Ch 2. User s Guide to the Sky Patterns and Cycles PDF

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Ioannis John Haranas...

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1.2.0 User’s Guide to the Sky: Patterns and Cycles 4 Questions ❖ How can you refer to stars by name, contellation, and brightness? ❖ How does the sky appear to move as Earth rotates and revolves ❖ What causes the seasons ❖ How does the motion of the moon produce…..?

1.2.1 The Stars Constellations ❖ Constellation: stellar patterns identified by name (usually mythical gods, people, animals, objects) ➢ Rep viewing direction/section of the sky ❖ Western culture constellations have Mesopotamian origin (~5000 years ago): Babylon, Egypt, Greece ❖ Asterism: grouping of stars that is not part of the official constellations (eg. Big Dipper, Great Square of Pegasus) ❖ Refer to Fig. 2.1 in tb

The Names of the Stars ❖ Names of constellations are in Latin or Greek ❖ Derived from ancient Arabic ❖ Another way to identify: Greek letters ➢ By brightness ■ Brightest (α) ■ Second brightest (β) ■ Etc. ➢ Eg. alpha Canis Majoris vs Sirius ❖ Ancient star names still in use by modern astronomers

Alpha Centauri system ❖ Brightest star in southern constellations of Centaurus

The Brightness of Stars ❖ Measured using magnitude scale ❖ Magnitude scale: astronomical brightness scale; larger the number, the fainter the star ❖ 6 brightness groups ➢ First-magnitude = brightest (100x brighter than 6th) ➢ Sixth-magnitude = faintest ❖ Hipparchus from Rhodes (190-120 BCE): first to categorize into brightness ❖ Claudius Ptolemy (140 CE): also used magnitude system

❖ Mid 19th century: discovery that brightness = intensity of light produced by star ❖ Apparent visual magnitude (mv): measure of brightness of star as seen by human eyes on earth ❖ Limitations of avm have prompted the creation of supplements ➢ Stars too bright extends into the negative scale ➢ Some stars emit ultraviolet/infrared lights that are invisible ■ Subscript v reminds us that we should only count light that is visible ❖ Flux: a measure of the flow of energy out of a surface ➢ F=L/(4πr2)

1.2.2 The Sky and Its Motions The Celestial Sphere ❖ Imaginary sphere of very large radius surrounding Earth to which celestial objects seem to be attached ❖ Important point ➢ Westward motion of sky = eastward rotation of Earth ➢ Appearance of sky depends on observer’s location ➢ Astronomical distances are measured in degrees ❖ Scientific model: concept that helps one think about some aspect of nature, but it isn’t necessarily true ❖ Zenith: point in sky directly above observer ❖ Nadir:point on the celestial sphere directly below observer; opp. of zenith ❖ Circumpolar Stars: stars that don’t rise or set ❖ Rotation: motion around an axis passing through a rotating body ➢ Eg rotation of earth ❖ Revolution: orbital motion about a point located outside the orbiting body ➢ Eg. revolution of Earth around the Sun (yearly cycle)

Procession ❖ Procession: the slow change in orientation of Earth’s axis of rotation that is gravity induced ❖ Precession cycle: 26k years discovered by Hipparchus

1.2.3 The Cycle of the Sun The Seasons ❖ Not caused by variation in the distance b/t Earth and Sun ❖ Caused by solar energy changes that Earth’s north and south hemispheres receive at different times of year ❖ Ecliptic: apparent path of the Sun around the sky ➢ 1TW=1012W

❖ Sun appears to move W → E on celestial sphere ➢ But in sky: E → W ❖ Celestial equator: imaginary line around sky directly above Earth’s equator ❖ North and South celestial poles: points on celestial sphere directly above Earth’s N and S poles !! look up solstice and equinox for sun cycles!!

1.2.4 The Cycles of the Moon ❖ Important points ➢ Moon keeps same side facing earth ➢ Changing shape of moon passing through its cycle of phases is produced by sunlight illuminating diff parts of the side of the moon as seen by an observer ➢ Orbital pd of moon around Earth is not the same as the length of a Moon phase cycle ❖ Location: geo longitude and latitude on Earth ❖ Waxing = “growing”; Waning = “shrinking”

1.2.5 Eclipses Solar Eclipses ❖ Sun and Moon have nearly equal apparent diameters ❖ Solar eclipse: sun is hidden (eclipsed), moon is “half way”/in front of sun ➢ 5 min. max. ❖ Umbra: region of shadow that is totally shaded ❖ Penumbra: portion of shadow that is only partially shaded ❖ Lunar eclipse: darkening of the Moon when it moves through Earth's shadow ➢ Darkens and turns copper-red bc of sunlight refracted through Earth’s atmosphere ➢ Always occurs at full moon, but not at every full moon ❖ Saros cycle: An 18-year, 111/3 - day period, after which the pattern of lunar and solar eclipses repeats

1.2.6 Stellar Coordinates ❖ Celestial sphere uses coordinate grid similar to longitude and latitude system on Earth ❖ Stellar coordinates: angular distances of an object on the celestial sphere ➢ Describes locations of celestial objects in the sky ❖ Declination � (degrees): angular N→S distance measured from the celestial equator ❖ Right ascension α(hr, min, sec.): angular E→W distance measured from the vernal equinox ❖ Centauri coordinates: α= 14hr 29min 42.948s, = -62 ° 40’ 46.141”

1.2.7 Timekeeping Timekeeping by Day ❖ Solar day: avg time b/t successive crossings of the Sun on the local meridian (24 hr) ❖ Sidereal day: time b/t successive crossings of any star on the local meridian (23hr 56min 4.09s) ❖ Earth moves about 1° per day around its orbit ❖ Synodic month: time for a complete cycle of lunar phases (~29.5 days) ❖ Sidereal month: time for Moon to orbit Earth once relative to any star (~27.3 days) ❖ Tropical (solar) year: time b/t successive spring (or autumnal) equinoxes ❖ Sidereal year: time for complete orbit relative to the stars ❖ Tropical year → 365.25 days minus 11 minutes ➢ Minus 11mn due to precession of equinoxes, seasonal cycle does not remain exactly synchronized eith the position of the Earth in its orbit around the Sun ❖ 365 days/year - Egyptian concept ❖ “Leap year” introduced by Julius Caesar (Julian calendar = old calendar) ❖ 1582: pope gregory xii introduced a slight adjustment - gregorian calendar used globally today...