## Compton Generator

Long before he won the Nobel Prize in Physics, and while still a Wooster undergraduate, Arthur Compton realized a third way to demonstrate Earth’s spin (after pendulums and gyroscopes).

Compton reported his results in a manuscript submitted to the journal Science on 1913 January 13 and published as “A Laboratory Method of Demonstrating the Earth’s Rotation”, Arthur Holly Compton, Physical Laboratory, University of Wooster, Science, 1913 May 23, volume 37, issue 960, pages 803-806.

Compton’s generator is nowadays often used as a text book example of the $\vec F_C = 2 m\, \vec v \times \vec \omega$ Coriolis pseudo-force deflecting the fluid in a circular tube as it is flipped in Earth’s rotating reference frame, which is analogous to the $\vec F_B = q\, \vec v \times \vec B$ magnetic force deflecting a moving charge. However, an inertial perspective is simpler. After viscosity has damped the fluid motion relative to Earth, the fluid farther from the rotation axis is moving faster relative to distant stars, so quickly flipping the ring reverses the speed gradient and induces transient circulation.

Flipping Compton’s ring in a spinning reference frame generates transient fluid flow like flipping a wire loop in a magnetic field generates transient electrical current, where the angular velocity plays the role of the magnetic field; indeed, the former is a motion generator and the latter is an electric generator!

Flipping Compton’s ring π = 180° causes the stationary fluid inside to circulate, generating motion and revealing Earth’s spin.

As in the above figure, assume Earth has radius $R$ and angular speed $\omega$. Assume the ring is at co-latitude $\theta$ and subtends an angle $2\delta$ from Earth’s center. Then the spread in the fluid’s inertial speeds is

$$\delta v = \omega R \sin[ \theta + \delta] - \omega R \sin[\theta - \delta] = 2\omega R \cos \theta\sin \delta.$$

If the ring’s radius $r \ll R$, then

$$1 \gg r / R \approx \delta \approx \sin\delta,$$

and so

$$\delta v = 2 \omega r \cos \theta = 2 \omega r \sin \lambda,$$

where $\lambda$ is the latitude. As checks, $\delta v = 0$ when $\lambda = 0$ (straddling the equator), and $\delta v = 2\omega r$ when $\lambda = \pi/2$ (straddling the north pole).

For Earth’s $\omega = 2\pi/\text{day}$ angular speed, flipping a quiescent $r = 1~\text{m}$ ring at $\lambda = \pi/4$ mid latitude generates an initial fluid speed $\delta v = 0.1~\text{mm/s}$. Compton amplified this motion by using a microscope to view the fluid through a window in a constriction of the tube.

## Analemma

Photograph the sky at the same time each day for a year and Sun will appear to execute a figure-8 path called an analemma, which is often inscribed on Earth globes and can be used as an almanac, as by Tom Hanks‘ character Chuck Nolan in the movie Cast Away.

The 2D animation below illustrates the formation of a 1D back-and-forth analemma for Planet whose spin axis is perpendicular to its orbital plane. Planet’s year divides into 7 equal sidereal days (when red arrow observer points left), 6 equal mean solar days  (when white ray from Sun crosses yellow lines), and 6 unequal apparent solar days (when red arrow points along white ray toward Sun).

Planet’s rotation (spin), which is constant, and eccentric revolution (orbit), which is faster nearer Sun where gravity is stronger, cause mean solar days to be nonuniformly distributed about the orbit, dense near aphelion (far point at left) and sparse at perihelion (near point at right). For an equatorial observer who experiences noon at perihelion, mean noon (when yellow dots record Sun’s direction) precedes apparent noon during the orbital top half and succeeds apparent noon during the orbital bottom half, with the constant rotation “falling behind” during the fast orbital right half and “getting ahead” during the slow orbital left half.

Formation of a back-and-forth analemma (yellow dots) for an un-tilted planet in an eccentric orbit.
(You may need to click to see the animation.)

The 3D animation below (with a wide-angle perspective) illustrates the formation of a 2D figure-8 analemma for an oblique Planet whose spin axis is tilted 45° downward. Planet’s year divides into 40 equal sidereal days, 39 equal mean solar days, and 39 unequal apparent solar days. The constant tilt and elliptical orbit cause an equatorial observer to oscillate above and below Sun. For an equatorial observer who experiences noon at perihelion, mean noon (when yellow dots record Sun’s direction) precedes apparent noon, with Sun above the equator, during the orbital top half and succeeds apparent noon, with Sun below the equator, during the orbital bottom half, forming a figure 8. (In practice, the precise shape and orientation of analemmas vary with time of day and latitude.)

