https://teddyrookbook.com/physics/astronomy/Recent content in Astronomy on Teddy Rook Bookhttps://teddyrookbook.com/img/share.jpghttps://teddyrookbook.com/img/share.jpgHugoenhttps://teddyrookbook.com/physics/astronomy/celestial-sphere/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/celestial-sphere/The sky looks like a dome that turns once a day around the celestial poles, because Earth spins. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/altitude-latitude/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/altitude-latitude/The North Star's altitude equals your latitude, and stars close enough to the pole never set. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/celestial-coordinates/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/celestial-coordinates/Declination and right ascension are the sky's latitude and longitude, fixed to the stars. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/solar-sidereal-day/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/solar-sidereal-day/Earth spins once in 23 h 56 min, but noon to noon takes 24 h because Earth also moves along its orbit. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/seasons/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/seasons/The Sun's declination swings between +23.4° and −23.4°, which sets the solstices, the equinoxes, the tropics and the polar circles. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/sun-noon-altitude/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/sun-noon-altitude/The Sun's altitude at noon is 90° − latitude + the Sun's declination. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/moon-phases/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/moon-phases/Each phase rises at its own time: a full Moon rises at sunset and a first-quarter Moon about noon, because of where the Moon sits relative to the Sun. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/synodic-month/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/synodic-month/The phases repeat every 29.5 days, longer than the Moon's 27.3-day orbit, because Earth moves along its orbit too. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/eclipses/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/eclipses/The Moon's orbit is tilted about 5°, so eclipses happen only near the two nodes, in eclipse seasons about six months apart. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/heliocentric-model/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/heliocentric-model/Retrograde loops and the phases of Venus favor Copernicus's Sun-centered model. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/kepler-first-law/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/kepler-first-law/Planets move on ellipses with the Sun at one focus, and eccentricity measures how stretched each one is. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/kepler-second-law/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/kepler-second-law/A planet sweeps out equal areas in equal times, so it moves fastest when nearest the Sun. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/kepler-third-law/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/kepler-third-law/P² = a³ when the period is in years and the size of the orbit is in astronomical units. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/weighing-with-orbits/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/weighing-with-orbits/Newton's form of Kepler's third law, M = a³/P² in solar masses, AU and years, gives the mass at the center. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/orbits-escape/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/orbits-escape/A low circular orbit needs about 7.9 km/s at Earth, and escaping takes √2 times the circular speed. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/tides/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/tides/When the Sun and Moon line up, their tides add to make spring tides; at the quarter Moons they partly cancel, making neap tides. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/light-travel-time/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/light-travel-time/Light takes time to reach us (8.3 minutes from the Sun), so the farther we look, the older the view. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/color-temperature/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/color-temperature/Hotter objects glow bluer: the peak wavelength is λ_peak = 2.9 × 10⁶ nm·K ÷ T. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/luminosity-radius-temperature/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/luminosity-radius-temperature/A star's power is L = 4πR²σT⁴, so in solar units L = R²T⁴. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/spectral-lines/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/spectral-lines/Hot, dense objects give a continuous spectrum; thin gases give bright lines or, in front of a hotter source, dark lines. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/doppler-light/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/doppler-light/A spectral line's shift gives the speed toward or away from us: Δλ/λ = v/c. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/telescopes/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/telescopes/A telescope's light-gathering power grows with the square of its diameter, and a bigger aperture shows finer detail. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/observing-the-spectrum/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/observing-the-spectrum/Radio, infrared, X-ray and gamma-ray telescopes see what eyes can't, and some must fly above the air. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/planet-families/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/planet-families/A planet's mass and radius give its average density, about 4 to 5.5 g/cm³ for rocky planets and 0.7 to 1.6 g/cm³ for the giants. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/solar-system-formation/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/solar-system-formation/A spinning cloud flattened into a disk; rock and metal condensed near the Sun and ices farther out. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/small-bodies/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/small-bodies/Comets on long, stretched orbits grow tails that point away from the Sun, and Earth passing through their dust makes meteor showers. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/exoplanets/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/exoplanets/A passing planet dims its star by (R_planet/R_star)², and a planet's tug makes its star wobble. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/sun-layers/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/sun-layers/Energy made in the core moves out through the radiative and convective zones to the glowing photosphere. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/sun-fusion/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/sun-fusion/Fusing hydrogen into helium turns about 0.7% of the mass into energy, by E = mc². Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/sun-lifetime/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/sun-lifetime/Fuel divided by the rate of use gives the Sun about 10 billion years on the main sequence. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/solar-activity/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/solar-activity/Magnetic fields make cooler, darker sunspots, and solar activity rises and falls about every 11 years. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/parallax/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/parallax/A star's distance in parsecs is 1 divided by its parallax angle in arcseconds. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/brightness-distance/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/brightness-distance/Apparent brightness falls off with the square of the distance: b = L/(4πd²). Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/magnitudes/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/magnitudes/Smaller magnitudes are brighter, and 5 magnitudes means 100 times the brightness. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/spectral-classes/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/spectral-classes/The classes O, B, A, F, G, K and M sort stars from hottest to coolest by their spectral lines. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/hr-diagram/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/hr-diagram/Plot luminosity against temperature to sort stars into the main sequence, giants and white dwarfs. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/binary-stars/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/binary-stars/Two stars orbiting each other reveal their masses, and the main sequence turns out to be a sequence of mass. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/star-formation/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/star-formation/Gravity pulls cold clouds of gas and dust into protostars until their cores are hot enough for fusion. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/main-sequence-lifetime/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/main-sequence-lifetime/A star's lifetime goes as its fuel over its luminosity, about M^(−2.5), so massive stars die young. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/red-giants/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/red-giants/When core hydrogen runs out a star swells into a red giant; a Sun-like star ends as an Earth-sized white dwarf. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/star-cluster-ages/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/star-cluster-ages/All stars in a cluster were born together, so the point where the main sequence ends gives its age. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/supernovae/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/supernovae/Massive stars fuse elements up to iron, then their cores collapse and explode, spreading heavy elements into space. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/neutron-stars/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/neutron-stars/A collapsed core can become a city-sized ball of neutrons spinning many times a second. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/black-holes/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/black-holes/Within the event horizon, R = 2GM/c², not even light can escape, yet far away a black hole pulls like any other mass. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/milky-way/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/milky-way/Our galaxy is a disk of a few hundred billion stars, with the Sun about 26,000 light-years from the center. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/galaxy-types/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/galaxy-types/Spiral, elliptical and irregular galaxies differ in shape, gas and the ages of their stars. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/dark-matter/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/dark-matter/Stars and gas far out in galaxies orbit too fast for the mass we can see, so most of a galaxy's mass is dark. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/standard-candles/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/standard-candles/Cepheid variables and type Ia supernovae have known luminosities, so how bright they look tells how far away they are. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/hubble-law/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/hubble-law/Distant galaxies recede at speeds proportional to their distance: v = H₀d. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/age-of-universe/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/age-of-universe/Running the expansion backward, 1/H₀ gives an age of about 14 billion years. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/big-bang-evidence/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/big-bang-evidence/The expansion, the cosmic microwave background and the amounts of hydrogen and helium all point to a hot, dense early universe. Astronomy · Physics Ladderhttps://teddyrookbook.com/physics/astronomy/dark-energy/Mon, 01 Jan 0001 00:00:00 +0000https://teddyrookbook.com/physics/astronomy/dark-energy/Distant supernovae show the expansion is speeding up, and we call the unknown cause dark energy. Astronomy · Physics Ladder