Astronomy Notes: Eclipses and Gravitational Lensing
Astronomy Observations & Turbulance
Recent observing sessions used one of the ten-meter telescopes from the observing room.
Initial observations were affected by significant atmospheric turbulence, leading to blurry images.
The blur circle size initially ranged from to , indicating poor image quality.
Conditions improved, reducing the blur circle to to , which, while not great, was better, allowing for decent data collection.
The unit for the blur circle size is arcseconds (Sons of Arc).
The speaker noted that things worsened again after expressing satisfaction, indicating worsening turbulence.
Current Research: Neutron Stars
The research focuses on a type of star known as a neutron star.
Neutron stars are approximately times the mass of the Sun.
They are incredibly dense, equivalent to half a million Earths squished into a ball the size of a city (roughly in radius).
When spinning, their magnetic fields generate electric fields that channel and accelerate particles in opposite directions, producing beams of radiation.
The specific object of study is a "black widow pulsar" (a type of neutron star).
This pulsar gained material quickly from an orbiting companion star, causing it to spin up and become very energetic.
It "repays the favor" by evaporating away the donor star with its particles and radiation, similar to how a female black widow spider eats the male.
The goal is to measure the orbital period and speed of the visible companion star.
From these measurements, the mass of the neutron star can be determined.
Neutron stars that gain material should be more massive than average neutron stars.
The research aims to determine the maximum mass a neutron star can sustain before collapsing into a black hole (i.e., the maximum mass of a neutron star and the minimum mass of a black hole).
Radial velocity can be determined from the spectrum of the companion star, showing lines of calcium and hydrogen.
Course Announcements
Office Hour: Today, a normal office hour, no new pickles for electrocution, which will happen Friday.
Star Party: Tonight, weather permitting; a recent Monday night star party had a large turnout, featuring the Usllar telescope (handled by Ben) and the Trefors telescope (handled by Anastasia).
Pizza Winners: Will arrange dinner at some point.
Quiz: Next week in discussion sections.
Will not cover material from the two lectures immediately preceding the discussion section.
Everything before those two lectures is fair game.
This allows students time to study the most recent material before a quiz, midterm, or final.
Format: Short answer, problem-solving, explanation (not true/false or multiple choice, as those are reserved for midterms and finals).
Homework Three: Due on Friday.
Talk Sessions: Optionally available tonight and tomorrow (late afternoon/evening) for homework help.
Total Solar Eclipses: Phases and Phenomena
Beginning (Non-Eclipse): Viewing the Sun through a filter, appearing yellow.
Partial Phases: The Moon gradually covers the Sun, taking about hour and minutes to completely block the photosphere (the bright disk of the Sun).
Diamond Ring Effect: Just before totality, the last tiny bit of the Sun's bright disk looks incredibly bright (overexposed in photos), forming a "diamond" against the dark sky, revealing the inner corona, chromosphere (red region), and prominences (tongues of gas). This is considered the most special part by the lecturer.
Totality:
The thin chromospheric layer and prominences are visible.
The corona extends quite far out; its shape changes due to varying solar magnetic field configurations, which charged particles follow. This unpredictability is a major attraction of eclipses.
Satellites observe the outer corona , but the lecturer avoids these images before an eclipse to preserve the surprise.
The sky darkens significantly, creating -degree twilight colors visible all around. This is because nearby atmospheric regions still see a partially eclipsed Sun (e.g., \%) and scatter sunlight, similar to sunrise/sunset, where violets, blues, and greens are scattered/absorbed, leaving yellows, oranges, and reds.
Bright planets (Jupiter, Venus) and some stars become visible.
Totality durations vary: shortest observed was seconds, theoretically longest is minutes (very rare), longest experienced by speaker was minutes.
Second Diamond Ring Effect: As totality ends, the Moon moves away, and the Sun's light reappears, creating a second diamond ring.
Post-Totality Partial Phases: Typically less dramatic, lasting about hours, with observers often celebrating.
Total Solar Eclipses: Observation Safety
Partial Phases: Requires a certified filter (e.g., CE-certified, blocking all but part in of visible light and \% UV/IR) or a pinhole camera to prevent eye damage.
