Lunar Phases
Introduction
The Moon orbits Earth from west to east—a “counterclockwise” direction. It reflects light from the Sun and is considered Earth’s only natural satellite. During the course of about one month on Earth, we view the Moon as it travels in an elliptical orbit (Earth is located at one of the two foci). The lunar surface facing the Sun is almost always illuminated. Since Earthlings don’t always see this entire illuminated side, humans have come to label the illuminated portions that are visible from Earth. These are called the lunar phases.
Concepts
- The Moon’s orbit
- Lunar eclipse
- Ellipses and foci
- Lunar formation
- Lunar phases
- Waxing vs. Waning
- Solar eclipse
Background
Helpful Definitions
- Lunar Phase—The amount and shape of the illuminated surface of the Moon visible from the Earth.
- Insolation—Solar radiation striking Earth or another planet. An arrow will represent insolation in this activity.
- Waxing—A term that describes a trend in lunar phases over a two-week period during which increasing amounts of the illuminated half of the Moon become more visible from Earth each night. Waxing phases begin with the New Moon phase and end with the Full Moon phase.
- Waning—A term that describes a trend in lunar phases over a two-week period during which decreasing amounts of the illuminated half of the Moon become less visible from Earth each night. Waning phases begin with the Full Moon phase and end with the New Moon phase.
- Crescent—A phase of the Moon where the illuminated portion seen from Earth resembles the shape of a crescent. Other objects shaped like a crescent are the Russian (hammer and) sickle and French croissant. Of the entire side of the Moon that faces Earth, less than half of the lunar surface is illuminated during a crescent phase.
- Gibbous—A phase with more than half of the lunar surface facing Earth illuminated.
- Quarter—As a three-dimensional object, the Earth can be split into four equal parts or quarters. Early descriptions of the lunar phases were labeled by their quarter number. The position of the Moon in its orbit during the First Quarter phase is 90° counterclockwise around its orbit from its starting point—the New Moon phase. The Second Quarter phase is called the Full Moon. The Third Quarter phase is located 270° from the New Moon phase. Finally, the fourth quarter would represent a full circuit—a return to the starting point, the New Moon position.
- New Moon—Traditionally a starting point for labeling the lunar phases. The Moon is not visible.
- Full Moon—The entire illuminated portion of the Moon is visible from Earth.
- “Top” View—A view from above looking down on something. Since there is no “up” or “down” in space, on diagrams where a “top” view is used it means the observer is positioned above the North Pole of Earth, looking down on Earth.
- “Profile” View—A view from the side of something.
- Radial—A pattern where lines are drawn outward from a center point (like the spokes of a wagon wheel).
The Moon’s position in the night sky is cyclic and predictable. It is difficult, however, to predict the Moon’s position due to the Moon’s inclined (tilted) plane of orbit around Earth. The Moon’s orbit is tilted about five degrees from Earth’s plane of orbit. Also, since the lunar orbit is inclined, it has a high point and a low point once a month—these positions also change throughout the course of a year. This results in a cyclic but complicated pattern for the motions of the Moon.
Occasionally, due to its inclined orbit, the Moon will pass directly between the Sun and the Earth. The Moon is usually too high or too low in its orbit for its shadow to be cast on Earth. When the Moon’s shadow is cast on the Earth (about 2–3 times a year), the Moon will block out part of the light from the Sun, creating a solar eclipse. The position of the Moon between the Sun and Earth usually yields a New Moon phase—except when the Moon’s orbital tilt places it precisely in the correct location to cast a shadow on Earth.
On the opposite side of its orbit, the Moon will occasionally pass directly behind the Earth and into its shadow. When the Earth blocks the illumination from the Sun, a lunar eclipse occurs. The surface of the Full Moon may appear to turn red. A lunar eclipse occurs on average 3–5 times a year.
