Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 9:30

Lecture 7: The Four Seasons

Key Ideas:

• Due to the tilt of the Earth's axis relative to the plane of its orbit.
• NOT due to changes in the distance of the Earth from the Sun!!!

• The amount of direct sunlight (Insolation)
• The length of the day

The Obliquity of the Ecliptic

• Tilt is about 23.5 degrees from perpendicular relative to the Ecliptic Plane.

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

• Currently points near Polaris.
• Changes slowly with time (as we'll see below)

The Equinoxes

• Axis is at right angles to the Earth-Sun line.
• The Sun is seen on the Celestial Equator.
• Day and Night are equal length (12 hours).

Vernal Equinox

• Occurs around March 21
• Northern Spring & Southern Autumn.

Autumnal Equinox

• Occurs around September 21
• Northern Autumn & Southern Spring.

Winter Solstice

• The Earth's northern axis is tilted away from the Sun.
• The Sun is at its maximum southern declination as seen in the sky

Northern Hemisphere Winter:

• The Sun is low in the sky.
• The day is shorter than the night

Southern Hemisphere Summer:

• The Sun is high in the sky.
• The day is longer than the night

Summer Solstice

• The Earth's northern axis is tilted towards from the Sun.
• The Sun is at its maximum northern declination as seen in the sky.

Northern Hemisphere Summer:

• The Sun is high in the sky.
• The day is longer than the night

Southern Hemisphere Winter:

• The Sun is low in the sky.
• The day is shorter than the night

(Click on the image to view at full scale [Size: 43Kb])

Insolation

Sun directly overhead (at Zenith):

• Maximum concentration of sunlight on the ground
• Approximately 1000 Watts/meter² of heating.

Sun 30° above the Horizon:

• Sunlight spreads out over 2 meter²
• Get only 500 Watts/meter² of heating.

The Earth-Sun Distance

Aphelion (greatest distance):

• Earth is 152.1 Million kilometers from the Sun
• Occurs in early July (e.g., 2005 July 5)

Perihelion (closest approach):

• Earth is 147.1 Million kilometers from the Sun
• Occurs in early January (e.g., 2006 Jan 4)

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

Hottest & Coldest Months:

• Hottest Month: July (average high/low 84/63° F)
• Coldest Month: January (average high/low 34/18° F)

Distance Doesn't Matter

As seen from Columbus:

• 7% more total solar radiation in January than July
• Insolation January is <50% what it is in July

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

Precession of the Equinoxes

• Slow westward drift of the rotation axis
• Takes ~26,000 years to complete 1 circuit
• Amounts to ~50"/year, or 1 degree in 72 years.
• Discovered by Hipparchus of Nicaea (c. 150BC), but may have been known to the Babylonians.
• Caused by tidal torques from the Moon & Sun.

The Age of Aquarius

Vernal Equinox:

• Now in Pisces, will enter Aquarius in 2597 AD ("The Age of Aquarius")
• 1 AD: in Aries ("First Point of Aries")

Summer Solstice:

• Now leaving Gemini & entering Taurus
• 1 AD: in Cancer ("Tropic of Cancer")

The "North Star"

(Click on the image to view at full scale [Size: 10Kb])

2000 AD:

• Polaris is 0.75 degrees from the North Celestial Pole.
• Gets closest to the NCP in 2099.

2700 BC:

• NCP was near the star Thuban in Draco.
• Thuban was the pole star of the Old Kingdom of Egypt.