Totality: It is safe and recommended to look directly at the totally eclipsed Sun without a filter, as the corona is very faint (less light than a full Moon) and emits no harmful rays. Binoculars or a telescope can also be used.
Diamond Ring: The lecturer suggests it's personally okay for students (with instructions) to look at the diamond ring with the unaided eye for one or two seconds immediately before and after totality.
This contradicts the official recommendation of the American Astronomical Society, which advises against it due to concerns about the general public staring for too long.
Using binoculars or a telescope during the diamond ring is extremely dangerous due to their light-gathering ability, which can burn the retina.
The lecturer advises exercising caution, especially before totality, as timing is difficult; it's better to miss the first diamond ring than risk eye injury by removing the filter too soon.
For the second diamond ring, observers should be prepared to quickly reapply filters or look away after a second or two.
Solar Eclipse Frequency and Travel
Total solar eclipses are rare at any given point on Earth, occurring roughly every years on average.
Therefore, one usually has to travel to witness one.
It's recommended to combine eclipse viewing with a longer trip (e.g., weeks) to an interesting location, so that if the eclipse is clouded out, the trip is still worthwhile.
Example Paths: Australia has several upcoming eclipses (2028, 2030, 2037, 2038).
The 2017 US eclipse was the only one in a -year period for the continental US.
A 2021 eclipse in Antarctica was clouded out, interesting to the speaker (first cloudy eclipse experience) but disappointing for first-time viewers.
The 2017 US eclipse was described as much more dramatic than anticipated by those who saw it.
Future US eclipses: 2044 (Montana, North Dakota), 2045 (Northern California to Florida).
The lecturer humorously threatens to retroactively fail students decades later if they had an opportunity to see a total solar eclipse within a couple hundred miles of the path of totality but didn't go, unless they had a wedding (in which case, they should rethink their priorities).
Annular Solar Eclipses
Defined as a special case of a partial eclipse.
Occurs when the Moon is farther than average from Earth in its elliptical orbit (apogee), making it appear smaller than the Sun's disk.
Instead of covering the Sun completely, the Moon creates a "ring of fire" (annulus) around its silhouette.
Earth's orbit around the Sun is also elliptical, causing the Sun's apparent size to vary (bigger around January 4th, smaller around July 4th).
This variation can also contribute to making the ring wider if the Moon is far and the Sun appears large.
Annular eclipses last longer than total eclipses, about hours from start to finish.
Terms like "perigee" (closest point) for the Moon lead to longer total eclipses; "apogee" (farthest point) leads to annular eclipses.
While interesting (e.g., creative photography by Dennis Mamana and Stephan Sip), they are not total eclipses, and the lecturer maintains that seeing a total eclipse is essential.
A recent annular eclipse was seen in parts of the US in 2023 (e.g., San Antonio, Texas).
Future US annular eclipses: February 5th, 2046, and June 11th, 2048.
Lunar and solar eclipses are entirely predictable to the second and kilometer, as orbital mechanics are well understood, unlike weather.
Eclipse Patterns & Predictability
While eclipse paths might appear random on a map, they are entirely calculable based on orbital mechanics.
Newton could have calculated these paths with the aid of computers.
Saros Cycle: Paths repeat approximately every .
The extra hours (one-third of a day) means the Earth rotates an additional degrees in longitude.
Thus, the overall eclipse path repeats but is shifted by degrees in longitude.
Examples include the 2021 and 2039 eclipses, which show a similar S-shaped pattern shifted.
Path Width Variation: The thickness of the path of totality differs due to the Moon's variable distance from Earth.
When the Moon is closer to Earth, the shadow cone is wider on the Earth's surface, leading to a longer and wider path of totality.
When the Moon is farther, the shadow is narrower or just barely touches Earth (making for shorter/annular eclipses).
Apparent widening near the poles in Mercator projection maps is partly an artifact of the projection (like the exaggeration of Greenland or Antarctica's size), but also a real effect due to the geometry of the shadow skimming the Earth's curved surface at a shallow angle, creating an oval patch.
