Astronomy

• 1000 BC. Ephemeris (“journal”) tabulates the trajectories of stars and planets. First in Babylon, then the Islamic Zig in 777, which influences the Spanish Tables of Toledo in 1080. Chinese tables in 1408.
• 1543. Nicolaus Copernicus publishes his heliocentric theory.
• 1572. Tycho Brahe observes the supernova of 1572.
• 1604. Kepler’s supernova visible by naked eye.
• 1609. Kepler’s laws of planetary motion: Astronomia nova
  • Orbit is an ellipse with the sun at one focus.
  • Line from planet to sun sweeps out equal areas in equal time.
  • centripetal force mr(2π/T)^2 = gravity GmM/r^2 -> T^2 ~ r^3.
• 1608. Galileo invents the refracting telescopes with up to 30x magnification.
  • 1610. Galileo sees the Galilean moons of Jupiter: Io, Europa, Ganymede and Callisto.
  • 1610. Galileo sees two round bumps around Saturn, its rings. He sends out an anagram enciphering “I observed that the highest planet has a triple form”. Kepler decodes the anagram as “be greeted, double-knob, children of Mars”. He believed that the Mars must have two moons, since Earth has one moon and Jupiter has four.
  • Galileo discovers that Venus has phases like the moon, and sends the anagram “The Mother of Loves imitates the figures of Cynthia”, the moon. Keplers decodes it incorrectly as a red spot on Jupiter.
  • The Roman Inquisition puts him under house arrest due to his defense of heliocentrism.
• 1656. Christiaan Huygens sees the rings of Saturn, and announces it via an anagram of “is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic”.
• 1660. Hooke’s law was announced with an anagram of “as the extension, so the force”
• 1781. Uranus discovered by William Herschel.
  https://en.wikipedia.org/wiki/History_of_astronomy
  https://en.wikipedia.org/wiki/Babylonian_astronomy
  https://en.wikipedia.org/wiki/Indian_astronomy

Cassegrain reflector

Cosmology

Currently the universe is 74% expansionary dark energy (a positive vacuum energy), 22% cold dark matter, 3.6% intergalactic gas, and 0.4% stars and objects. The average density of ordinary matter is ~0.2 hydrogen atoms per cubic meter.

• Big Bang occurred 13.8 Gya.
• In the theoretical Planck epoch at 1e-43 s, all forces are unified. The universe is at the Planck length and the temperature is ~1e32 K.
• Cosmic inflation epoch from 1e-37 to 1e-33 s, expanding by 1e26 in each linear dimension. Inflation slows down after this. Produces an isotropic universe in the present at distance scales greater than 100 Mpc. At 1e-36 s, the strong nuclear force separates.
  • This strongly implies that ours is just one universe in a multiverse.
• At 1e-32 s, the universe reheats to produce a quark–gluon plasma and relativistic particles.
• At 1e-12 s, electroweak symmetry breaks.
• 1 μs Photons and neutrons form. Mass annihilation removes all antimatter, leaving just 0.01 ppm of matter. Energy density dominated by photons.
  1 s. Free electrons form, creating an opaque fog with Thomson scattering.
• 1 m. Big bang nucleosynthesis forms hydrogen and helium nuclei at 1 billion K.
  • Hans Bethe, 1967 Nobel Prize in Physics.
  • 1948. Alpher–Bethe–Gamow paper.
• 47 kyr. Energy density of matter dominates and universe is a plasma with Compton scattering of photons. Overdensities of matter cause heat pressure which drives an expanding shell of baryonic matter, the baryon acoustic oscillations (BAO). These shells expand to half a million light years (about 490 million light years today), until the plasma froze into neutral atoms and influenced the clustering of galaxies, quasars, and the intergalactic medium. Dark matter stays at the center of each shell.
• 378 kyr or redshift z = 1100. Recombination epoch. Electrons and nuclei combine into neutral hydrogen, and the universe becomes transparent.
• Cosmic microwave background radiation consists of photons emitted from recombination at 10,000 K to 3,000 K.
• 9.8 Gyr. Dark energy dominates, causing accelerating expansion of the universe.

In the cosmic distance ladder, parallax measures distances within the spiral arm. A Cepheid variable star has pulsation period proportional to luminosity. Cepheids and Typa Ia supernovae are used to measure other galaxies, and redshift is used to measure quasars and very distant galaxies.

