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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 7: The Four Seasons

Key Ideas:

The Four Seasons The tilt of the Earth's axis affects Precession of the Equinoxes

The Obliquity of the Ecliptic

The Earth's rotation axis is tilted relative to the plane of its orbit around the Sun:

The Earth's axis points towards the same direction in space as we orbit around the Sun:

The Equinoxes

In March & September:

Vernal Equinox

Autumnal Equinox

Winter Solstice

Occurs around December 21:

Northern Hemisphere Winter:

Southern Hemisphere Summer:

Summer Solstice

Occurs around June 21:

Northern Hemisphere Summer:

Southern Hemisphere Winter:


What matters for solar heating is how directly the rays of the sun hit the ground:

Sun directly overhead (at Zenith):

Sun 30° above the Horizon:

The Earth-Sun Distance

The Earth's orbit is slightly elliptical, so that the Earth is closer to the Sun at some times, and farther away at others.

Aphelion (greatest distance):

Perihelion (closest approach):

The difference in distances is about 5 Million kilometers. While this seems like a lot (it is about 13 times the mean Earth-Moon distance), overall it makes only a ~7% difference in the total amount of solar radiation hitting the Earth.

Summer vs. Winter in Columbus

June 21 (Summer Solstice) December 21 (Winter Solstice)
Sun's altitude at noon: 73.5° Sun's altitude at noon: 26.5°
Insolation: 960 W/m2 Insolation: 450 W/m2
Average high/low temperature: 80/58° F Average high/low temperature: 39/25° F
Length of the Day: 15h Length of the Day: 9h
Distance from the Sun: 152 Million km Distance from the Sun: 147 Million km

Hottest & Coldest Months:

Distance Doesn't Matter

The Earth is 5 Million kilometers closer to the Sun in January than in July, yet January is the coldest month in the North!

As seen from Columbus:

Differences in seasonal insolation beat variations in solar radiation due to differences in the Earth-Sun distance every time.

Seasonal temperature variations have nothing to do with changes in the distance of the Earth from the Sun.

Precession of the Equinoxes

The Earth's rotation axis slowly wobbles or precesses about the Ecliptic Pole.

The Age of Aquarius

Precession causes the Equinoxes & Solstices to drift westward over time.

Vernal Equinox:

Summer Solstice:

The "North Star"

Precession also changes which star is the northern pole star, if any, over time. 2000 AD:

2700 BC:

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