Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 6: The Four Seasons

Key Ideas

Solstices and Equinoxes: Equator and Tropics
The Four Seasons: Due to the 23.5 degree tilt of the Earth's axis of rotation relative to the plane of the ecliptic
The tilt affects:: The amount of direct sunlight (insolation); The length of the day
Right Ascension and the Vernal Equinox
Precession of the Equinoxes

Solstices

A Solstice occurs when the Sun is at its maximum northern or southern declination along the Ecliptic.: From Latin sol sistit for "sun stands"
Summer Solstice:: Maximum northern declination of the Sun; Occurs in June; Sun is directly overhead on the Tropic of Cancer
Winter Solstice:: Maximum southern declination of the Sun; Occurs in December; Sun is directly overhead on the Tropic of Capricorn

Equinoxes

Equinoxes occur when the Sun crosses the Celestial Equator: From Latin equi noctis for "equal night"
Happens twice a year in March and September
Vernal Equinox:: Sun crosses the Celestial Equator Northward in March
Autumnal Equinox:: Sun crosses the Celestial Equator Southward in September

Length of the Day

Vernal and Autumnal Equinoxes:: Sun rises due East and sets due West; Day and Night are equal (12 hours)
Summer Solstice:: Sun rises in Northeast, sets in Northwest; Day is longer than Night.
Winter Solstice:: Sun rises in Southeast, sets in Southwest; Day is shorter than Night

Equinoxes

In March and September:: Axis is at right angles to the Earth-Sun line.; The Sun is on the Celestial Equator.; Day and Night are equal length (12 hours)
March: Vernal Equinox: Northern Spring and Southern Autumn; September: Autumnal Equinox; Northern Autumn and Southern Spring

Winter Soltice

In December:: The Earth's North Pole tilts away from the Sun; The Sun is at its maximum southern declination
Northern Winter: The Sun is low in the sky; The day is shorter than the night
Southern Summer: The Sun is high in the sky; The day is longer than the night

Summer Soltice

In June: The Earth's North Pole tilts toward the Sun; The Sun is at its maximum northern declination
Northern Summer: The Sun is higher in the sky; The day is longer than the night
Southern Winter: The Sun is low in the sky; The day is shorter than the night

Insolation

What matters for solar heating is how directly the rays of the sun hit the ground
Sun directly overhead (at Zenith):: Maximum concentration of sunlight; Get approximately 1000 Watts/meter² of heating
Sun 30 degrees above the Horizon: Sunlight spreads out over 2 square meters; Get only 500 Watts/meter² of heating

The Earth-Sun Distance

The 5x10⁶ km difference between perihelion and aphelion makes only a ~7 percent difference in the amount of solar radiation we receive from the Sun
In Columbus, the insolation in January is less than 50 percent the insolation in July
Seasonal temperature changes are NOT due to the Earth-Sun distance.

Equinox and Right Ascension

Right Ascension (RA or α) is the celestial equivalent of longitude
Declination (Dec or δ) is the celestial equivalent of latitude
The location of the Sun on the Vernal Equinox defines the celestial equivalent of the Prime Meridian (00 hours of RA)
The location of celestial objects in RA is defined by the number of hours they rise after 00 hours RA

Precession of the Equinoxes

Wobble of the Earth's rotation axis about the Ecliptic Pole.
Slow westward drift of the rotation axis
Takes about 26,000 years to complete 1 circuit
Amounts to 50'' per year, or 1 degree in 72 years.
Discovered by Hipparchus of Nicaea (c. 150 BC)
Caused by tidal torques from the Moon and Sun

The Age of Aquarius

Precession causes the Equinoxes and Solstices to drift westward over time.
Vernal Equinox:
Now in Pisces, will enter Aquarius in 2597 AD
1 AD: in Aries ("First Point of Aries")
1900 BC: in Taurus
Summer Solstice:
Now leaving Gemini and entering Taurus
1 AD: In Cancer ("Tropic of Cancer")

The "North Star"

Precession also changes which star is the northern pole star.
2000 AD:
Polaris is 0.75 degrees from the North Celestial Pole.
Gets closest to the NCP in 2099.
2700 BC:
NCP was naer the star Thuban in Draco
Pole star of the Old Kingdom of Egypt

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.
[ Return to the Astronomy 171 Main Page | Unit 2 Page ]

Updated: 2007 January 7
Copyright © Paul Martini All Rights Reserved.