Hubble’s law. Galaxies move away from the Earth at speeds proportional to their distance to the Earth (Hubble constant).

• Light from remote galaxies is redshifted.
• Late universe measurements based on distance ladders measure 73 (km/s)/Mpc.
• Early universe measurements based on cosmic microwave background measure 68 (km/s)/Mpc.

Stars

A nebula is a luminescent cloud of ionized, neutral, or molecular hydrogen.

• An emission nebula consists of gases ionized by high-energy ultraviolet photons from a nearby star. An H II region consists of atomic hydrogen (H+). The Orion Nebula is the brightest in the sky. In a pulsar wind nebula, flows of relativistic plasma from a pulsar create synchrotron emissions.
• A reflection nebula has carbon, iron, or nickel dust that reflect light from nearby stars.
• A dark nebula or absorption nebula consists of cold dust coated with CO and N, which are opaque to visible light but transparent to radio and infrared.

A galaxy is a system of interstellar gas and stars bound by gravity. The galaxy color-magnitude diagram plots color vs. luminosity or mass, showing:

• Red non-star forming galaxies are more elliptical, with random orbits.
  • Lenticular galaxies are disc galaxies without spiral arms. They have little interstellar matter and star formation.
• Blue star forming galaxies are more spiral, thin, dense, and rotate rapidly.

An active galactic nucleus (AGN) is the luminous accretion disk of a supermassive black hole. The most powerful are called quasars, with disks covering 1 billion square km. Microquasars have accretion disks of 1,000 square km. Seyfert galaxies are less luminous and look like spiral galaxies in visible light. The Eddington luminosity is the maximum achievable luminosity, beyond which outward radiation forces exceed gravity.
A radio galaxy has radio lobes up to 1e39 W powered by synchotron radiation from charged particles orbiting the magnetic field of the nucleus.

Stellar evolution is visualized on the Hertzsprung–Russell diagram of luminosity versus temperature. A protostar condenses from opaque H II regions. An accretion disk of diffuse material orbits the star, and friction produces infrared radiation that clears away the cloud. It becomes a true star when it ignites hydrogen fusion in its core. Population I (metal-rich) stars are younger than Population II stars, which formed in the metal-poor early universe.
In the main sequence, thermal pressure balances against gravity. Stars fuse hydrogen using the proton-proton chain below 1.5 solar mass and the CNO cycle above that mass.

• Stars smaller than the sun have a nonconvective core with a strong temperature gradient.
• Stars from 1 to 8 solar masses have a convective core and exhaust all its hydrogen at once, stopping fusion and contracting for several million years, becoming brighter and warmer. A hook occurs when the star becomes hot enough to ignite hydrogen in a shell. The star expands, dims, and cools as a subgiant.
• The largest, brightest, hottest stars are blue giants of 100+ solar mass and 5 million solar luminosity.
  A subgiant star has an inert helium core. Its hydrogen shell begins to expand and cool. After around 2 billion years, the helium core becomes degenerate and the star increases its energy production. Low-mass stars produce most carbon and nitrogen.
  Red giant branch. Stars from 0.4 to 0.9 solars eventually reach a degenerate helium core. This increases temperature causing faster fusion in its hydrogen shell. The outer atmosphere inflates, reducing the surface temperature. UV radiation from the core ionizes the atmosphere and makes the star brighter.
  Horizontal branch occurs when a red giant reaches a helium core mass of 0.5 solar and ignites helium fusion via the triple-alpha process. A degenerate core becomes nondegenerate explosively in the helium flash. The star becomes hotter and less luminous. Stellar nucleosynthesis was proposed in B2FH paper (1956).
  When a red giant runs out of hydrogen, the core collapses into a white dwarf. It ejects its outer layers to form a planetary nebula, an expanding shell of ionized gas.
  A Type II supernova is the core-collapse explosion of a star of 8-40 solar masses, emitting a spectrum containing hydrogen. When the core exceeds the Chandrasekhar limit, the star implodes within seconds, reaching 23% of c and producing elements heavier than iron. The collapse stops due to nuclear force and neutron degeneracy pressure and rebounds outward over one second, expelling most of its mass. Supernova nucleosynthesis is responsible for most elements with masses from silicon 28 to nickel 56. Cosmic rays are high-energy charged protons or nuclei from supernovas and AGNs. Supernova gamma-ray bursts are the most powerful electromagnetic bursts.
  Brown dwarfs have mass at least 0.01 solar, enough to fuse deuterium, but less than 0.08 solar. They never get hot enough to fuse hydrogen. Sub-brown dwarfs are even smaller.
  A white dwarf is a star remnant consisting of a gas of electrons supported only by electron degeneracy pressure. All quantum states are filled, so pressure comes from quantum exclusion. The mass cannot exceed the Chandrasekhar limit of 1.44 solar. After 1e15 years, it will theoretically cool to a black dwarf with minimal radiation.
  A neutron star is the remant of a supergiant star of 10 to 25 solar masses. It consists of neutrons supported by neutron degeneracy pressure. Its mass cannot exceed the Tolman–Oppenheimer–Volkoff limit of ~2.4 solar. A pulsar is a spinning neutron star that generates a magnetic field over 100 million teslas and which emits radiation from its magnetic poles. Relativistic jets are collimated outflows of ionised matter at near light speed.
  A cataclysmic variable star is a white dwarf accreting from a donor star or merging with another white dwarf in a binary system. The core can repeatedly reach carbon detonation from carbon fusion. This Type Ia supernova has a consistent peak luminosity and can be used as a standard candle.

