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# astronomyessentials (Essentials ** )

### Essential astronomical information

Essential information:

• 1 mile = 1.6 kilometers (multiply the number of kilometers by 2/3 to get the equivalent number of miles).
• 1 astronomical unit (AU) = the average Earth-Sun distance = 93,000,000 miles.
• 1 light-year = distance traveled by a photon of light in 1 year, traveling at 186,000 miles per second = 6 trillion miles.
• 1 light-year = 63,000 AU.
• 1 par-sec (PC) = 3.26 light years.
• 1 kiloparsec = 1000 PC = 3,260 light years.
• Brightness ratio = (2.512)^magnitude difference, or I↓a/I↓b = (2.512)^M↓b – M↓a
• Magnitude difference and brightness ratio:  0 – 1, 1 – 2.5, 2 – 6.3, 3 – 16, 4 – 40, 5 – 100, 6 – 250, etc.
• The orbits of planets are elliptical.
• Planetary nebulae are glowing shells of gas, the gently-expelled remains of aged stars.
• Angles are divided into degrees, minutes, and seconds.  There are 360 degrees in a circle, 60 minutes in a degree, and 60 seconds in a minute.
• A quarter viewed face-on from 100 years equals 1 minute of arc.
• A dime viewed from edge-on from 100 yards equals 7 seconds of arc.
• A piece of paper viewed edge-on at arms’ length equals 30 seconds of arc.
• The Sun and the Moon are approximately 0.5 degree or 30 minutes in diameter.
• The pointer stars on the Big Dipper are 5 degrees apart.  The bowl of the Big Dipper is 30 degrees from Polaris.
• An arms’ length hand span is about 15 to 20 degrees on the sky.
• An arms’ length fist span is about 10 degrees on the sky.
• Our Solar System is moving at 130 miles per second or 470,000 miles per hour.
• Since the Earth completes 1 revolution of 360 degrees around the Sun every year in 365 days, a line from the Sun to the Earth sweeps through about 1 degree per day.
• The solar day we use in every day life is about 4 minutes longer than the true period of Earth’s rotation, which is a sidereal day (a day measured with respect to the distant stars).
• EQUINOX: the two times each year when the Sun appears to cross Earth’s equator.  It is the time when day and night are everywhere of equal length.  The spring equinox is March 21 and the autumn equinox is September 23.
• SOLSTICE: the times when the Sun is farthest north or south of Earth’s equator.  The summer solstice is June 22 and the winter solstice is December 22.
• On a clear night, you can see stars down to the 6th magnitude with the naked eye.  Large telescopes can see stars down to the 25th magnitude.
• The three fundamental forces in the universe, along with their relative strengths, are: 1) nuclear force   2) electromagnetic force   3) gravitational force .
• 90% of all the matter in the universe is hydrogen, and hydrogen and helium make up 99% of all the matter in the universe.
• The planets formed from condensation of the solid matter that circled our newly-formed Sun.  Moons were condensations of matter around the new planets.
• Rotation is the turning of a celestial body around an axis, such as the Earth spinning on its own axis.  Revolution is the motion of a body around a point located outside the body, for example, a planet revolving around a Sun.
• Three definitions of the ecliptic:
• The apparent path of the Sun around the sky
• The projection of the Earth’s orbit on the sky
• The plane of the Earth’s orbit
• The Sun traveling 360 degrees around the ecliptic in 365 days, travels 1 degree eastward every day (relative to the stars).  The Sun is 0.5 degrees in diameter, so it travels 2 times its diameter every day.
• Because the Earth is tipped 23.5 degrees relative to its orbital plane, the ecliptic is tipped 23.5 degrees relative to the celestial equator.
• Precession is the slow change in the direction of Earth’s axis of rotation.  It is caused by the gravitational pull of the Sun and the Moon, and because Earth is not a perfect sphere.  Due to precession, Vega will be the polar star 13,000 years from now.
• Two reasons why summers are warmer than winters:
• The summer Sun is above the horizon for more hours each day than the winter Sun
• The summer Sun stands higher in the sky and we receive sunlight at a more direct angle
• The season are not related to the variations in the Earth-Sun distance.  Remarkably, Earth reaches perihelion, its closest point to the Sun, on January 3 and aphelion, its furthest distance from the Sun, on July 6.
