The first solar eclipse of 2023 will span Australia and southeast Asia into the Pacific Ocean region.
Mark your calendars. The first eclipse season of 2023 is about to begin on Thursday, April, 20th, with a rare hybrid annular-total solar eclipse.
Solar Eclipse Primer
Eclipses occur when the Moon passes between the Earth and the Sun, casting its shadow across the surface of the planet. The Moon’s path is inclined five degrees relative to the ecliptic plane, and misses the Sun on most passes. Otherwise, we’d see two eclipses—one lunar and one solar—per month. For an eclipse season to occur, New and Full Moon need to fall very near an intersection node of the Moon’s orbit and the ecliptic. This happens about twice a year.
Total eclipses occur when the Moon completely covers the Sun, plunging those standing in the shadow of the Moon into an eerie darkness and revealing the pearly white solar corona. This is the kind of eclipse most folks will get on a plane and head to an exotic location for. Though we often marvel at how the Moon seems to be a great fit versus the Sun as seen from the Earth, this isn’t always the case. If New Moon is headed towards apogee and the Sun is a few months within perihelion, the inner umbral shadow fails to reach the surface of the Earth, and an annular eclipse occurs. Observers are then treated to a brilliant ‘ring of fire’ eclipse.
Animation for the April 20th eclipse. Credit: NASA/GSFC.AT Sinclair.
Bizarre Hybrid Solar Eclipse
But something stranger still happens on April 20th. The Moon’s umbral shadow barely brushes the Earth on one part of the track, only to liftoff again on the other. This is the hybrid portion of the eclipse, which transitions from a broken annular, to totality, then back to annular again.
Types of solar eclipses: total (left), annular (center) and partial (right). Credit: NASA/Joseph Matus/Bill Dunford/Bill Ingalls.
The 49-kilometer wide path touches down at sunrise over the Indian Ocean. The eclipse only brushes land briefly at three points. First landfall occurs over the extreme northwestern tip of Australia along the Ningaloo Coast and the tiny town of Exmouth. The shadow then crosses the Timor Sea and touches the eastern tip of the island nation of East Timor near the capital of Dili, and then crosses a scattering of Indonesian islands including Kisar, the Schouten Islands and Western New Guinea.
Circumstances with times in UT, and partial eclipse percentages for the April 20th hybrid eclipse. Credit: Michael Zeiler.
Maximum duration for totality is only 1 minute and 16 seconds, just south of the Indonesian island of East Timor in the Timor Sea.
Rare (and Remote) Event
How rare is a hybrid eclipse? Well, there are only seven hybrid eclipses in the 21st century, or 3.1% of solar eclipses overall. Annulars are actually more common than totals in the current epoch, and will continue to become even more so over the next few hundred million years as the Moon slowly recedes from the Earth, until all central solar eclipses are elusively annular.
Starship gave us quite a show during the first flight test of a fully integrated Starship (S24) and Super Heavy rocket (B7) from Starbase in Texas.
On April 20, 2023 at 8:33 a.m. CT, Starship successfully lifted off from the orbital launch pad for the first time. The vehicle cleared the pad and beach as Starship climbed to an apogee of ~39 km over the Gulf of Mexico – the highest of any Starship to-date.
With a test like this, success comes from what we learn, and we learned a tremendous amount about the vehicle and ground systems today that will help us improve on future flights of Starship.
This is the vehicle trajectory and mission control audio without any additional commentary. There may be very long periods of silence. For our full hosted webcast, visit
When two black holes collide, they don’t smash into each other the way two stars might. A black hole is an intensely curved region of space that can be described by only its mass, rotation, and electric charge, so two black holes release violent gravitational ripples as merge into a single black hole. The new black hole continues to emit gravitational waves until it settles down into a simple rotating black hole. That settling down period is known as the ring down, and its pattern holds clues to some of the deepest mysteries of gravitational physics.
Gravitational wave observatories such as the Laser Interferometry Gravitational-Wave Observatory (LIGO) have mostly focused on the inspiral period of black hole mergers. This is the period where the two black holes orbit ever closer to each other, creating a rhythmic stream of strong gravitational waves. From this astronomers can determine the mass and rotation of the original black holes, as well as the mass and rotation of the merged black hole. The pattern of gravitational waves we observe is governed by Einstein’s general relativity equations, and by matching observation to theory we learn about black holes.
General relativity describes gravity extremely well. Of all the gravitational tests we’ve done, they all agree with general relativity. But Einstein’s theory doesn’t play well with the other extremely accurate physical theory, quantum mechanics. Because of this, physicists have proposed modifications to general relativity that are more compatible with quantum theory. Under these modified theories, there are subtle differences in the way merged black holes ring down, but observing those differences hasn’t been possible. But a couple of new studies show how we might be able to observe them in the next LIGO run.
The modified Teukolsky equation. Credit: Li, Dongjun, et al
In the first work, the team focused on what is known as the Teukolsky Equation. First proposed by Saul Teukolsky, the equations are an efficient way of analyzing gravitational waves. The equations only apply to classical general relativity, so the team developed a way to modify the equations for modified general relativity models. Since the solutions to both the Teukolsky and modified Teukolsky equations don’t require a massive supercomputer to solve, the team can compare black hole ring downs in various gravitational models.
The second work looks at how this would be done with LIGO data. Rather than focusing on general differences, this work focuses on what is known as the no-hair theorem. General relativity predicts that no matter how two black holes merge, the final merged black hole must be described by only mass, rotation, and charge. It can’t have any “hair”, or remnant features of the collision. In some modified versions of general relativity, black holes can have certain features, which would violate the no-hair theorem. In this second work, the authors show how this could be used to test general relativity against certain modified theories.
LIGO has just begun its latest observation run, so it will be a while before there is enough data to test. But we may soon have a new observational test of Einstein’s old theory, and we might just prove it isn’t the final theory of gravity after all.
Reference: Li, Dongjun, et al. “Perturbations of spinning black holes beyond General Relativity: Modified Teukolsky equation.” Physical Review X 13.2 (2022): 021029.
Reference: Ma, Sizheng, Ling Sun, and Yanbei Chen. “Black hole spectroscopy by mode cleaning.” Physical Review Letters 130.2 (2023): 141401.
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