Historical Significance: Einstein's General Relativity
Eclipses are crucial for studying the Sun's corona.
The most significant historical use was providing the first test of Einstein's General Theory of Relativity in 1919.
General Relativity Concept (Qualitative): Mass (or energy) warps the fabric of spacetime (space and time combined).
Space is not flat; it's curved or bent by massive objects like the Sun.
Light rays follow natural paths through this warped spacetime.
Prediction: Einstein predicted that light from stars, passing close to the Sun, would be bent by the Sun's gravity.
This bending would make the apparent positions of stars (as observed from Earth) appear slightly offset from their true positions.
The degree of displacement would be greater for stars whose true positions are closer to the Sun.
The Problem: Normally, stars cannot be seen during the day due to the bright sky scattered by Earth's atmosphere, making the prediction seem untestable.
The Solution: Total Solar Eclipse: During a total solar eclipse, the Moon blocks the Sun's bright disk, creating a dark sky on Earth, allowing stars to be visible and photographed.
Eddington's Experiment (1919): Arthur Eddington organized two expeditions (Africa and Brazil) to photograph stars in the direction of the Sun during a total solar eclipse.
These photographs were compared to earlier photographs of the same stars taken six months earlier at night (when the Sun was not in that part of the sky and could not bend the light).
The initial data, though not perfectly confident, suggested that stars were indeed displaced as predicted by Einstein.
Confirmation (1922): Astronomers from Lick Observatory (Berkeley) organized an expedition to Australia in 1922, reproducing and improving on Eddington's setup.
They more accurately confirmed Eddington's results, verifying Einstein's prediction.
Significance: This was a crucial scientific triumph.
It validated a new theory (General Relativity) not just by explaining known phenomena but by successfully predicting the outcome of future observations/experiments.
This predictive power is a hallmark of robust scientific theories.
Science aims for increasingly accurate models of the universe that agree with observations and experiments, rather than claiming absolute "Truth" or "Reality."
Total Lunar Eclipses
Definition: A "true" eclipse, where one object (the Moon) passes into the shadow of another object (Earth).
A solar eclipse is technically an "occultation" (when one object blocks another of comparable angular size) or a "transit" (for a small planet blocking a star) rather than the Sun being in a shadow.
Appearance:
The Moon gradually enters Earth's curved shadow; this curvature was an early indication of Earth's spherical shape.
During totality, the Moon can still be seen and often appears a shade of yellow, orange, or red.
Reason for Redness: Sunlight that skims Earth's atmosphere is refracted (bent) and scattered.
Violets, blues, and greens are scattered much more significantly by the atmosphere (explaining blue sky).
This leaves yellows, oranges, and reds to pass through the atmosphere, where they are bent towards the Moon.
These "warmer colors" then illuminate the Moon, reflect back to Earth, making the Moon appear red.
Atmospheric particles (wildfires, volcanic eruptions, dust storms) enhance this effect by scattering/absorbing blues/greens even more.
The effect is similar to sunset/sunrise colors, but the light passes through Earth's atmosphere twice (to the Moon, then reflected back to Earth), enhancing the scattering/absorption.
Variability: The darkness and specific shade of red vary, depending on how deep the Moon goes into the shadow (closer to the center means darker) and the amount of particulate matter in Earth's atmosphere.
Visibility:
Visible from the entire dark (nighttime) side of Earth because the full Moon is up at night.
Intrinsically about as rare as total solar eclipses, but much more likely to be seen by an individual because it's visible over a much wider area (approx. \% of Earth's surface) and lasts longer (up to an hour or more).
Frequency:
Occurs only during a full Moon.
Does not happen every full Moon due to the Moon's orbital tilt (approx. ) relative to Earth's orbit around the Sun.
An eclipse only occurs when the Moon is at the intersection point of its orbital plane and Earth's orbital plane (the "node") and it is a full Moon.
Minimum Time Between Solar and Lunar Eclipse:
A solar eclipse happens at new Moon.
The soonest a full Moon can occur is two weeks later (half the lunar orbital cycle).
Therefore, the minimum time between a solar and a lunar eclipse is two weeks.