MS-ESS1-2NGSS*ScienceEarth and Space ScienceGrades 6-8
MS-ESS1-2: Modeling How Gravity Holds the Solar System and the Galaxy Together
In plain English: MS-ESS1-2 is the NGSS middle school performance expectation that asks students to develop and use a model to describe the role of gravity in the motions within galaxies and the solar system. Students use a football field scale model, orbit simulations and size proportions to explain how gravity holds planets, moons and stars in orbit. It is usually taught in middle school Earth and space science.
Develop and use a model to describe the role of gravity in the motions within galaxies and the solar system.
Clarification Statement: Emphasis for the model is on gravity as the force that holds together the solar system and Milky Way galaxy and controls orbital motions within them. Examples of models can be physical (such as the analogy of distance along a football field or computer visualizations of elliptical orbits) or conceptual (such as mathematical proportions relative to the size of familiar objects such as students' school or state).
Assessment Boundary: Assessment does not include Kepler's Laws of orbital motion or the apparent retrograde motion of the planets as viewed from Earth.
Next Generation Science Standards · Disciplinary Core Idea: Earth's Place in the Universe · Official standard
Students build and use models to describe what gravity does in the solar system and in the Milky Way galaxy. Gravity is the pull between any two objects that have mass. An orbit is the curved path of one object around another, and a galaxy is a huge group of stars, gas and dust held together by gravity. Students walk a football field model of planet distances, use a computer orbit simulation to see what happens when gravity is switched off or the launch speed changes, and shrink the Milky Way to the size of a state and their school. With each model they explain the same idea: gravity pulls each object toward the body it orbits, and the object's sideways motion keeps it from falling in, so the path bends into a loop.
The NGSS Clarification Statement says: "Emphasis for the model is on gravity as the force that holds together the solar system and Milky Way galaxy and controls orbital motions within them. Examples of models can be physical (such as the analogy of distance along a football field or computer visualizations of elliptical orbits) or conceptual (such as mathematical proportions relative to the size of familiar objects such as students' school or state)." The Assessment Boundary says: "Assessment does not include Kepler's Laws of orbital motion or the apparent retrograde motion of the planets as viewed from Earth." So this page uses no orbit laws or formulas for gravity: students describe the role of gravity in words and use simple proportions for scale. Distances are published values, rounded. One AU (astronomical unit) is Earth's average distance from the Sun, about 150 million km, and one light-year is the distance light travels in a year, about 9.46 trillion km. Simulation data are labeled as such.
Learning Objectives
By the end of this lesson, students will be able to:
Use a model to explain that an orbit is the result of gravity pulling an object toward another while the object moves sideways
Use a computer orbit simulation to describe how gravity bends a path into a circle or an oval (ellipse), and what happens without it
Build a football field or school scale model of solar system distances and use proportions to place objects
Describe gravity as the force that holds the Milky Way together and keeps the Sun and other stars orbiting its center
Critique a model of gravity by naming what it shows well and what it leaves out
Prior Knowledge Required
Students should already be comfortable with:
Earth's gravity pulls objects down, toward Earth's center 5-PS2-1
Gravity is an attractive force that depends on the masses of the objects MS-PS2-4
Earth orbits the Sun once a year and the Moon orbits Earth about once a month
Multiplying and dividing decimals and writing a ratio as a fraction
Draw a large circle on the board with the Sun in the middle and Earth on the circle, with an arrow showing which way Earth is moving.
Warm-Up Prompt
"Imagine the Sun's gravity switched off at this exact moment. Draw the path Earth would take next. Then explain in one sentence what the Sun's gravity was doing to Earth before it switched off."
Collect drawings. Many students draw Earth flying straight out, away from the Sun, or spiraling outward. Ask: "Which way was Earth moving just before? Does anything push it outward?" Lead students to see that without a pull, Earth would keep going straight in the direction it was already moving. Keep the drawings to check against the simulation later.
Direct Instruction15 minutes
Key words. A model is a simplified version of a real system that we use to explain and predict. A scale model shrinks every distance by the same factor, so the proportions (the comparisons by division) stay true. An ellipse is an oval shape; many orbits are ellipses that are close to circles. Every object moving through space keeps going in a straight line at the same speed unless a force acts on it. In space, that force is usually gravity. Use Diagram 1 with the first example and Diagram 2 with the second.
Football field model of planet distances
Put the Sun on one goal line and Neptune, 30.07 AU away, on the other goal line, 100 yards away. Each AU becomes 100 ÷ 30.07 ≈ 3.33 yards. Earth (1.00 AU) goes at 3.3 yards, Jupiter (5.20 AU) at 17.3 yards and Uranus (19.19 AU) at 63.8 yards.
