitational theories about the Earth’s movements around the Sun as well as the Moon-Earth interactions, to calculate the timing of solar eclipses over time as seen from our planet. They then calculated where on Earth they’d be visible from, and compared the results to records of eclipses from ancient Babylonians, Chinese, Greeks, Arabs and medieval Europeans.

“We obtained historical, relevant records from historians and translators of ancient texts,” explained Morrison for the AFP.

“For example, the Babylonian tablets, which are written in cuneiform script, are stored at the British Museum and have been decoded by experts there and elsewhere.”

They found discrepancies between the points eclipses should have been observable from and where they were actually seen. This discrepancy can only be caused by a rotational speed different from the one that the team used in their model (the present one.)

“This discrepancy is a measure of how the Earth’s rotation has been varying since 720 BC” when ancient civilisations started keeping eclipse records, they wrote.

Earth’s rotation speed can be influenced by the Moon’s breaking effect, electro-magnetic interaction inside the planet (between the solid core and the mantle that floats over it), as well as mass shifts on the planet — changes in sea level, shrinking polar caps since the last Ice Age, large reservoirs, and so on.

And while most of us probably won’t ever notice this increase, it’s vital that scientists know about it. This information can be used in adjusting high-precision clocks for example, which underpin our navigational systems.

The full paper “Measurement of the Earth’s rotation: 720 BC to AD 2015” has been published in the journal Proceedings of the Royal Society A.