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In this lab, you're going to observe how the stars, the sun, and the moon move throughout the night and over the course of several days or weeks. We make these measurements in terms of angles, and for this lab, it will be sufficient to use the following approximate measurements. Begin by holding your arm outstretched toward the part of the sky in which you're taking a measurement.
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across. You can compare this distance to the
width of your closed fist (remember to keep your arm outstretched).
In this part of the lab, you will follow the motion of a star to determine the period of the earth's rotation about its axis, and the period of its revolution about the sun. Remember to keep an accurate record of all of your observations, always including the date, time (to the nearest minute), and observing location.
The daily rising and setting of the sun and stars is due to the earth's rotation about its axis. The apparent motion of the sun, from east to west across the sky during the day, is familiar to us all. But have you ever noticed that the stars complete a similar motion at night? Probably not, unless you have gone out and spotted your favorite constellation then returned a few hours later to look for it again.
To calculate how long it takes the earth to spin once on its axis (its period of rotation), you will measure how fast stars appear to move overhead.
Go outside and, using your star chart, find a star that lies near the
celestial equator (the celestial equator is the imaginary line that
runs around the sky 90 down from the north star). Whichever star
you decide to use, be sure to record its name. Now find a place from
which to make your observations. This place should be located so that
your star appears to be balancing on some structure, like a building,
a telephone pole, or electric power lines. Make sure that this
structure is rigid, not something flimsy like a tree or a branch,
which can be blown about by the wind. Also make sure that this
structure is far enough away that your star still appears to balance
on it if you take a small step to the left or right. We'll call this
structure your ``reference point''. Make sure that your spot is one
that you will be able to relocate accurately about a week later. It
would be ideal if you could mark your spot with a stone and not have
it be moved.
When you have your star balancing on your reference point, record the date and time (to the nearest minute). Come back to your spot an hour later (exactly where you were before) and relocate your star. Measure how many degrees it has moved from its original position on your reference point. Record this result, along with the time to the nearest minute.
The angular velocity of the star in the sky during your observation is:
Calculate w in units of degrees/minute. Now that you have the angular velocity of stars in the sky, calculate how long it would take for a star to travel 360 degrees, or all the way around the sky.
Since the star is ``moving'' because of the earth's rotation, this period is the amount of time it takes for the earth to spin once.
If you are familiar with the night sky, you have probably noticed that there are different constellations visible in the summer and winter. This seasonal change is due to the earth's revolution around the sun. In this section, you will again follow the motion of a star, but this time you will use it to measure the earth's period of revolution.
To measure the earth's period of revolution, you will make another measurement on the same star you used before, but a few days later. If you were to return to your spot at the same time a few days after your first measurement, your star would appear to have moved westward relative to your reference point. You could then determine the star's motion in degrees per day by measuring how far your star had moved. For this lab, however, we'll use a slightly more accurate method.
A few days, say three to ten, after you've completed the observations of the star in the earth's rotation part of the lab, return to your spot. This should be exactly the same spot as before, but arrive about 45 minutes earlier. Note that your star is just east of your reference point. Wait at your spot until your star is again balancing on your reference point. When this happens, record the date and time to the nearest minute.
The star came to balance earlier than it did the first time. In other words, if you waited until the same time of night as when you made your first measurement, the star would be further to the west by an amount:
where w is the angular velocity you found in the earth's rotation part of the lab.
Calculate this shift. Now you know the westward shift of the star in degrees and the number of days it took to make this shift. Use these values to calculate a new angular velocity, v, in degrees/day, due to the earth's motion around the sun. How many days would it take for the star to move around 360 degrees? This is the earth's period of revolution around the sun.
You are all probably aware that the sun rises in the east and sets in the west due to the earth's rotation. What you may not have noticed is that the exact location at which it rises and sets, and the path that the sun follows through the sky changes throughout the year. In this section of the lab, you will observationally investigate the nature of these changes.
The observations for this portion of the lab will require you to do multiple observations of the sunset throughout the course of the quarter.
Over the course of one day, the moon will rise and set just as the sun and stars do, rising in the east and setting in the west. Unlike the sun which rises in the morning and sets in the evening (by definition), the moonrise and moonset times change drastically over the course of a month.
In this section, you will observe the motion and changing phase of the moon over a portion of the moon's orbital period (27 days).
This section requires that you make multiple observations over the course of about a week and a half, starting a few days after new moon (January 28). It is possible, and even probable, that you will be clouded out for at least a portion of this window of opportunity, so it is imperative that you remain aware of the weather conditions so that you can optimize observations on nights that are not COMPLETELY cloudy. Since you are observing the moon, you will be able to do the necessary observations even on cloudy nights.
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) around the earth?