Result: All four rocky planets sit within about 5 yards of the Sun, and the giant planets are spread over the rest of the field. The Sun's gravity reaches across the whole field: it is the pull that keeps even Neptune, 4.5 billion km away, moving around the Sun instead of off into space.
Computer visualization: gravity on and off
In an orbit simulation, a planet starts 1 AU from a star like the Sun, moving sideways at 30 km/s. With gravity on, its path bends into a loop that is almost a circle. When the student switches gravity off partway around, the planet moves off in a straight line, in the direction it was moving at that moment.
Result: Gravity does not push the planet forward: the planet's own motion does that. Gravity pulls it toward the star, bending its path, again and again, into a closed loop. Without gravity there is no orbit. (Diagram 2 shows the paths for three launch speeds.)
Orbits inside orbits
The Moon goes around Earth about every 27.3 days at about 384,400 km. At the same time, Earth and the Moon travel together around the Sun once every 365.25 days, at about 150 million km.
Result: Two pulls work at once. Earth's gravity is strong at the Moon's distance and keeps the Moon in its loop around Earth. The Sun's gravity pulls on Earth and the Moon together and keeps both of them on their path around the Sun. The Sun's distance is about 390 times the Moon's (150,000,000 ÷ 384,400 ≈ 390).
Gravity in the galaxy
The Sun is about 26,000 light-years from the center of the Milky Way and moves around it at about 220 km/s. One trip around the center is about 2 × π × 26,000 light-years long.
Result: At that speed one trip takes roughly 220 million years. The Sun has not flown off in a straight line because the combined gravity of the galaxy's billions of stars, gas and dust pulls it toward the center, just as the Sun's gravity pulls on the planets. Every star in the galaxy is in orbit.
Proportion model: the galaxy as a state
The Milky Way's disk is about 100,000 light-years across. Shrink it to the size of a state about 1,200 km across. Then each light-year becomes 1,200 ÷ 100,000 = 0.012 km, or 12 m.
Result: The Sun sits 26,000 × 0.012 ≈ 312 km from the center of the state. The whole solar system out to Neptune's orbit, about 60 AU or 0.00095 light-years across, becomes about 1.1 cm: smaller than a coin, and gravity holds it together inside a galaxy the size of the state.
Planet distances from the Sun (published average values, rounded) and positions on a 100-yard football field
Planet
Distance from Sun (AU)
Football field position (yards)
Mercury
0.39
1.3
Venus
0.72
2.4
Earth
1.00
3.3
Mars
1.52
5.1
Jupiter
5.20
17.3
Saturn
9.54
31.7
Uranus
19.19
63.8
Neptune
30.07
100.0
Ask students what the field shows that a textbook picture of the planets usually does not: how far apart they are, and how empty the space between them is. Then ask what holds such a spread-out system together. Note that this page does not use the laws that describe orbit shapes and timing (Kepler's laws), which are beyond this standard.
Guided Practice15 minutes
Pairs analyze results from an orbit simulation (simulation data). The star has the Sun's mass. A planet starts 1 AU from the star every time and is launched sideways, straight up the page, at a different speed in each run. The table gives the other end of each path.
Orbit simulation runs (simulation data, rounded): planet launched sideways 1 AU from a star like the Sun
Run
Launch speed (km/s)
What the path does
Distance from star at the other end of the path (AU)
1
0
Falls straight into the star
0
2
20
Narrow oval, swings close to the star
0.29
3
25
Oval inside the starting distance
0.54
4
30
Almost a circle
1.03
5
35
Oval that swings outward
2.23
6
40
Long oval
9.18
7
45
Never comes back
none
Sketch the paths for runs 2, 4 and 6 around one star, all starting from the same launch point. Compare your sketch with Diagram 2.
Describe the pattern: what happens to the path as the launch speed goes up?
Explain run 1. What does it show about the direction of gravity's pull?
Explain run 7 in terms of gravity. Is gravity switched off in that run? (No: the pull is there, but the planet moves too fast for the pull to bend its path into a closed loop.)
Expected answers: with no sideways speed, gravity pulls the planet straight in, so the pull points toward the star. A little sideways speed gives an oval that dives toward the star; about 30 km/s gives a near circle; more speed makes the oval swing farther out; at 45 km/s the path never closes. Gravity controls every path, and the launch speed decides its shape.
