Seasonal Motion

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

Another Complication: Axis Tilt!• The Earth’s rotation axis is tilted 23½ degrees

with respect to the plane of its orbit around the sun (the ecliptic)

• It is fixed in space sometimes we look “down” onto the ecliptic, sometimes “up” to it

Path around sun

Rotation axis

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

Precession of the Equinoxes

Precession period about 26,000 years

“The dawning of the age of Aquarius”

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.)

The Seasons• Change of seasons

is a result of the tilt of the Earth’s rotation axis with respect to the plane of the ecliptic

• Sun, moon, planets run along the ecliptic