Solar System

The Solar System formed 4.6 Ga from the gravitational collapse of a giant molecular cloud. Planets clear their orbits.

The inner solar system contains the terrestrial planets Mercury, Venus, Earth, and Mars, and the asteroid belt. The sun’s habitable zone includes Venus and Mars.
Venus has a thick carbon dioxide atmosphere causing a runaway greenhouse effect and all water to evaporate into space. Sulfur-based phototrophic life is theoretically possible in its sulfuric acid clouds.
Mars lost its magnetosphere 4 Ga, and solar wind removed most of the atmosphere, causing all liquid water to evaporate into space.

4.5 Ga The Moon forms after Theia, a Mars-sized dwarf planet, collides with the earth. Lunar rock has an identical stable isotype ratio for oxygen-16, 17, and 18. The moon forms lighter crust and mantle fragments that left the Roche limit, and has a smaller iron core. Energy from the collision causes an anomalously high angular momentum in the Earth-Moon system, a former lunar magma ocean, and vaporization of lunar volatile elements.

There are four giant planets.

The heliosphere is the 120 AU bubble of interplanetary medium inflated by the solar wind, which shields cosmic ionizing radiation.

• Trans-Neptunian objects are small solar system bodies, mostly ices of methane, ammonia, and water. The Kuiper belt is a disc from Neptune (30 AU) to 50 AU that contains most of the dwarf planets. The scattered disk reaches to 100 AU, and detached objects reach to 500 AU.
• The Oort cloud is a band of planetesimals extending from 2,000 AU to 200,000 AU (1 parsec or 3.2 ly), the limit of the sun’s gravitation influence. The cloud generates long-period comets that enter the inner solar system.
  Local Interstellar Cloud is 9.2 pc wide and has a density of ~0.3 hydrogen atoms/cm^3.
  Local Bubble is 300 pc wide and has a density of ~0.05 atoms/cm^3. It was created by supernovae around 10 mya. It is in the inner rim of the Orion Arm, about halfway down its length, 8 kpc from the galactic center.
  The Milky Way galaxy is a barred spiral galaxy 300 pc thick, 27 kpc in diameter, with a speed of 600 km/s and a density of 0.5 atoms/cm^3. It contains 100 billion stars. Sagittarius A* is the galactic center, a black hole of 4 million solar masses. Stars orbit at around 220 km/s. It is 90% dark matter extending to 613 kpc in diameter. The Zone of Avoidance is the area of sky that is obscured by the Milky Way.
• The Sagittarius Dwarf Spheroidal Galaxy is the nearest satellite galaxy at 60 kly.
• The Large Magellanic Cloud is a satellite galaxy around 1% of the mass of the Milky Way. It was a spiral galaxy before it was disrupted by Milky Wave gravity.
  The Local Group is 3 Mpc and 1e12 solar masses. It contains the Milky Way galaxy and the Andromeda Galaxy, which are 800 kpc apart and getting closer.
  The Virgo Supercluster is ~1e15 solar masses and one of 10 million in the observable universe. The Laniakea Supercluster is ~1e17 solar masses centered on the Great Attractor. The Pisces–Cetus Supercluster Complex is our galaxy filament.
  Large-scale structure
• observable universe is 93 bly in diameter.
• cosmological principle: universe should be isotropic and homogeneous at 1 bly scale. End of greatness at 0.3 bly.
• Big Ring has a diameter of 1.3 bly, 9 bly away
• Giant Arc (2021) is a 3.3 bly long, 9 bly away
• Hercules-Corona Borealis Great Wall (2013) is 10 bly long and 2σ.
  https://en.wikipedia.org/wiki/Observable_universe