Independent Practice10-15 minutes
Nested orbits model. Each student draws a conceptual model with four levels: the Moon around Earth, Earth around the Sun, the Sun around the center of the Milky Way, and a small galaxy around the Milky Way. For each level they draw an arrow for the pull of gravity, an arrow for the direction of motion, and write the time for one orbit when it is known (27.3 days, 365.25 days, about 220 million years). Then partners trade drawings and check three things: every gravity arrow points toward the object being orbited, every motion arrow is sideways to it, and the drawing says what would happen if the pull stopped.
Closure5 minutes
Return to the warm-up drawings and ask students to fix them using the simulation: the path should leave in a straight line in the direction Earth was moving. Exit ticket: "Name one thing gravity does in the solar system and one thing it does in the Milky Way, and the model from today that shows each." Look for answers that use the idea of a pull toward the center plus sideways motion.
Differentiation Strategies
For Struggling Students
Give the frame: "Gravity pulls ___ toward ___. ___ is moving sideways, so instead of falling in, its path ___."
Pre-mark the football field positions on a strip of adding-machine tape before going outside, so students can check their own
Pair the simulation with a short video clip of a planet's orbit before students change any settings
For Advanced Students
Ask students to use the simulation to find the slowest launch speed at 1 AU that never comes back, and explain the result in terms of gravity
Have students build the football field model for the Large Magellanic Cloud, a small galaxy about 160,000 light-years away that orbits the Milky Way
Ask students to compare the spandex-sheet model of gravity with the simulation and decide which shows the role of gravity more honestly
Assessment Guidance
What to Look For
Check that students describe an orbit as a pull toward the center plus sideways motion, not as a balance in which the planet is pushed outward, and not as "no gravity in space." Look for proportions set up the same way each time (distance × scale factor) with units. Students should name gravity as what holds the galaxy together, not only the solar system. Students do not need orbit laws: if they ask why outer planets take longer to orbit, say that the laws that describe this come in high school.
02
Classroom Activities
3 Activities
1
Football Field Solar System
25 minGroups of 3-4
The class builds the planet-distance model on a football field, with a cone for the Sun on one goal line and a cone for each planet. Groups then walk the model and use it to talk about how far the Sun's gravity reaches.
Procedure
Each group takes one or two planets, finds the yard position in the table and places a labeled cone there. Use the yard lines for the outer planets and a measuring tape for the inner planets.
Walk from the Sun to Neptune and count your steps.
Stand at Jupiter and look back at the Sun's cone. Then stand at Neptune.
Safety: walk, don't run, stay on the field, and check the ground for holes and sprinkler heads first.
Discussion Questions
The four rocky planets are inside the first 5.1 yards. What does that tell you about where most of the solar system's space is?
Neptune is 30 times as far from the Sun as Earth. The Sun's pull is much weaker out there, yet Neptune still orbits. What does that tell you about the Sun's gravity?
The model shows distances but not sizes. At this scale the Sun would be only about 3 cm wide. What does the model leave out, and why might that matter?
Modification for Distance Learning
Students use a hallway, a sidewalk or a strip of paper 1 m long for 100 yards (so 1 yard becomes 1 cm), place labeled sticky notes and post a photo.
2
Orbit Simulation Investigation
20 minPairs
Pairs use a free orbit simulation, such as PhET's Gravity and Orbits, to test what gravity does to a moving planet. They change one setting at a time and record the shape of the path.
Procedure
Start with a star and one planet. Run the simulation and trace the path on paper.
Turn gravity off while the planet is moving. Record the direction it leaves in.
Reset. Make the planet's starting speed smaller, then larger. Trace each path.
Reset. Make the star's mass larger and keep the planet's starting speed the same. Trace the path.
Discussion Questions
When you switched gravity off, did the planet fly straight out from the star, or keep going in the direction it was moving? What does that tell you about what gravity was doing?
When you made the star's mass larger, the pull got stronger. How did the path change?
The simulation is a model. What would you check against real data before trusting it?
Challenge Variation
Pairs add a moon to the planet and find a setting where the moon stays with the planet as both go around the star. They explain which pull keeps the moon nearby.
3
Shrink the Galaxy to Our State and Our School
20 minGroups of 3-4
Groups use proportions to shrink the Milky Way, first to a state about 1,200 km across and then to the school grounds, and mark objects on a map. They use the maps to explain how gravity holds the galaxy and its neighbors together.
Procedure
State model: 1 light-year = 0.012 km. Find the scaled distance from the galaxy's center to the Sun (26,000 light-years), to the edge of the disk (50,000 light-years) and to the Large Magellanic Cloud, a small galaxy that orbits the Milky Way (about 160,000 light-years).
