ASTRO 112 Naked Eye Observing Project
Note: This is a project that can be done with the naked eye under great weather and
light conditions, so please assess the weather and the amount of light pollution at
your location. The use of binoculars will also help, but is optional.
The Earth is constantly rotating about its axis like a top. The Earth takes about 24
hours to make one spin. Because of this rotation, different parts of the sky are
constantly coming into our view while other parts are constantly leaving our view.
This rotation causes the Sun, Moon, and stars to periodically show up in our sky
then leave our sky. This motion is too slow to readily perceive with your naked
eyes but can be obvious when looking through a telescope.
The Earth is rotating counterclockwise when viewed from the north pole. More
directly, San Diego is heading eastward (at about ~800 mph). This is why “new”
sky appears above the eastern horizon and the “old” sky disappears below the
western horizon (like the Sun).
For this task, pick a constellation that is visible on the horizon about one hour after
sunset. Remember, stars set on the western horizon, so its best to pick something
on the eastern horizon. If there are not any easily recognizable constellations to the
east, the souther horizon is another good place to look. In the late Summer and
early Fall, two good constellations to use are Scorpius and Sagitarrius. In late
Winter and early Spring, try Canis Major and Orion.
To get better idea which constellations will be up during what times, download an
evening skymap from www.skymaps.com/downloads.html and use the skymap
from the current month in the Northern Hemisphere. Note that the skymaps
typically show you what the sky looks like around 8 – 9 PM at night. See your
instructor if you need help on how to use an evening skymap.
Part I Procedure (Do this early in the semester!!):
0. Read the questions first before proceeding. These will help you get an idea how
detailed your observations need to be.
1. Find a nice spot where you can see the constellation and a fixed point of
reference. The point of reference can be anything large and stationary; telephone
poles, buildings, a mountain, etc. It maybe be helpful to mark your spot with a
rock, chalk, or some tape so you can be sure you are standing in the same place
later.
2. Using the Motion of the Sky sheet, draw a picture of the constellation and your
reference point. In your drawing, make sure you write down where you are
standing, which direction you are facing (N, E, S, W or NE, NW, SE, SW) and
what time.
3. Wait about an hour or two, then go back to the same exact spot you were when
you drew your first picture. Draw a picture of where the constellation is now, using
the same point of reference. Do not forget to write down the time of your second
observation.
4. On a separate sheet of paper, draw a bigger portion of the sky (wider field of
view) such that you can easily refer to it several weeks later. This time, draw an
additional three constellations that are currently visible. Again, write down the
time, location, and direction you are facing. You will be using this to compare with
Part II.
Questions: Please answer in clear, complete sentences and use diagrams/drawings
if you need to!
1. Approximate in degrees how far your chosen constellation is above the horizon
and how far it has moved from its initial position. As a general rule of thumb, you
can use this:
2. Based on your two observations and your answer above, at what time do you
estimate the constellation set that night? How did you come up with this estimate?
(Hint: How long does it take for stars to rise from the eastern horizon and set on
the western horizon?)
3. Why might this motion be more obvious through a telescope?
Part II Procedure (Do this several weeks after Part I):
0. Read the questions first before proceeding. These will help you get an idea how
detailed your observations need to be.
1. Simply sketch the wide region of the sky, with the same constellations you
included from Part I. Note that you should make these observations from the same
place and around the same time (make sure to account for Daylight Savings
Time!).
Questions: Please answer in clear, complete sentences and use diagrams/drawings
if you need to!
1. Compare your observations from several weeks before. Due to the Earth’s
motion around the Sun, all the stars in the sky, rise about four minutes earlier each
day. Over several weeks, you will be able to see those four minute intervals add up
to a significant displacement in the sky. Have the constellations moved closer to
the eastern, or western horizon?
2. Is this what you expect? Why?
3. Why can’t we see winter constellations at night in the summer time? Where are
the winter constellations in the summertime?
4. What fraction of the year is the interval between your two observations (for
example, 3 months is 1/4th of a year)?
5. Approximately what fraction of the sky have the constellations moved across?

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