The Phases of the Moon |
(Use Your Planetarium Software)
If you can do this activity with the real Moon, please try to make the real outdoor observations in addition to this activity. It is highly useful to see the simulation on the computer, but more satisfying to see it in the sky.
For Directions on When and Where to Look in the Real Sky, Click the Button.
Your local conditions and times will vary, so we will set this computer activity up for a particular location and time. Please either print out or copy the instructions below if you cannot switch back and forth between this file and the Planetarium software.
1) Open your Planetarium software.
2) Across the top you should see the options:
File - Edit - Set - Field - Center - Animation - Miscellaneous - Help .
From this bar click on "Field," then "Chart Mode" and finally "Local Horizon."
3) Again from the "Field" option, select "Field Size." Enter "30" and then click "OK."
4) From the "Set" option, select or click on "Geographic Location" and then "USA: Denver CO."
4) From "Set" again, select "Time" and then "Enter Local Time." In the appropriate boxes, enter the data for October 19, 1999 at 18 hours and 0 minutes (6:00 p.m.), then click "OK."
5) From the "Center" option, select "Solar System Object," then "Moon."
6) The Moon should now be centered in your view. It's color will depend on the color you have set in the "Set Sky Colors" option. Note its position relative to the stars around it. If you have set things correctly, it should be just above a star called Delta Capricornus. This star is at one corner of the roughly triangular-shaped constellation of Capricornus. Click here for an image: Oct. 19 image .
Place your cursor exactly over this star and press the left mouse key. A dialog box should open up to give you various pieces of information about the star. Do the same thing with the Moon. The dialog box should tell you that the Moon at this time is at RA (Right Ascension) of about 21:47, meaning 21 hours, 47 minutes (rounded); and DEC (Declination) -15 o 30', or minus 15 degrees, 30 minutes. Perhaps a bit more meaningful for you at this time is the azimuth (angular distance measured clockwise along the horizon from due north) and altitude (number of degrees above the horizon). In this case it should read 148 o 05' for azimuth, and 28 o 37' for altitude. This means that the Moon is in the east-southeast sky, and roughly one-third of the way up in the sky (90 o is straight overhead).
7) Use a bit of tape or the corner of a sticky note to mark the position of the Moon on your screen. Next, select "Set," "Time" and "Enter Local Time again. Change the date to "20" and click on "OK."
8) Now the screen will flicker a second and you will note that the Moon has moved a distance to the left (east). This is how much it moves in a single 24-hour period. Now it is a roughly kite-shaped portion of the constellation Aquarius. Click here for an image: Oct. 20 image.
If you place your cursor over the original position (the one you marked), then press down the right mouse key and hold it as you move the mouse to the new position of the Moon, when you let go an informational box will appear. This box will tell you how many degrees the Moon moved from the first day to the next. It should read approximately 13 degrees. The average from day to day is about 13.19 degrees. It gets really interesting when you divide 360 degrees, which is how many degrees the Moon must move in one orbit relative to the star, and divided by 13.19. You will get 27.3, which is exactly the sidereal month (the Moon's orbital period relative to the stars) in days!
9) You should now experiment by observing the Moon at various points in its orbit to see how it varies. If you do this with care, you will find that the number of degrees the Moon moves per day varies slightly throughout the orbit. Can you think of why this would be? (Hint: remember Kepler's 2nd Law.)
You can find the dates and times of the Moon phases in various places, including some of those listed in our "Links" file.
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New MoonThe New Moon is so close in line with the Sun that it cannot be seen at all. It rises with the Sun in the east and sets with it in the west. A few days before New Moon, you can see a thin crescent in the east just before sunrise. A few days after New Moon you can see a thin crescent in the west shortly after sunset. The crescent Moon before sunrise is called a "Waning Crescent Moon," and after sunset it is called a "Waxing Crescent Moon." This later phase progresses toward First Quarter. |
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First Quarter MoonThe First Quarter Moon occurs about a week after New Moon, rising at about mid-day and setting at about midnight. It can be seen in the evening, but not in the morning. You can see this Moon in broad daylight, in the east or southeast in the afternoon. By sunset, it is high in the southern sky. It is shaped somewhat like the letter "D," which is more or less on its back when rising, upright when it is in the southern sky, and turned with the curved side down when it is near to setting in the west or southwest. It is a "quarter" Moon because all you see is one quarter of the Moon's surface (you have to count the back side, too). |
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Full MoonWith the Full Moon you see all of the visible side of the Moon. In the sense that the quarter moons are only quarters, this Moon is really a half Moon! You will see it rising in the east at about sunset, high in the south at midnight, and setting in the west at about sunrise. The exact orientation of features depends on where it is in the sky. This depiction shows the Full Moon as it would appear high in the southern sky, at roughly midnight. |
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Last Quarter MoonThe Last Quarter Moon is often called the Third Quarter Moon. It rises at about midnight and can be seen in the predawn sky. It is high in the south at sunrise, and can be seen for a time in the southwest even after sunrise. Its shape is like that of a backwards letter "D." |
Note that this illustrates only the 4 major phases of the Moon. Chances are that you will observe it in one of the intermediate stages. The phases themselves are given for precise moments in time, but it is unlikely that you will be observing at the exact moment of, say, First Quarter. Still, the Moon phases should appear much like this on the days listed for each phase, and gradually melding into the next as days pass. The Moon will appear a bit farther East each day.