Formation of a figure-8 analemma (yellow dots) for a tilted planet in an eccentric orbit. Showing the planet at mean solar day increments reduces bandwidth but hides most of its spin.
(You may need to click to see the animation.)

## Perseverance, Ignition, Breakeven

Overcoming decades of enormous physics and engineering challenges, and despite persistent pessimism, skepticism, and criticism, the National Ignition Facility has achieved an historic milestone for controlled nuclear fusion, a target energy gain factor of $Q > 1$.

Last week, NIF focussed the world’s most powerful laser pulse on a small gold cylinder that converted the incident ultraviolet light into x-rays and caused an enclosed diamond-coated deuterium-tritium pellet to implode and convert some of its matter to energy: 2.05 MJ of energy went into the target, and 3.15 MJ came out.

I remember the fusion goals of my childhood as ignition and breakeven.  On 2021 August 8, NIF achieved ignition by surpassing the Lawson criterion (roughly, the triple product $\text{density} \times \text{temperature} \times\text{confinement time}$) to create a self-sustaining “burning plasma”, where fusion heating exceeded all cooling processes, but with a target energy gain of only $Q \approx 0.72$. Finally, on 2022 December 5, after 16 months of additional hard work, including increasing and balancing the laser power and thickening the pellet’s diamond coating, NIF achieved both ignition and breakeven with a record target energy gain of $Q \approx 1.5$.

Although nowadays the “ignition” and “breakeven” criteria are often conflated, as in this morning’s formal announcement, by any standard NIF appears to have at last achieved the grand but elusive goal embodied in its name. Like LIGO’s success in detecting gravitational waves, NIF’s success in achieving ignition and breakeven is a tribute to perseverance and diligence in the face of a daunting challenge.

The physics works; the rest is engineering.

The Artemis 1 mission’s Orion spacecraft has successfully entered and exited a distant retrograde orbit about Moon. DRO is a stable and easily accessible orbit requiring a low velocity change $\Delta V$. In DRO, Earth‘s non-negligible gravity contributes to a 3-body problem that makes the inertial space orbit non-Keplerian: an ellipse centered — not focussed — on Earth.

The attached animation, which I generated by numerically integrating the 3-body motion equations, displays a DRO in reference frames fixed relative to distant stars [left pane] and rotating with Moon about Earth [right pane]. From the north celestial hemisphere, Orion [red] orbits anti-clockwise relative to Earth [cyan], like our solar system’s planets, but clockwise (and hence retrograde) relative to Moon [white].

For Artemis 1, Orion spent almost a week in DRO, completing a half revolution about Moon (and a quarter revolution about Earth), and is currently returning to Earth. The crewed Artemis 2 will use a free return trajectory for safety, and future Artemis missions will use Near Rectilinear Halo Orbits to avoid Moon periodically eclipsing Earth.

Far from Earth [cyan], the Orion spacecraft [red] entered a distant retrograde orbit about Moon [white]. Relative to Earth [left pane] Orion orbits one way, but relative to Moon [right pane] Orion orbits the opposite way. (You may need to click to start the animation.)

From a distant retrograde orbit, a camera at the tip of an Orion solar panel photographs Earth and Moon, 2022 November 28.

## Artemis Is the Sister of Apollo

I stayed up late last night and early this morning to watch the successful uncrewed launch of Artemis 1. In Greek tradition, Artemis was the twin sister of Apollo, and the Artemis program hopes to return humans — including the first woman — to Moon as preparation for sending them onward to Mars.

As a child of the Apollo program, I am convinced that a future with humans living and working in space, on Moon and Mars, is much more exciting than one with humans confined to Earth, and in this regard the half-century gap between Apollo and Artemis has been deeply disappointing.

Today real hope exists for realizing Apollo’s promise to permanently extend the bounds of human experience beyond low-Earth orbit. Later this decade, the crewed Artemis 2 should circumnavigate Moon and the crewed Artemis 3 & 4 should land on Moon’s unexplored South Pole. I hope these and subsequent increasingly ambitious missions and their diverse explorers will excite and inspire a new generation, the Artemis generation; I know they will excite and inspire me.

