Orbital Properties of the Earth
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 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
- 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
- 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
- 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.
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
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
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
- 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: