Quadrants, Ecliptic & Starmaps

“Motion” Debriefing Stars circle NCP counterclockwise –For circumpolar stars: E  W if above Polaris, but W  E if below Polaris Stars move E  W but also up (rising) and down The sun AND the stars move around the observer, so the sun stays (approx.) fixed amongst the stars

iSkylab: Sun Option What: Determine how the height of the sun above the horizon at a specific time is changing as the days pass by measuring the length of the shadow it casts with a gnomon (essentially a stick in the ground). Time: Once you know how to do it, this only takes a minute per observation. Commitment: Do this over several, not necessarily consecutive days, at exactly the same time. Weather: Need to see the shadow for a minute, so can do on partly cloudy, possibly hazy but not overcast days. Shadow Gnommon To Sun

iSkylab: Moon Option 1 What: Determine the height of the moon above the horizon with the help of a quadrant (essentially a bob dangling from a protractor), and see how it changes as the days go by. Time: Once you know how to do it, this only takes a minute per observation. Commitment: Do this over several, not necessarily consecutive days, at exactly the same time. Weather: Need to see the moon for a minute, so can do on partly cloudy, possibly hazy but not overcast days.

iSkylab: Moon Option 2 What: Determine the position of the moon with respect to the stars by sketching the position and the shape of the moon and the bright stars in the sky. Document changes as the days go by. Time: Once you know how to do it, this takes several minutes per observation. Commitment: Do this over several, not necessarily consecutive days, exact time does not matter. Weather: Need to see the moon and the stars for several minutes, so it needs to be a cloudless night with good seeing.

Axis Tilt  Ecliptic The Earth’s rotation axis is tilted 23½° with respect to the plane of its orbit around the sun This means the path of the sun among the stars (called ecliptic) is a circle tilted 23½° wrt the celestial equator Path around sun Rotation axis pointing to NCP, not SCP

Position of Ecliptic on the Celestial Sphere Earth axis is tilted w.r.t. ecliptic by 23 ½ degrees Equivalent: ecliptic is tilted by 23 ½ degrees w.r.t. equator!  Sun appears to be sometime above (e.g. summer solstice), sometimes below, and sometimes on the celestial equator

Is the sun rising in the East? Typically NOT! See for yourself! –Study variation of the rising/setting points of the sun over time –Need at least 10 sunrises or sunsets; more is better –Measure time and azimuth (angle relative to North) –Note position of sunrise/sunset on horizon –Measure angle to that position relative to some fixed landmark (mountain, etc.)

Precession: Pattern in the Sky We notice that position of the NCP changes wrt stars We notice that the position of the vernal equinox, i.e. the intersection point of celestial equator and ecliptic changes wrt stars These are very slow & subtle changes! Hipparchus discovered them 130 BC by making an accurate, naked-eye star catalogue

Precession of the Equinoxes Precession period about 26,000 years “The dawning of the age of Aquarius”

Understanding and using Star Maps The night sky appears to us as the inside of a sphere which rotates Problem: find a map of this curved surface onto a plane sheet of paper Let’s explore our turning star map!

Fixed and unfixed Stuff The stars are “fixed” to the rotating sky globe  They move from East to West and also from near to the horizon to higher up in the sky The Solar System bodies (Sun, Moon, Planets, Asteroids, Comets) move with respect to the fixed stars SSB’s have complicated paths: their own motion is added to the overall motion of the celestial sphere  they cannot be printed on a star map!

Star Maps Celestial North Pole – everything turns around this point Zenith – the point right above you & the middle of the map 40º 90º