Some imaginationrequired!

When I Heard the Learn'd Astronomerby Walt Whitman

When I heard the learn'd astronomer,When the proofs, the figures, were ranged in columns

before me,When I was shown the charts and diagrams, to add,

divide, and measure them,When I sitting heard the astronomer where he lectured

with much applause in the lecture-room,How soon unaccountable I became tired and sick,Till rising and gliding out I wander'd off by myself,In the mystical moist night-air, and from time to time,Look'd up in perfect silence at the stars.

Alternate models

• geocentric • heliocentricWhich model is “true”?

demo

Diurnal motion• Daily motion of all the objects in the sky

Motion of the Earth• Rotation about axis once every

23h56m (sidereal day)• Revolution about sun once every

365.25 days• Precession of axis every 26,000 yr• All are counterclockwise (seen

looking down from North Pole)

Precession

Takes 26,000 years

What causes seasons?• Northern and Southern hemispheres

have opposite seasons• Earth closest to sun in January• Varying distance of sun only

changes intensity of light coming to earth by a few percent

Summer Solstice• Sun highest in sky

• shortest shadows• Longest day• Most direct sunlight

• warmest• Sun is at the northernmost point of

the eclipticWinter Solstice just the

opposite!

Equinoxes• Vernal (spring) and autumnal (fall)• Equal hours of daylight & darkness

• True everywhere on Earth!• Sun is at the point where ecliptic

crosses celestial equator• Length of day changing fastest

analema

What phase is the moon in today?

Waxing crescent

animation

Eclipses• Caused by shadows of earth &

moon• Don’t occur every month because

of 5° inclination of moon’s orbit

Web sitesAstronomy Picture of the Dayhttp://antwrp.gsfc.nasa.gov/apod/astropix.html

Comins & Kaufmann (textbook)http://whfreeman.com/dtu6e

othersspace.comastronomydaily.comskyandtelescope.com