Orbital Properties of the Earth

There are two important attributes about the earth's orbit which manifest themselves through visual observations.

Lunar Phases:

Lunar Phases are relatively easy to understand but they nevertheless are a source of constant confusion to the public at large.

The moon shines by reflected sunlight - its phase as viewed form the earth depends only on the moon-earth-sun angle. In the figure above, the sun is off to the right. There are 8 positions shown.

For more on Lunar Phases, watch the animation

Seasonal Variations on the Earth:

The rotation axis of the Earth is inclined with respect to its orbital plane by 23.5 degrees. This is shown in the following:

Because the earth's axis is tilted with respect to the incoming sunlight, the hemispheres receive unequal distributions of solar radiation at certain times of the year. This is shown in the following:

Seasonal variation is caused by the earth's tilted rotational axis.

The 23.5 degree tilt of the earth's rotational axis is manifest by the position of the sun in the sky. Over the 6 month period June 21 - Dec 21 the sun moves from +23.5 degrees N. Latitude to -23.5 degress S. Latitude.

The following figure shows the position of the Sun at noon with respect to an observer at 45 degrees N. Latitude on Dec 21 and June 21. The path of the sun (yellow dotted arcs) in the sky is much higher on June 21 than Dec 21. On Dec 21 the highest point in the sky above the horizon will be 21.5 degrees as shown: