Use the Starry Night Enthusiast \({ }^{\mathrm{TM}}\) program to observe the
apparent change in size of the Moon as seen from the surface of the Earth.
Click the Home button in the toolbar. Stop the time flow by clicking the Stop
time button (the button with a square icon in the Time Flow Rate section of
the toolbar). Click each field of the Time and Date display pane in the
toolbar and use the keyboard to set the time to \(12: 00: 00 \mathrm{~A} .
\mathrm{M}\). and the date to \(\mathrm{O}\) tober 16,2006 . You can now set up
the view of the Moon as if you were observing from a transparent Earth in
continuous darkness. Open the Options pane by clicking its tab on the left
border of the view window. In the Local View layer of the Options pane,
uncheck the boxes beside the Daylight and Local Horizon options. Open the Find
pane and click the menu button at the left of the entry for the Moon and
select Magnify from the menu to display a stationary image of the magnified
Moon in the center of the view. Notice the Moon's phase. (a) Select Options >
Solar System > PlanetsMoons... from the menu. In the Planets-Moons dialog box,
click the slide control next to the Show dark side label near the top of the
window and adjust the control all the way to the right (Brighter) side of the
scale. Then click the OK button to close the dialog box. Note that the image
of the Moon now appears full because Starry Night Enthusiast TM has
artificially brightened the dark side of the image of the Moon, effectively
removing the appearance of the Moon's phases.
(b) Use the + button in the Zoom section of the toolbar to adjust the field of
view to about \(55^{\prime} \times 45^{\prime}\). Starry Night Enthusiast \({
}^{\mathrm{TM}}\) can display a reference field of view (FOV) upon this sky.
Open the FOV pane by clicking its tab. Select the 30 Arcminutes option. Then
click the FOV tab to close the pane. (c) The final view is of the Moon, its
dark side artificially brightened, as it would be seen from your home location
if the Earth were airless and transparent, surrounded by a yellow FOV circle
30 arcminutes in diameter. Note the size of the Moon relative to this
reference circle. Set the Time Flow Rate in the toolbar to 1 minute. Run Time
Forward for least 24 hours and observe the apparent size of the Moon relative
to the reference circle. (c) Note that the apparent size of the Moon changes
somewhat over the course of a day (of simulated time). Explain in terms of the
Earth's rotation. (Hint: In this view, Starry Night Enthusiast \({ }^{\text {Tm
}}\) has made the Earth transparent, so you can see objects that would normally
be below the horizon. As the Earth rotates, your observing location on the
surface is carried along and your distance from the Moon changes.) (d) Change
the Time Flow Rate to 1 day and again click the Run Time Forward button. Does
the apparent size of the Moon always stay the same, or does it vary? Explain
what this tells you about the shape of the Moon's orbit.
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