Orbits

• low Earth orbit (LEO): below 2,000 km (one third Earth radius)
  • period under 2 hours
• medium Earth orbit (MEO): below geosynchronous
• geosynchronous orbit: at 35,786 km, period of 1 sidereal day
  • geostationary orbit is a circular geosynchronous orbit
• acceleration of gravity: g = -GM/R^2
• Orbital period = pendulum period: (T/2π)^2 = R^3 / GM = R/g
  • Earth mass M = 6e24 kg
  • Earth radius R = 6,370 km
  • G = 6.7e-11 N m^2 /kg^2
  • g = 9.8 m/s^2
  • T = 84 min
    https://en.wikipedia.org/wiki/Low_Earth_orbit

Orbital elements relative to the reference plane (equatorial plane). Less conveniently, an orbital state vector consists of position and velocity.

• Semi-major axis (a): average of apsis (furthest) and periapsis (nearest) distance
• Ellipse eccentricity (e)
• Orbital plane inclination (i): angle from the reference plane
  • For Earth, there is zero apigee drift at the critical inclination of 63.4°
  • A polar orbit has a 90° inclination
  • Convention is that inclination over 90° represents a retrograde orbit, moving against Earth’s rotation
• Longitude of the ascending node (Ω): where the orbit crosses the equator from south to north
• Argument of periapsis (ω): orientation of the ellipse in the orbital plane, from ascending node to periapsis
• True anomaly (ν, θ, or f) at epoch (t0): position along the ellipse
  Other bodies, air resistance, and a nonspherical primary body can perturb the orbit.

Missions

1944 Wernher von Braun develops the V-2 rocket, the first long-range guided ballistic missile and the first to reach space. 12,000 concentration camp prisoners died building 3,000 V-2s, which killed around 9,000 people. The 125 L/s turbopump increases pressure for centrifugal injection of liquid ethanol-oxygen into a mixing chamber, a converging nozzle for mixing, and the combustion chamber. The exhaust nozzle is cooled with a film of alcohol. It is guided by a Pendulous Integrating Gyroscopic Accelerometer (PIGA).
1945 Operation Paperclip hires 1,600 German engineers. It moves Wernher von Braun to NASA, where he becomes director of the Marshall Space Flight Center in Alabama in 1960.
1949 Albert II the rhesus monkey becomes the first mammal in space, aboard a US-launched V-2 rocket. He dies after a parachute failure.
1957 Sputnik 1 becomes the first artificial satellite.
March 1961 Valentin Bondarenko burns to death in a 50% oxygen altitude chamber.
April 1961 Yuri Gagarin becomes the first person to travel into space.
1961 Soyuz 1 flight. Vladimir Komarov dies after a parachute failure.
1971 Soyuz 11 depressurised in space while preparing for reentry, killing all three crew. It marks the only deaths to occur in space and the only crewed mission to the first space station, Salyut 1.

In May 1961, Alan Shepard becomes the second person to travel into space in Project Mercury using a modified Redstone rocket. Mercury sends seven people, the Mercury Seven, including Gus Grissom (2nd) and John Glenn (3rd).

Project Gemini flies 8 pairs of astronauts into low Earth orbit. It develops two-week missions, extravehicular activity (EVA) methods, and orbital maneuvers for docking.

The Apollo program lands the first man on the moon. In May 1961, Kennedy declares: “We choose to go to the Moon in this decade and do the other things, not because they are easy, but because they are hard.” The John F. Kennedy Space Center in Florida and the Johnson Space Center in Houston are built for the program.

The program chooses a Lunar Orbit Rendezvous (LOR) mission profile. It launches with the three-stage Saturn V rocket. The command and service module (CSM) orbits the moon while the Lunar Module (LM) lands on the moon and takes off. The command module (CM) is a 6 ton conical crew cabin designed for an ocean landing, with an ablative heat shield, reaction control system engines, and parachutes. The service module (SM) is a 25 ton cylinder with engines, propellant, liquid hydrogen and oxygen fuel cell, and high-gain S-band antenna for lunar communication. The 2 ton lunar module has separate descent and ascent stages, each with its own engine.

In 1967, the Apollo 1 cabin fire kills all three crew members, including Gus Grissom. During a launch rehearsal, the high-pressure pure oxygen atmosphere ignited and the plug door hatch sealed them in.

In 1969, Apollo 11 is the first moon landing. Commander Neil Armstrong declares: “That’s one small step for a man, one giant leap for mankind”. Lunar Module Pilot Buzz Aldrin joins him, describing the landscape as “magnificent desolation”.

Apollo 13. An explosion empties both oxygen tanks on the service module, which also destroys electrical power needed for propulsion and life support. Commander Jim Lovell and two other crew members transfer to the lunar module as a lifeboat. They improvise an adapter to use the crew module carbon dioxide scrubbers on the lunar module. Astronauts survive four days of cold and damp conditions while dehydrated due to scarce water.
A second stage engine shut off due to pogo oscillation, where a spike in combustion chamber pressure increases back pressure against the fuel coming into the engine, reducing fuel flow and back pressure.

Apollo 14. The abort switch was faulty, with loose solder falsely triggering the switch. The solution was to put the computer in “abort in progress” mode during landing to ignore incoming abort signals while allowing the astronauts to pilot the lunar module.

In 1973, Skylab is the first US space station, occupied for half a year.
In 1975, the Apollo and Soyuz capsules dock together in the first crewed international space mission.

The Space Shuttle program flies 135 missions with 355 astronauts to low Earth orbit. It builds and crews the International Space Station (ISS). It launches the Hubble Space Telescope and performs four service missions. It launches from the Kennedy Space Center, with first launch in 1981.
In 1986, the Space Shuttle Challenger disaster kills all seven crew after an O-ring seal failure in record-low temperatures blows up the solid rocket booster propellant tank.
In 2003, the Space Shuttle Columbia disaster kills all seven crew on reentry, after the heat shield fails. During launch, a piece of the insulating foam from an external fuel tank damaged the thermal protection tiles on the orbiter’s left wing.
In 2011, STS-135 is the final launch.

NASA Flagship missions
Planetary Science Division ($1B+)

• 1976 Viking program probes land on Mars.
• 1977 Voyager program studies Jupiter, Saturn, Uranus, and Neptune. In 2012, Voyager 1 crosses the heliopause into interstellar space. They carry the Voyager Golden Record created by Carl Sagan. Voyager has a 3.7 m dish which transmits 160 bits/s at 23 W and 8.3 GHz.
  • In 1983, the Pioneer probe is the first to leave the Solar System.
  • In 2014, the New Horizons probe studies Pluto and leaves the Solar System. These probes will likely outlive the sun.
• 1990 Ulysses probe orbits the sun after a gravity assist maneuver around Jupiter. In 1974, the Helios probes orbited the sun. In 2016, the STEREO mission observes the sun from Earth-like orbits.
• 2010 Living With a Star program. Solar Dynamics Observatory in GEO in 2010. Van Allen probes in HEO in 2012. Parker Solar Probe in 2018 is the fastest object ever built, approaching within 10 radii of the Sun. Solar Orbiter in 2020.
• 1995 Galileo probe orbits Jupiter and studies its moons and is intentionally destroyed in Jupiter’s atmosphere. It is powered by a radioisotope thermoelectric generator (RTG). Juno is orbiting Jupiter since 2016.
• Mars Exploration Program
  • 1998 Mars Climate Orbiter is destroyed after inserting too low in the atmostphere because Lockheed Martin thrust impulse calculations incorrectly returned results in imperial units.
  • 2003 Mars Exploration Rovers Spirit and Opportunity.
  • 2011 Curiosity rover in the Mars Science Laboratory mission.
  • Mars 2020 Perseverance rover and Ingenuity helicopter.
• 2004 Cassini-Huygens studies Saturn and Titan

Astrophysics (Great Observatories)

