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

• Changing angle of the Sun in the Sky
• Changing Phases of the Moon

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.

• Position 1 - New Moon - the moon-earth-sun angle is 0 degrees and the moon is in the same part of the sky as the sun and hence can't be seen from the earth because the illuminated side is facing away from the earth.

  Note that if the orbital planet of the earth-moon system and that of the earth-sun system were exactly the same, then each new moon would be marked by this event . In practice, the earth-moon plane is inclined by about 5 degrees so that full solar eclipses are rare. Note the angular sizes of the moon and the sun are both about 1/2 a degree thus making it possible for total solar eclipses to occur. This similarity of size is a coincidence.

• Position 2 - one stop counterclockwise: The moon-earth-sun angle is about 45 degrees. We see a quarter moon in the western sky at sunset.

• Position 3 - The moon-earth-sun angle is now 90 degrees and we see 1/2 a moon. As the sun sets on the western horizon this 1/2 moon should be relatively high overhead.

• Position 4 - The moon-earth-sun angle is now 135 degrees and the 3/4 moon is in the eastern sky at sunset. By now should notice the relation between lunar phase and where the moon is in the sky relative to the setting sun.

• Position 5 - Full Moon> The moon-earth-sun angle is now 180 degrees meaning the moon is opposite in the sky as the sun. A full moon always rises at sunset and a full moon is always overhead at midnight.

• Positions 6-8 are exactly the same positions 4-2 except that the word sunset should be replaced by sunrise.

• The lunar cycle is 29.5 days and so only roughly conincides with our calendar in which the average month is 30.5 days. Hence there will be some months in which there are two full moons - don't panic you will survive.

• The moon is tidally locked to the earth so that it keeps the same side pointed the earth at all times. Thus the moon rotates once on its axis every 29.5 days to insure this.

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.

• Position 1 = Winter in the Northern Hemisphere and Summer in the Southern Hemisphere. The Northern Hemisphere is tilted away from the sunlight.

• Position 3 = Summer in the Northern Hemisphere and Winter in the Southern Hemisphere. The Northern Hemisphere is tilted towards the sunlight.

• Positions 2 and 4 are intermediate when both hemispheres are illuminated with approximately the same amount of solar radiation

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.

• If you lived at the equator (Latitude = 0 degrees) the sun would be within 23.5 degrees of overhead at all times - this is why its hot year around at the equator

• If you lived at 45 degrees N latitude then on June 21, when the sun is at 23.5 degrees N Latitude it is only 21.5 degrees from being straight overhead or (90 - 23.5 = 66.5 degrees) above the horizon.

• If you lived at 45 degrees N latitude then on Dec 21, when the sun is at -23.5 degrees S Latitude then it is 45 -(-23.5) = 68.5 degrees from overhead or (90 -68.5) 21.5 degrees above the horizon

• Hence in the summer the path of the Sun is higher in the sky and it takes longer to go form horizon to horizon. The days are long and the temperatures are high.

• In the winter, the path of the Sun is lower and hence the days are shorter and the colder.

• If you lived at latitude higher than 90 -23.5 = 66.5 degrees then in the winter, the Sun will always be below the horizon and in the summer it will always be above the horizon.

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: