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Earth-Moon-Sun System (seasons, moon phases, and tides)
Though the Sun emits a constant amount of energy, different places on Earth receive
different amounts of solar energy because of the rotation and revolution of Earth. The
center of rotation of Earth is known as its axis. It takes Earth 24 hours to one complete
spin, or rotation, on its axis. As it rotates, different sides (or hemispheres) of Earth face
the Sun; that is, when one hemisphere faces the Sun, the other is in darkness. Earth’s
rotation is what creates daytime and nighttime. Though one would think that its axis
would be perpendicular to the plane of the Earth’s orbit around the Sun, it is actually
tilted. It tilts 23.5 degrees away from this perpendicular, and always faces the same
direction, pointing towards the North Star (Polaris).
While Earth rotates on its axis, it also moves, or revolves, around the Sun on a path
known as its orbit. This orbit is elliptical (not perfectly circular) in shape, and the Sun
does not sit directly in the center, but slightly to one side of the ellipse. When Earth is
closest to the Sun, it is ~147 million km away, and at its farthest point it is ~152 million
km away from the Sun. Many believe that the distance to the Sun is what causes the
seasons. In other words, when Earth is closest to the Sun it is summer and when it is
farthest it is winter, but Earth is closest to the Sun in January and furthest away in July.
The seasons are really caused by the tilt of Earth’s axis.
As Earth revolves around the Sun, its axis can be tilted toward or away from the Sun
depending on its location along its orbital path. It is because of the tilt of Earth’s axis and
the fact that it always points in the same direction as Earth revolves around the Sun that
we have seasons. For example, when Earth’s axis is tilted towards the Sun, the northern
hemisphere receives more solar energy than the southern hemisphere. This is when the
northern hemisphere has summer and the southern hemisphere has winter. When Earth’s
axis is tilted away from the Sun, the northern hemisphere receives less energy from the
Sun than the southern hemisphere, leading to the northern hemisphere’s winter and
southern hemisphere’s summer.
The length of time that an area receives solar radiation (daytime), as well as the angle at
which the Sun’s beams hit an area’s surface, vary with distance from the equator. Clearly
greater time in the Sun yields more solar energy, but why does the angle of the Sun’s
beams impact the amount of energy an area receives? If sunlight strikes the surface at an
angle it will spread over a wider area than if it hits the surface directly. Think of a
flashlight. If you point a flashlight directly at the ground, a small but bright circle of light
hits the ground. If you point the flashlight down at the ground at an angle, the light will
spread out over a wider area and will be less intense. The equator, no matter what time of
year it is, experiences 12 hours of daytime and 12 hours of nighttime. In addition, the
Sun’s rays will always hit the equator’s surface directly (perpendicular), which heats the
ground efficiently. Areas near the equator, since they receive a constant amount of solar
energy year round, do not experience seasons. With distance from the equator, seasons
become more distinct.
During the northern hemisphere’s summer (when the axis points towards the Sun), it
receives sunlight for a longer time (longer days, shorter nights) as well as more directly.
(The Sun is higher in the sky, so it hits the surface more directly.) Moving from the
equator to the poles, the length of the day increases, and the sunlight becomes less direct.
For example, in the middle of summer the North Pole has almost 24 hours of daytime, but
it does not get as warm as regions closer to the equator because the Sun barely rises
above the horizon; thus the North Pole is not receiving very direct or strong solar energy.
During the northern hemisphere’s winter (when the axis points away from the Sun), it has
longer nights and shorter days, and the Sun does not rise as high in the sky before it
begins to set, so it does not heat the surface as efficiently as it does during the summer.
During spring and fall, the axis does not point toward or away from the Sun, so the
southern and northern hemispheres receive equal amounts of solar radiation and sunlight
(daytime and nighttime are equal in length everywhere on Earth).
While Earth orbits the Sun, the Moon orbits Earth. Both of these celestial bodies, the Sun
and the Moon, exert a gravitational pull on Earth. Their gravities pull on Earth’s surface
water as it rotates on its axis. The gravitational pull of both the Moon and the Sun on
Earth’s water causes the water to rise and fall; this is known as tides. Tides are most
noticeable in larger bodies of water, particularly oceans, seas, and large lakes (like the
Great Lakes). The Moon’s gravity contributes the most to the creation of tides, and
because of this we can predict high and low tides using the lunar cycle. During the course
of a month we see many different faces of the Moon. This is because the Sun illuminates
the Moon’s surface as the Moon rotates around Earth. This change in the illuminated
surface of the Moon is called the lunar cycle. This cycle has eight different phases. First
is the new moon, which occurs when the Moon lies in between Earth and the Sun. The
side of the Moon that we see on Earth receives no sunlight, so it appears dark. As the
Moon rotates around Earth and more of its surface receives sunlight, we see more of its
surface. This process of the Moon appearing to “grow” is known as waxing. After the
new moon phase is the waxing crescent moon phase, where the moon “grows” into a
crescent (looks like a thumbnail). Next is the first quarter moon, where only a quarter of
the whole Moon is illuminated. This looks like a half circle in the sky. It is referred to as
first quarter because it represents a quarter of the time it takes for the entire lunar cycle to
occur. A waxing gibbous moon is when the majority of the moon is illuminated, but not
quite all of it. Half way through the lunar cycle is the full moon, when the entire Moon’s
face that we see is illuminated and looks like a white circle in the sky. At this point, the
lunar cycle has entered its waning period, when the Moon appears to be “shrinking.”
From the full moon phase, what we see shrinks to waning gibbous (majority but not all
illuminated), last quarter (half the surface is illuminated, and only the last quarter of the
time for a full lunar cycle remains), and waning crescent (sliver illuminated). From here,
the cycle is back at new moon and starts over again.
The lunar cycle is critical to understanding our ocean tides. Tides are when two bulges
occur in Earth’s oceans on opposite sides of Earth. The surface water on the side closest
to the Moon forms a bulge because this is where the Moon’s gravitational force is
greatest. Another bulge occurs on the opposite side of Earth from this location because
the Moon’s gravitational force is weakest here, allowing the force from Earth’s rotation
on its axis (centrifugal force) to push water outward from the surface. Because water
bulges in these two areas (high tides), in the area in between these bulges are depressions
(low tides) to compensate. High tide is when sea level reaches its highest vertical level,
which corresponds to the furthest it reaches inland. Low tide is when sea level reaches its
lowest vertical point, which corresponds to the furthest out to sea the shoreline reaches.
Tidal range is the difference in sea level between high and low tide. The Sun’s
gravitational pull also causes a second, though smaller, tidal effect in addition to the
Moon’s. The Sun can work to either magnify or minimize the effect of the Moon on our
tides. When the angle of the Moon to Earth is perpendicular (90 degrees) to the angle of
the Sun to Earth (this occurs during first or last quarter), we experience less intense high
and low tides. (That is, high tide is not as high as normal, and low tide is not as low as
normal.) This is because the Sun’s gravitational pull is interfering with the Moon’s pull.
This is called neap tide. During neap tide, tidal range is very small, in some instances
only a few inches. Conversely, when the Sun is on the same or opposite side of Earth as
the Moon (this occurs during new moon or full moon, respectively), we have extra high
high tides and extra low low tides. This is because the Sun’s gravitational pull is adding
to the Moon’s gravitational pull. This is called spring tide. During spring tide the
difference between high and low tide, or the tidal range, is greatest. The Sun-Earth-Moon
system of rotation and revolution is responsible for the length of our day, the seasons, and
our tides.