1.7: Accessible Descriptions

Figure 1.1.1 The Scientific Method

A circular diagram showing the six repeating steps of the scientific method.

Diagram Elements

• Six rounded boxes are arranged in a circle around the center label "Scientific method."
• Starting at the top and moving clockwise, the boxes read: "Observation / question," "Research topic area," "Hypothesis," "Test with experiment," "Analyze data," and "Report conclusions."
• Gray arrows connect each box to the next, circling back from "Report conclusions" to "Observation / question" to show that the cycle repeats.

Figure 1.1.2 How Science Works

A flowchart illustrating how the scientific method leads to laws and theories over time.

Flowchart Elements

• An "Observation and curiosity" box has an arrow pointing to a box labeled "Form hypothesis; make prediction."
• A curved arrow labeled "Next" connects this box to a box labeled "Perform experiment; make more observations."
• Another arrow, labeled "Results not consistent with prediction," points back from the experiment box to the "Form hypothesis; make prediction" box.
• An arrow labeled "Results are consistent with prediction" points from the "Perform experiment" box to a box labeled "Contributes to body of knowledge."
• An arrow labeled "Further testing does not support hypothesis" points from "Contributes to body of knowledge" back to "Form hypothesis; make prediction."
• An arrow labeled "Much additional testing yields constant observations" leads from "Contributes to body of knowledge" to a box labeled "Observation becomes law."
• An arrow labeled "Much additional testing supports hypothesis" leads from "Contributes to body of knowledge" to a box labeled "Hypothesis becomes theory."

Figure 1.2.1 Stonehenge

A photograph of Stonehenge, a prehistoric monument in southwest England used to track the motions of the Sun and Moon.

Image Elements

• Large upright stone pillars are arranged in a circle on a grassy field, with some pillars topped by horizontal stone lintels.
• Several stones have fallen or lie flat on the ground within and around the circle.
• Some of the standing stones align with the direction of sunrise and sunset on the summer and winter solstices.

Credit: Adriano Aurelio Araujo

Figure 1.2.2 El Caracol

A photograph of El Caracol, a Mayan observatory at Chichen Itza in the Yucatan, Mexico.

Image Elements

• A round stone tower with a domed, snail-shell-like upper structure sits atop a wide rectangular platform.
• A long stone staircase leads up the front of the platform to a doorway at the base of the tower.
• Small window openings near the top of the tower allowed Maya astronomers to observe the sky.

Credit: wiredtourist.com/Flickr

Figure 1.2.3 Earth's Round Shadow

A multiple-exposure photograph of a total lunar eclipse, showing Earth's curved shadow crossing the Moon.

Image Elements

• Nine images of the Moon are shown side by side, left to right, tracking the progress of the eclipse over time.
• On the far left and right, the Moon appears as a bright, full white disk with a small dark bite taken out of one edge as Earth's shadow begins and ends crossing it.
• Moving toward the center, the dark shadow covers more of the Moon, and the curved edge of the shadow is clearly visible.
• In the three center images, the Moon appears dull red rather than fully dark, because sunlight refracted through Earth's atmosphere still reaches it.

Credit: Brian Paczkowski

Figure 1.2.4 Epicycles

A diagram of Ptolemy's model of planetary motion, showing a planet moving on a small circle that itself orbits Earth.

Diagram Elements

• Earth is shown as a dot slightly left of center.
• A yellow dot labeled Center marks the middle of the diagram.
• A large circle called the Deferent surrounds Center, with an arrow showing the planet's overall path moving counterclockwise.
• A yellow dot labeled Equant point sits to the right of Center. An arrow labeled y runs from the Equant point to a point on the Deferent.
• A small circle called the Epicycle is centered on that point on the Deferent, with a counterclockwise arrow.
• The planet, a yellow dot labeled x, lies on the Epicycle, so its actual position combines motion around the Epicycle with motion of the Epicycle around the Deferent.

Figure 1.3.1 The Heliocentric Model

Copernicus' original diagram of the Sun-centered Solar System, with the orbits labeled in Latin.

