MoonPhases

General Description

This program illustrates how the appearance of the Moon changes depending on the position of Moon relative to the Earth and Sun. The right window shows Earth (at the center) and the Moon, as well as a circle tracing out the Moon's orbit. Sun is far to the right in this picture and therefore the right side of the Earth and the Moon are bright while the left sides are dark. A glowing band of light (green) shows the portion of the Moon visible from the Earth, as well as the portion of the Earth from which the Moon is visible. The angle between the Sun and the Moon (as seen from the Earth) can be adjusted. A green dot indicates an observer on Earth and the observer's local (mean solar) time can be set, which effectively changes the observer's location on the Earth in this picture.

The left window shows the appearance of the Moon as seen from Earth when Moon is in the position shown in the right window. Note that the darkness of the dark part of the Moon changes depending on the loction of the Moon in its orbit. This is because the Moon receives reflected light from Earth, a phenomenon known as Earthlight, and the amount of reflected light received is connected to the location of the Moon in its orbit.

Visual Elements

Orbit Frame

• Blue/black disk: Earth (greatly exaggerated in size compared to Moon's orbit).
• Green dot: shows location of observer on Earth.
• White/black disk: Moon (also exaggerated in size).
• White circle: Moon's orbit around Earth.
• Yellow arrows: indicate the direction of incoming sunlight. Sun is far to the right.
• Transparent green band (light cone): shows the portion of Moon that is visible from Earth as well as the portion of Earth from which Moon is visible. When the observer (green dot) is in the lightcone (green band), he or she can see the Moon.

Moon View Frame

• Shows view of Moon as seen from Earth when Moon is in the position shown in the Orbit Window. Illustrates the phase of Moon as well as the Earthlight phenomenon (see above).

Options

• Show Moon View: select to show view of Moon as seen from Earth.
• Show Observer: select to show the location of an observer on Earth. The local time for the observer is given at the bottom of the window and can be adjusted using the slider.
• Show Light Cone: select to show light cone indicating portions of Moon that can be seen by Earth and portions of Earth that can see Moon.

Controls

• Play/Pause Button: play/pause Moon's motion along its orbit.
• Step Button: steps the Moon's motion along its orbit forward one time step.
• Reset Button: resets teh simulation.
• Angle: slider controls the angle between Sun and Moon, as seen from Earth. This effectively moves Moon around in its orbit as seen in the Orbit Window.
• Local Time: sets the local mean solar time for an observer (indicated by the green dot) on Earth. Time is in hours, with 0 corresponding to midnight.