• 1990 Hubble Space Telescope: near-IR, visible, and near-UV in LEO. 7’ diameter mirror.
• 1991 Compton Gamma Ray Observatory
• 1999 Chandra X-ray Observatory
• 2003 Spitzer Space Telescope (SST): infrared

Earth Science

• remote sensing
• 1999 Earth Observing System (EOS). Landsat satellites image the Earth at 15 m resolution every 16 days. Terra satellite in LEO in 1999. ICESat in 2003. A-train includes OCO-2, GCOM-W1, and Aura (Microwave Limb Sounder MLS and Tropospheric Emission Spectrometer TES), and previously PARASOL, CloudSat, CALIPSO, and Aqua.
• 2011 Joint Polar Satellite System for weather prediction and climate monitoring
• 2015 Magnetospheric Multiscale (MMS) Mission in HEO
• 2014 European Space Agency Philae achieves the first comet landing.

L1 Lagrange point

• 1995 Solar and Heliospheric Observatory (SOHO) by the European Space Agency (ESA)
• 1997 Advanced Composition Explorer (ACE)
• 2004 Wind satellite
• 2015 Deep Space Climate Observatory (DSCOVR) launched by Falcon 9

L2 Lagrange point

• 2001 Wilkinson Microwave Anisotropy Probe (WMAP) measures CMB in radio frequency.
• 2009 ESA Planck mission measures CMB in microwave and infrared.
• 2013 ESA Gaia measures 3D position and motion of objects in extended visual bands.
• 2021 James Webb Space Telescope (JWST): infrared and red light. 21’ diameter mirror. $10B. First use of NIRISS aperture masking interferometry (AMI) in space.

In 1989, the Magellan space probe maps the surface of Venus using synthetic-aperture radar.

Optical reflecting telescopes

• Hobby-Eberly Telescope in Texas: 10 m aperture.
• Keck telescope (1993) at Mauna Kea: 10 m aperture.
  • Adaptive optics adjust for blurring from atmospheric turbulence. Tip-tilt first order corrections using a natural star. Measure higher-order atmospheric aberration using a laser guide star, then used to adjust mirror segments.
  • Aperture synthesis combines amplitude and phase using optical interferometry.
  • Speckle imaging combines many short exposures using shift-and-add image stacking. Can also apply speckle interferometry via Fourier analysis.
    • In lucky imaging, only the 10% of exposures least affected by the atmosphere are stacked.
    • Aperture masking uses around 20 small holes allowing speckle interferometry via closure phase.
• The Large Binocular Telescope (LBT) in Arizona has two 8.4 m aperture monolithic (nonsegmented) mirrors equivalent to an 11.8 m aperature with a lower diffraction limit and higher resolution.
• Event Horizon Telescope uses very-long-baseline interferometry.
• Green Bank Telescope (2000) in West Virginia is the largest fully steerable radio telescope.
• Sloan Digital Sky Survey (SDSS) (2000) is a redshift survey with a 2.5 m optical telescope in New Mexico.
• Dark Energy Spectroscopic Instrument (DESI) measures 5,000 spectra from 360 nm to 980 nm to determine redshift and distance. It maps the 3D position and velocity of 40 million galaxies over 5 years to measure the expansion history of the universe. It computes the size of the baryon acoustic oscillations (BAO) over seven time intervals. Initial results suggest that dark energy density or vehemence w is not a constant -1.

Laser Interferometer Gravitational-Wave Observatory (LIGO) observes gravitational waves.

Global Navigation Satellite System (GNSS): MEO

• Global Positioning System (GPS): 32 operational satellites and 6 spares
• Russia Global Navigation Satellite System (GLONASS)
• China BeiDou
• European Space Agency Galileo

SATCOM
Deep Space Network uses three sites

• Canberra Deep Space Communication Complex (CDSCC) in Canberra has a 70 m sterrable parabolic antenna weighing 3,000 tons which is used to communicate with Voyager 2. Earth to deep space is hard because the earth is very close in angle to the sun, which emits a large amount of radiation.
  Tracking and Data Relay Satellite System (TDRSS) supports NASA crewed flights.

apolloinrealtime.org
https://old.reddit.com/r/space/comments/11w017u/my_homebuilt_observatorygrade_telescope_that_fits/

https://en.wikipedia.org/wiki/Copernican_principle