Diagram Elements

• The Sun, labeled Sol, sits at the center of a series of concentric circles.
• Moving outward from the Sun, the circles are labeled, in order: Mercury, Venus, Telluris (Earth, with the Moon's orbit included), Martis (Mars), Jovis (Jupiter), and Saturnus (Saturn).
• The outermost circle is labeled Stellarum Fixarum, representing the fixed stars beyond the planets.

Credit: Nicolai Copernici

Figure 1.3.2 Retrograde Motion

A two-part diagram showing why an outer planet like Mars appears to move backward in the sky as Earth passes it.

Left Panel: Orbits

• The Sun is at the center, surrounded by two circles: an inner orbit for Earth and an outer orbit for Mars.
• Earth and Mars are each marked at five positions, labeled A through E, along their orbits.
• Because Earth travels faster than Mars, Earth's five positions are spread evenly around its orbit, while Mars's five positions are bunched closely together.
• A line drawn from each Earth position through the corresponding Mars position points toward that planet's apparent position among the stars.

Right Panel: Apparent Path Among the Stars

• The path traced by Mars as seen from Earth is plotted against a background of stars.
• From position A to B, Mars appears to move upward (eastward).
• From B to C to D, as Earth overtakes Mars, the planet appears to reverse and move downward (westward) before continuing to D.
• From D to E, Mars resumes its normal upward (eastward) motion.
• Overall, the path traced makes a sideways loop, or "Z" shape, in the sky.

Figure 1.3.3 Phases of Venus

A diagram of Venus's orbit around the Sun, showing how its phases change as seen from Earth.

Diagram Elements

• Earth is shown at the bottom of the diagram, orbiting the Sun, which is at the center.
• Venus is shown in six positions along its orbit around the Sun.
• On the side of the orbit closer to Earth, Venus appears as a large crescent, and its crescent narrows further as it moves toward the position directly between Earth and the Sun.
• On the side of the orbit farther from Earth, Venus appears increasingly full, moving from half-illuminated to gibbous as it passes behind the Sun.
• The two positions labeled Half show Venus roughly half-illuminated, similar to the Moon's first or last quarter.

Figure 1.3.4 Conic Sections

A diagram of a cone sliced at different angles to produce the four conic section curves.

Diagram Elements

• A cone is drawn with its circular base at the bottom and its apex at the top.
• Near the top, a plane parallel to the base produces a Circle, shown in orange.
• Below that, a plane angled through the cone without touching the base produces an Ellipse, shown in red.
• Further down, a plane angled through the cone that touches the base produces a Parabola, shown in teal.
• Near the bottom, a plane nearly perpendicular to the base and touching it produces a Hyperbola, shown in blue.

Figure 1.3.5 Drawing an Ellipse

Two panels demonstrating how to draw an ellipse using string and thumbtacks.

Panel (a)

• On the left.
• Two thumbtacks are pushed into a drawing board with a loop of string around them.
• A hand holds a pencil against the string, pulling it taut, and traces the resulting curved ellipse.

Panel (b)

• On the right.
• The same setup is shown with the center of the ellipse marked by a red dot between the two thumbtacks.
• Each of the two semimajor axes, labeled a, is marked as the distance from the center to the farthest edge of the ellipse on either side.

Figure 1.3.6 Kepler's Second Law: The Law of Equal Areas

A diagram of a planet's elliptical orbit around the Sun, illustrating that it sweeps out equal areas in equal times.

Diagram Elements

• The Sun sits at one focus of a blue elliptical orbit, with an arrow showing the planet's counterclockwise motion.
• A shaded wedge-shaped region labeled A is swept out on the far side of the ellipse, from time 1 to time 2, where the planet moves a short distance because it is far from the Sun.
• A second shaded wedge-shaped region labeled B is swept out on the near side of the ellipse, from time 3 to time 4, where the planet moves a much greater distance because it is close to the Sun.
• Both wedge-shaped regions, A and B, have the same area, showing that the planet moves faster near the Sun and slower far from the Sun to sweep out equal areas in equal times.

Figure 1.3.7 Astronauts in Free Fall

A photograph of four astronauts floating weightlessly together inside the International Space Station.

Image Elements

• Four astronauts in blue and tan shirts are gathered in a cluster near a round window, each oriented at a different angle.
• Their relaxed, floating postures and mixed orientations show that there is no fixed "up" or "down" inside the orbiting station.

Credit: NASA