• A series of 13 zodiacal constellations lie along the ecliptic, but Ophiuchus is ignored.  “Zodiac” comes from the Greek ‘zodiakos kyklos’, which means ‘circle of animals’.  All zodiac constellations are named after living creatures except Libra (the Scales).
• The planets move counter clockwise in their orbits, and the farther from the Sun they are, the slower they travel.
• PARALLAX: The change in the apparent position of an object due to a change in the position of the observer.
• The luminosity of a star depends on its temperature and diameter (surface area).
• Iron has the most tightly-bound nucleus of all elements, so it and any elements with heavier nuclei cannot release energy by fusion.
• Two pounds of hydrogen converted entirely to energy would produce enough power to raise an average mountain 6 miles into the air.
• The average neutrino could pass unhindered through a lead wall one light-year thick.
• 10100 = 1 googal         and 10 raised to the googal power equals one googal plex
• Auroras occur when electrons, which flow away from the Sun in the solar wind, spiral down the magnetic field lines of the Earth.  In other words, auroras are produced when a planet’s magnetic field guides charged particles down into its atmosphere.
• Kepler’s Laws of planetary motion:
• The orbits of the planets are ellipses with the Sun at one focus point
• An imaginary line drawn from the planet to the Sun sweeps through equal areas in equal periods of time
• A planet’s orbital period squared is proportional to its average distance from the Sun cubed (p2 years = a3 in AU)
• Light is one form of electromagnetic radiation.  It travels through space at 186,000 miles per second.  A photon is a “particle” of light, or bundle of waves.  The shorter the wavelength (w) the more energy the photon contains (E = hc/w).
• The highest part of our atmosphere absorbs x-rays, gamma rays, and some radio waves.  The ozone layer absorbs ultraviolet radiation.  Water vapor absorbs infrared radiation.
• 1 angstrom = 10-10 meter, 1 nanometer = 10-9 meter, 10 angstroms = 1 nanometer, 1 micron (micrometer) = 10-6 meter, visible light has wavelengths between 4000 and 7000 angstroms.
• X-rays are high-energy photons that are produced by violent space events.
• Only in the coolest stars are collisions gentle enough to allow chemical bonds.
• Electrons can only occupy certain, permitted energy levels or orbits.
• 21 centimeter radiation, which is so important in radio astronomy, comes from hydrogen atoms whose electrons are at ground state.  Thus only cool unexcited hydrogen, such as that in cold interstellar space, can emit 21 centimeter photons.
• The spectra of stars cooler than 3000 degrees Kelvin contain dark bands produced by molecules such as titanium oxide.  These molecules can absorb photons at many wavelengths, producing numerous closely-spaced spectral lines that blend together to form bonds.  Only in cool stars can these molecules avoid the collisions that would break them up in hotter stars.
• Spectral lines of a star approaching Earth are slightly shifted toward blue, and spectral lines of a star receding from Earth are slightly shifted toward red.  This is known as the Doppler effect.
• Nearly all stars have a composition similar to our Sun, that is, 92% hydrogen, 7.8% helium, and traces of heavier elements.
• Parallax and distance — Parallax is the apparent change in the position of an object due to a change in the position of the observer.  The nearest star to our Solar System, Alpha Centauri, has a parallax of only 0.76 seconds of arc.  The distance (in parsecs) = 1 / p (stellar parallax).  One parsec = 206,265 AU or 3.26 light-years.  Since we cannot measure p when it is smaller than 0.01 seconds of arc, the farthest stars whose distance we can measure by parallax is approximately 100 parsecs away.  Half a million stars are within this distance, but only 10,000 have had their parallaxes measured.  The Hubble space telescope can measure parallax to 0.001, and distances to 1000 parsecs.
• The flux of energy we receive from a light source depends inversely on on the square of the distance to the source (doubling the distance equals a drop in flux by 22, or 4.
• Absolute visual magnitude is the apparent visual magnitude a star would have if located 10 parsecs away.
• Luminosity  is the total amount of energy a star emits per second (all wavelengths of energy).  Our Sun emits 4 x 1026 joules per second, and Canopus emits 4 x 1028 joules per second.