Seasons

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

The Sun appears sometimes among the stars above the Celestial Equator, and sometimes amongst the southern stars

• March 21 – March (vernal) equinox

• June 21 – northern Solstice

• September 23 – September (autumnal) equinox

• December 21 – southern Solstice

Is the sun rising in the East?• Typically NOT, only Mar 21/Sep23! 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

The Seasons: Earth axis always points in the same direction

• However, as Earth moves around the sun, the axis points sometimes towards and sometimes away from the sun.

Ecliptic horizontal, Celestial equator and Earth axis tilted

The reason for the seasons

Daily path of the Sun in Winter and Spring

Daily path of the Sun in Summer and Fall

What changes?

• Sun is closer to the celestial north pole in the northern summer– Therefore it is higher in the sky

• Therefore the sunlight hits us more directly– Therefore the energy deposit per unit area is higher

» Therefore the temperature goes up

– Therefore the sun is above the horizon longer• Therefore there is more sunlight per day

– Therefore the temperature goes up

What does not change

• The direction of the Earth’s axis (still points to the CNP)

• The distance to the sun (there is a 1.7% change of the distance to the Sun, but we get closest to the Sun in early January!)

• The fact that we are about an Earth radius closer to the Sun at noon in the summer is negligible (6300km compared to 150 mill.km is 0.004%)