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Part I: Rotation of the Earth-
The Diurnal Motion of Stars
Background
If you track the position of a star at night, you will notice that
it will change over the course of several hours. The reason for this is that
the Earth is rotating. It takes approximately 24 hours for the Earth to complete
one full rotation. Because the Earth is rotating, the star appears to move over the course of the night. The movement
of the stars due to the Earth’s rotation is called diurnal motion or daily motion. Due to the 24-hour counterclockwise rotation of
the Earth, all the stars in the sky will follow a predictable path. The paths
that they follow depend upon the which latitude on Earth that you are observing
the stars from and the position of the North Star in the sky. The reason for
this is because the altitude (angular distance from the horizon to the star) of
the North Star changes depending upon where the observer is located. The altitude
of the North Star is, however, always equal to the observer’s latitude. For
example, if you lived at 36 deg latitude, the North Star would be located at an altitude of 36 deg. The other stars will appear to revolve around the North Star due to the rotation of the Earth. If you live at the North Pole, the North Star would be located at 90 deg altitude, or at the zenith which is the point in the sky directly above you.
If you have a sensitive enough camera, you can take photos of the
night sky using long exposure times. When you observe the photograph, the stars
do not appear as points of light but rather as long streaks of light. The planetarium
can simulate the star trails that you would typically observe in such a photograph.
In this part of the lab, you will observe and sketch the star trails, and in doing so, you will obtain a better understanding
of the diurnal motion of stars at different latitudes on the Earth.
Part II: Revolution of the Earth- The Analemma
Background
Contrary to popular belief, the seasons are not caused by the changing distance of the Earth to the Sun as it moves
in its orbit around the Sun. According to this scenario, the Earth would be warmest
(summertime) when it is closest to the Sun and coolest (wintertime) when it is farthest away from the Sun. Furthermore, the Earth would exhibit the same seasons in both
the northern hemisphere and the southern hemisphere.
However, as we are all well aware, this is not the case. When it is wintertime
in the northern hemisphere, it is summertime in the southern hemisphere and vice versa.
In fact, in the northern hemisphere, the Earth is actually closest to the Sun during winter time and farthest away
from the Sun during summertime.
The actual explanation for the seasons results from the combined effects of the Earth’s revolution and the 23.5
degree tilt of the Earth’s axis of rotation relative to the plane of our orbit around the Sun, which is called the ecliptic plane. It takes the Earth one
year to complete one full revolution around the Sun. During that year, the Earth
experiences four seasons. For one half of the year, the northern hemisphere is
tilted towards the Sun, whereas for the other half of the year, the northern hemisphere is tilted away from the Sun. During the months when the northern hemisphere is tilted towards the Sun, we
experience summer in the northern hemisphere and winter in the southern hemisphere.
During the months when the northern hemisphere is tilted away from the Sun, we experience winter in the northern hemisphere
and summer in the southern hemisphere. As the Earth revolves around the Sun,
the maximum height that the Sun reaches in the sky changes, and thus the total amount of daylight hours also changes. This is the reason why we have seasons and why there are temperature changes through
the course of a year.
Have you ever observed a globe and seen a figure which looks like an 8 on the globe and wondered what it meant? In this lab, you will come to understand that figure 8 shape, which we call the analemma. The analemma is a shape that
represents the height of the Sun, which is measured at the same time of day during different times of the year. The shape of the analemma is caused by two factors. The height of the analemma is due to the Earth’s tilted axis of rotation, whereas the width is caused
by the eccentricity of the Earth’s orbit around the Sun. In this lab, you
will track the apparent motion of the Sun in the sky during different days of the year at the same clock time, and in doing
so, you will generate your own analemma.
Part III: Precession of the Earth
Background
When you spin a top, you might notice that, as the top loses energy, the top will start to wobble; that is its axis
of rotation will start to trace a circle. The Earth also exhibits this wobbling
as it spins upon its axis. However, since the timescales of the wobbling are
rather huge, we don’t feel this effect in our day to day life. It takes
about 26,000 years for the Earth’s spin axis to trace a complete circle! The
circular motion of the Earth’s spin axis is called precession. The Earth’s speedy 24-hour rotation rate causes the Earth’s shape to deviate from spherical. Instead, the Earth is slightly thicker across the equators than it is across the poles. The gravitational “tugs” of the Sun and the moon on the Earth’s
equatorial bulge causes the Earth’s axis of rotation to precess with a period of 26,000 years.
Because of the precession of the Earth, the north polar axis will not always point toward the direction of Polaris;
thus, Polaris will not always remain the “North Star”. Depending
on which direction the north polar axis points to, the North Star will change. There
will also be times when the north polar axis will not line up with any bright stars in the sky. During these times, there will be no North Star. Indeed the
North Star used by Greek sailors for navigation was a different one than we use today.
Because of the tilt of the Earth’s axis of rotation, the ecliptic and celestial equator are inclined to each
other by 23.5 degrees. These two circles intersect at two points called the equinoxes
(“equal nights” in Latin). Since the rotation axis is precessing,
the celestial equator also precesses with time which means that the position of the equinoxes will also change with time.
Thus, the precession of the Earth may also be referred to as the precession of the
equinoxes.
Furthermore, the precession of the Earth may be responsible for extreme temperature conditions exhibited on Earth over
a long period of time. According to the Milankovich theory, the precession of
the Earth could partially explain the occurrence of ice ages! In this part
of the lab, we will observe the precession of the Earth’s axis of rotation and determine which star (if any) is closest
to being the North Star at different epochs.
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