As currently planned, Artemis 3 & 4 will require the development of a brand-new revolutionary launch vehicle — a true 21st century super-heavy lift rocket, which I have highlighted before and expect to soon discuss again.

A camera attached to the tip of the Orion capsule’s service module photographed Artemis 1 departing Earth orbit on 2022 November 16

## Zero-G Indicator

When Crew 5 rocketed to orbit last week aboard the SpaceX Dragon “Endurance” bound for the International Space Station, I was curious to see their zero-gravity indicator. A tradition SpaceX crews have adopted from Russian cosmonauts, the zero-g indicator is usually a stuffed animal whose first float announces the free-fall of Earth orbit. Was that a plush Einstein doll?

On orbit, Crew 5 pilot and physicist Josh Cassada explained that the doll demonstrated Einstein’s happiest thought, that people falling can not feel their own weight. This insight, essentially the equivalence of inertial mass and gravitational charge, permits gravity to be interpreted as geometry in the theory of general relativity, still our best description of gravity. Crew 5 is experiencing Einstein’s happiest thought not merely momentarily but continuously!

Einstein doll zero-g indicator floats (top right) aboard the SpaceX Crew Dragon “Endurance” illustrating Einstein’s happiest thought. (NASA TV)

## For the dinosaurs!

The dinosaurs didn’t have a space program, but we do.

I just watched live the first kinetic-impact asteroid-redirection test as NASA’s Double Asteroid Redirection Test spacecraft collided with the asteroid-moon Dimorphos of the asteroid Didymos. Below is the last image DART transmitted, truncated by the impact itself!

The goal is to measurably change the speed of Dimorphos as it orbits Didymos to test asteroid planetary defense. Ground-based telescopes cannot resolve the system, which is only a few hundred meters across. (To DART Dimorphos looked like a spheroidal rubble pile). However, the system undergoes mutual eclipses as seen from Earth and its brightness dips periodically when one asteroid blocks or shadows the other. The head-on collision should have slowed Dimorphos, lowering its orbit and reducing its orbital period by several minutes, which should be noticeable over the next few weeks. Confounding variables include momentum exchanged with ejecta and the consequent gravity change due to their reshaping.

(In 2005, NASA’s Deep Impact space probe released an impactor into comet Tempel 1 not to redirect it but to excavate the interior for remote analysis, like taking a core sample from a tree.)

Final image transmission from DART cut-off by its impact with the asteroid Dimorphos

## Great Plains Solstice Twilight

Last month I drove across the United States, coast-to-coast back-and-and forth diagonally, 8000 miles through 18 states, as in the animation below. Amazing was driving through the Great Plains of the North American flatland with uninterrupted 360° horizons as the sun slowly set and civil, nautical, and astronomical twilight seemed impossibly long. The explanation was the nearness of the summer solstice, when Earth’s tilt extends the sunset by spinning an Earth-bound observer diagonally across the day-night terminator, which is blurred by atmospheric refraction, as illustrated in the diagrams below.

My 2022 July road trip, back-and-forth across the continental US, 8000 miles across 18 states.
(You may need to click to start the animation.)

At the equinoxes Earth is not tilted to (or from) Sun and its spin sweeps earthlings rapidly through twilight (short thick black line segment at left). But at solstices, Earth is tilted to (or from) Sun and its spin takes longer to sweep earthlings through twilight (long thick black line segment at right).

## Signature Spikes

Nearly a quarter century in the making, I was tremendously excited and relieved last week by the release of the first images from the James Webb Space Telescope. I remember the trials, tribulations, and triumph of the Hubble Space Telescope and am now confident that Webb’s gallery of images and spectra will meet or exceed Hubble’s and begin a new chapter in astronomy.

To distinguish Hubble and Webb images, look for their signature diffraction spikes: Due to their primary mirror shapes and secondary mirror support strut configurations, bright stars in Hubble and Web space telescopes images display distinctive 4 and 6 + 2 = 8 diffraction spikes.

Hubble and Webb images of the galaxy cluster SMACS 0723 with signature diffraction spikes around the bright foreground star