Sun-Earth Relations

Thanksgiving has come and gone, and we are moving into late autumn and starting to think about winter snows.  As we enjoy autumn activities, the earth is moving ever closer to the sun.  In January, the earth will be at its closest approach to the sun than any other time of the year.  So, why are we in the deep freeze at that time?

The earth’s axis is a straight line passing through both the north pole and the south pole.  In relation to the sun, the axis is tilted 23.5˚ to one side.  Referring to our diagram above, called “Earth Axis”, the subsolar point is where the sun’s rays directly hit the earth at right angles.  The northern hemisphere is tilted away from the sun during winter in our area, when the light and warmth of the sun is striking the earth at an oblique angle, compared to the much more direct sunlight striking the earth in the southern hemisphere’s summer.

The tilt of the axis changes slightly every year causing a wobbling movement.  The wobble is called axial precession and can be visualized by watching a spinning top as it moves across a floor, not quite perfectly smooth.  As the top moves in circles, so does the wobble of the earth, taking about 26,000 years to return to the same orientation.  Earth’s spin drifts slowly east and west, but never very far.  The farthest movement observed from the center of the axial spin, since recording began in 130 B.C., has been 37 feet.  This will not affect daily life but does affect GPS and satellite observations.

There are many theories about what causes axial precession, but data from the last five to seven years shows a strong possibility that movement of water around the world is responsible.  Changes in the amount of water and ice can be counted as changes in mass and weight.  These types of changes near the poles have no significant effect on the earth’s spin, but where these changes occur in the middle latitudes, around 45˚, greatly impacts the stability of the spin.  Losses of water in the Europe-Asia-India areas has created an imbalance in mass and weight sufficient to alter earth’s spin and account for some of the recent wobble movements.  There are strong correlations with other water movements back through the last 2,150 years of history.

As the earth moves through a one year orbit around the sun, the tilt of earth’s axis does not change.  As shown in the diagram labeled “Earth Orbit”, this means that for a quarter of the year, the northern hemisphere is leaning towards the sun.  In mid-June, the summer solstice occurs when earth’s tilt and orbit brings the northern hemisphere closest to the sun, and we experience the longest daylight period.  The winter solstice, occurring in mid-December, is when the southern hemisphere is closest to the sun, and the northern hemisphere experiences the shortest day of the year.  The two in-between periods, vernal (spring) and autumnal (fall) equinox are when the sun’s orbital path crosses the equator, and day and night hours are about equal.

If you would like to learn more about the seasons, consider the following resource links: National Geographic Earth-Sun Relationships;  SUNY Earth-Sun Relation;  U of I Extension Ecosystems; and U of I Global Temperature Learning.