According to current theory, the Moon was created about four billion years ago from a catastrophic collision between a Mars-sized object and Earth. The resulting material that “splashed” out from Earth quickly cooled to form the Moon. From the remaining debris orbiting Earth came the millions of meteors that bombarded the fresh surface of the Moon, leaving it cratered and pockmarked to this day. Although the Earth was also bombarded by meteors, these vestiges have been mostly weathered away due to Earth’s dynamic atmosphere. Based on this violent beginning, Lunar rotation matches its rate of revolution around Earth. Strangely, this motion keeps the same side of the Moon facing the Earth at all times. In fact, it wasn’t until a Soviet probe orbited the Moon and first photographed the “far side” that humans finally got to see it!
Experiment Overview
In this activity, a complete lunar orbit will be graphed.
Materials
Lunar Orbit Graph Pencil Ruler, metric
Safety Precautions
Although this activity is considered relatively safe, please follow normal laboratory safety guidelines. Never look directly at the Sun—even during a Solar Eclipse.
Procedure
Part I. Drawing the Lunar Positions on the Lunar Orbit Graph
- Obtain a Lunar Orbit Graph. The Earth is labeled in the center of the graph. The bold line is the ellipse formed by the lunar orbit (longitude) of the Moon. The outer ring represents the solar longitude.
- Use the Lunar Coordinates Data Table to plot the Lunar longitude at the appropriate location on the Lunar Orbit Graph (see example for July 1st on the graph). Place a dot on the bolded lunar orbit longitude ellipse shown on the graph for each date listed for July. These dots will represent the location of the Moon in relation to the Earth. Note: The values on the following table correspond to celestial longitude values found on the Lunar Orbit Graph paper.
{12644_Procedure_Table_1_Lunar Coordinates Data Table}
- Label each of the lunar longitude positions with the appropriate July date.
- Draw arrows on the lunar orbit ellipse showing the direction of motion of the Moon around Earth.
- Draw an arrow representing the direction of Solar Longitude for each of the solar longitude degrees listed in the Lunar Coordinates Data Table. This is the direction of incoming solar radiation each day in July. Place the arrow outside the radial graph pointing past Earth (notice that the arrow for July 1st has already been completed).
- Label each insolation arrow with the appropriate date.
Part II. Locating and Drawing the New Moon, Full Moon, First Quarter and Third Quarter Phases
{12644_Procedure_Figure_1}
- Locate the July 1st position (dot) of the Moon between the Sun and Earth on the Lunar Orbit Graph.
- Enlarge this dot to a 1.0 cm diameter circle (this will represent the position of the New Moon in its orbit).
- Shade in the portion of this circle (with a pencil) that would not receive sunlight on its surface in the Lunar Phase Diagram or the Lunar Orbit Graph. Positions on the Lunar Orbit Graph do not currently show lunar phases. Lunar phases are dependent upon the observer seeing the Moon from Earth. Since you, the observer, are viewing the relative positions of the Earth, Moon and Sun from a position “above” them, by definition you are not actually viewing the illuminated portions of the Moon’s surface from Earth.
- Rotate the Lunar Orbit Graph so that you can view the Earth (remember, the Earth is in the center of the graph), the July 1 position of the Moon and the general direction of insolation (incoming solar radiation). Note: The insolation arrows will not line up exactly with the Moon and the Earth. To an observer on Earth, the entire side of the Moon facing Earth would be in shadow. To represent this entire unlit portion, shade in the dashed-line circle shown on the Lunar Orbit Graph (see Figure 2).
{12644_Procedure_Figures_2-5}
- Label this the New Moon Phase.
- Repeat steps 1–6 on the appropriate Lunar Phase Diagrams for July 13th (Full Moon Phase—see Figure 3), July 7th (First Quarter Phase—see Figure 4) and July 19th (Third Quarter Phase—see Figure 5).
