Quadrants, Ecliptic & Starmaps

“Motion” Debriefing

• Stars circle NCP counterclockwise – For circumpolar stars: EW if above Polaris,

but WE 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

Daily and yearly motion intertwined

Solar vs Siderial Day – Earth rotates in 23h56m

– also rotates around sun

needs 4 min. to “catch up”

Consequence: stars rise 4 minutes earlier each night (or two hours per month, or 12 hours in ½ year)

After 1/2 year we see a completely different sky at night!

Seasonal Motion

• Daily Rising and Setting:– Due to the rotation of the

Earth around its axis– Period of rotation: 1

siderial day= 23h56m4.1s – 1 solar day (Noon to Noon) =24h

– Stars rotate around the North Star – Polaris

• Seasonal Changes:– Monthly differences caused

by Earth’s orbit around sun

The Zodiac throughout the Year

Example: In Winter sun in Sagittarius, Gemini at night sky; in summer sun in Gemini, Sagittarius at night sky

Zodiacal signs vs. Constellations

- 360/12=30, so each zodiacal sign is exactly 30 degrees “long”- 0 degrees: Aries, 30 degrees: Taurus, 60 degrees: Gemini, 90

degrees: Cancer, etc.

•“Constellation” is a modern, well-defined term

- Some constellations are big, some are small on the celestial sphere

•“Zodiacal sign” is the old way of dividing the year and the Sun’s path into 12 equal parts

Reminder: iSkylab 1 due in two weeks, Sep 24

• Observe!

• Ask questions!

• Already demonstrated Option 1 measurement (shadow of a stick altitude of the Sun)

• Will construct a quadrant

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.

Activity: Building a Quadrant from scratch with office supplies

• Pick up yardstick, string, tape, push-pin

• Make a protractor by dividing angles into two, starting with right angle: 90, 45, 22.5,11.25, etc.

• Does not have to be accurate

• Measure the alt. angle of a tree from classroom

• Write up results and turn in with names of group members

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.)

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