The Cycles of the Sky

Celestial Sphere Imaginary dome/sphere surrounding the Earth Horizon – where the sky meets the ground Serves as a simplified model for the placement of stars and planets Stars all appear to be the same distance, but ARE NOT

Constellations Fixed patterns of stars Ancients made pictures of the patterns and related them to certain events of different seasons Sun in Pisces & Aquarius = dangerous sailing Sun in Virgo = harvest time Ex. Big Dipper, Little Dipper, Orion

Celestial Poles 2 points of sphere that do not move Lie directly above the North & South Poles Polaris (North Star) lies very near North Pole Used for navigation because it doesn’t move Stars move from east to west (think Sun) Ancients thought the celestial sphere was turning Actually caused by the Earth’s rotation (spinning) Move counterclockwise around the Celestial poles

Celestial Equator Lies above the Earth’s equator Here stars move due east to due west Northern hemisphere – stars move northeast to northwest Southern hemisphere – stars move southeast to southwest Some stars are far enough north/south that they can not be seen in the other hemisphere North Star, Southern Cross

Annual Motion of Sun (yearly cycle) Constellations change with the seasons due to the Earth moving (revolving) around the Sun Any stars in the celestial sphere toward the Sun are blocked by its glare Ancients used appearance of certain stars/constellations to order their lives Ex. Sirius near Sun told Egyptians the Nile would be rising Ex. Same stars are always present when it’s time to plant crops – They didn’t rely on weather, it can be tricky

Ecliptic The line the sun traces on the celestial sphere It passes through 12 constellations called the zodiac Greek zoidion = “little animal”; kyklos = “circle” Aries (ram), Taurus (bull), Gemini (twins), Cancer (crab), Leo (lion), Virgo (maiden), Libra (scales), Scorpion, Sagittarius (archer), Capricornus (seagoat), Aquarius (water bearer), Pisces (fish)

Seasons - Cause Earth spins on its Rotational axis – imaginary line from N to S pole Axis isn’t perpendicular (90) to sun Tipped 23.5 from vertical Tilt remains constant throughout Earth’s revolution Portion tilted toward sun receives more direct light and more hours of light Seasons are opposite in Northern & Southern Hemispheres

Seasons – Ecliptic Tilt The celestial equator and the sun’s ecliptic differ due to the Earth’s tilt Equinox (“equal night”) – when sun is on the celestial equator Day & night are equal length Marks beginning of spring (vernal) & fall (autumnal) Usually March 20 & September 22 Solstice – sun is at its extreme north/south position & pauses to change direction Sun lies 23.5 north/south of celestial equator Marks beginning of summer & winter Usually June 21 & December 21 Lag of seasons – coldest/warmest is usually 6 weeks after solstice due to time for land/water to change temperature

Seasons – Sun’s Position Zenith – point directly overhead Ancients built structures/temples that aligned w/ Sun’s movement From winter to summer solstice – sun travels north; from summer to winter solstice – sun travels south Causes a change in sunrise & sunset

Seasons – Sun’s Position At a North latitude of 40 To determine how far above horizon At summer solstice 40-23.5 = 16.5 away from zenith 90-16.5 = 73.5 above horizon At winter solstice 40+23.5 = 63.5 away from zenith 90-63.5 = 26.5 above horizon At equinox 90-40 = 50 above horizon

Moon – Lunar Phases Caused by Moon’s orbital motion around Earth About 29 ½ days – origin of “month” ½ of Moon is always lit by sun Phase is caused by our position compared to the Moon’s position with the Sun

Moon – Lunar Phases New Moon – btw us and Sun We can’t see the lit half Will rise about sunrise (6am) & set about sunset (6pm) Waxing Crescent 1st Quarter – seen about 12pm – 12am (noon-midnight) Waxing Gibbous Full Moon – we’re btw Sun & Moon We see all of the lit half Seen btw sunset (6pm) & sunrise (6am) Waning Gibbous 3rd Quarter – seen btw 12am – 12pm (midnight-noon) Waning Crescent

Eclipses Lunar eclipse – Earth moves btw Sun & Moon and shadows the Moon Solar eclipse – Moon passes btw Sun & Earth and blocks Sun Annular eclipse – Moon is farther away from Earth & won’t cover entire sun Partial eclipse (only a portion is covered) vs Full eclipse Only occur when the Moon’s orbit aligns w/ the Earth’s It is actually tipped compared to us Known as eclipse seasons – twice a year