Part III. Waxing and Waning Trends in Lunar Phases
As the Moon revolves around Earth in a counterclockwise direction (from West to East), there is a period of about two weeks (13–15 days) where Earthlings see illuminated portions of the lunar surface increasing each night. Then, for another period of about two weeks (13–15 days), Earthlings view the amounts of illuminated surface of the moon diminishing each night. These trends in the phases are called “waxing” and “waning.” Lunar phases are considered “waxing” between the New Moon and Full Moon positions. Each phases has a little more of the illuminated half of the Moon that is visible from Earth. Lunar phases are considered “waning” if they are located between the Full and New Moon phases (see Figure 6). Each phase displays a little less of the illuminated half of the Moon visible from Earth each night.
{12644_Procedure_Figure_6}
A. Locating the Waxing Phases
- Draw a dot on the Lunar Orbit Graph halfway between the New Moon and the First Quarter Phase. Calculate the Lunar longitude of this point by using the lunar longitude values in the Lunar Coordinates Data Table.
- Enlarge the dot to a 1.0 cm diameter circle.
- Label this position “Waxing …………” on the appropriate Lunar Phase Diagram. The second word will be filled in later.
- Draw a dot on the Lunar Orbit Graph halfway between the First Quarter Moon and the Full Moon Phase. Calculate the Lunar longitude of this point by using the lunar longitude values in the Lunar Coordinates Data Table.
- Enlarge the dot to a 1.0 cm diameter circle.
- Label this position “Waxing …………” on the appropriate Lunar Phase Diagram. The second word will be filled in later.
B. Locating the Waning Phases
- Draw a dot on the Lunar Orbit Graph halfway between the Full Moon and the Third Quarter Phase. Calculate the Lunar longitude of this point by using the lunar longitude values in the Lunar Coordinates Data Table.
- Enlarge the dot to a 1.0 cm diameter circle.
- Label this position “Waning …………” on the appropriate Lunar Phase Diagram. The second word will be filled in later.
- Draw a dot on the Lunar Orbit Graph halfway between the Third Quarter Moon and the New Moon Phase. Calculate the Lunar longitude of this point by using the lunar longitude values in the Lunar Coordinates Data Table.
- Enlarge the dot to a 1.0 cm diameter circle.
- Label this position “Waning …………” on the appropriate Lunar Phase Diagram. The second word will be filled in later.
C. Locating the Crescent Phases
The Moon appears in a crescent phase while in orbit on either side of the New Moon position. A crescent phase is when the Moon is in orbit halfway between a Quarter phase and the New Moon phase position. Whether or not the amount of illumination we see from Earth is increasing or decreasing, a crescent phase will have another term attached: “waxing” or “waning.”
- Draw the Waxing Crescent Phase on the appropriate Lunar Phase Diagram (see Figure 7).
{12644_Procedure_Figure_7}
- Complete the label for this phase on the appropriate Lunar Phase Diagram so it reads “Waxing Crescent” Phase.
- Draw the Waning Crescent Phase on the appropriate Lunar Phase Diagram (see Figure 8).
{12644_Procedure_Figure_8}
- Complete the label for this phase on the Lunar Phase Diagram so it reads “Waning Crescent” Phase.
D. Locating the Gibbous Phases
The Moon appears in a gibbous phase while in orbit on either side of the Full Moon position. A Gibbous Phase is when the Moon is in orbit halfway between a Quarter phase and the Full Moon phase position. Whether or not the amount of illumination we see from Earth is increasing or decreasing, a gibbous phase will have another term attached: “waxing” or “waning.”
- Draw the Waxing Gibbous Phase on the appropriate Lunar Phase Diagram (see Figure 9).
{12644_Procedure_Figure_9}
- Complete the label for this phase on the appropriate Lunar Phase Diagram so it reads “Waxing Gibbous” Phase.
- Draw the Waning Gibbous Phase on the appropriate Lunar Phase Diagram (see Figure 10).
{12644_Procedure_Figure_10}
- Complete the label for this phase on the appropriate Lunar Phase Diagram so it reads “Waning Gibbous” Phase.
|