Mark the center, the Sun and the edge on the state map. Decide where the Large Magellanic Cloud goes.
Pick a width for your school grounds, such as 300 m, and repeat with that scale.
Expected Results
State model: the Sun is 312 km from the center and the edge is 600 km out, the state's border. The Large Magellanic Cloud is 1,920 km from the center, well outside the state. With a 300 m school (1 light-year = 0.003 m), the Sun is 78 m from the center.
Discussion Questions
The Large Magellanic Cloud is outside our state map, yet it orbits the Milky Way. What force connects them across that gap?
Every star on your map is moving at hundreds of kilometers per second. Why hasn't the galaxy flown apart?
Which scale, the state or the school, makes the size of the galaxy easier to understand? Why?
Modification for Distance Learning
Groups mark the scaled distances on an online map with the measure-distance tool and share a screenshot.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: The Solar System on a Football Field
A physical scale model of distances: the Sun is on one goal line and Neptune is on the other, 100 yards away, so 1 AU is about 3.33 yards. The zoom shows the first 6 yards, where the four rocky planets sit. Planet sizes are not to scale.
Diagram 2: Simulated Paths for Three Launch Speeds
Simulation data drawn to scale: a planet launched sideways 1 AU from a star like the Sun at 25, 30 and 35 km/s. Gravity bends every path into a closed loop; the launch speed decides its shape. With gravity off, the planet goes straight on.
04
Homework Assignment
~30 min
MS-ESS1-2 Homework: Models of Gravity in Space
Directions: Use the models from class. Distances are published values, rounded; data marked invented or simulation data are made up for practice. 1 AU is about 150 million km and 1 light-year is about 9.46 trillion km. Show each proportion with its scale factor and units, and end each problem with one sentence about what gravity does.
Part 1: Gravity in the Solar System (Problems 1-3)
Your school hallway is 60 m long. Put the Sun at one end and Neptune (30.07 AU) at the other. (a) Find the scale factor in meters per AU. (b) Find where Earth (1.00 AU), Jupiter (5.20 AU) and Saturn (9.54 AU) go. (c) At this scale the Sun, 1.39 million km wide, is about 1.9 cm wide. Explain why a model can't easily show both sizes and distances.
Io, one of Jupiter's moons, goes around Jupiter about every 1.8 days. (a) Draw Io's orbit and mark one point P. Draw an arrow for Jupiter's pull on Io at P and an arrow for Io's motion. (b) Draw the path Io would take if Jupiter's gravity switched off at P. (c) Explain what Jupiter's gravity does to Io's motion.
Halley's comet was closest to the Sun in 1986, at about 0.6 AU. It reached its farthest point, about 35 AU, in late 2023, and it will be closest again in 2061. (a) Sketch its long oval path around the Sun, to scale with 1 AU = 2 mm. (b) What pulls the comet back toward the Sun after it reaches 35 AU? (c) Use your sketch to explain why the comet does not just keep going out into space.
Part 2: Gravity in the Galaxy (Problems 4-6)
Shrink the Milky Way (100,000 light-years across) to a school campus 200 m across. (a) Find the scale factor in meters per light-year. (b) Find the scaled distance from the center to the Sun (26,000 light-years) and to the nearest star, Proxima Centauri (4.24 light-years from the Sun). (c) The Andromeda galaxy is about 2.5 million light-years away. How far is that in your model, in kilometers?
In an invented model galaxy, every star moves at 220 km/s. Stars at 5,000, 10,000, 20,000 and 40,000 light-years from the center take about 43, 86, 171 and 342 million years to go around once. (a) Graph time for one orbit against distance. (b) Describe the pattern, and predict the time for a star at 30,000 light-years. (c) Every star in the table keeps going around the center. What does that tell you about gravity in a galaxy?
A classmate says, "Gravity holds the planets around the Sun, but stars in a galaxy just drift, because they are too far apart to pull on each other." Write an argument against this claim. Use at least two pieces of evidence from this unit and one labeled sketch of a model.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Model
Sketch or scale model is labeled, uses one scale factor throughout and shows the pull and the motion
Model is present but mixes scales or leaves out the pull or the motion
Proportions
Scale factors and scaled distances are correct, with units
Method is right but a value or unit is wrong
Role of Gravity
Explains that gravity pulls toward the central object and bends the moving object's path into an orbit
Names gravity but does not explain how it controls the motion
Evidence and Reasoning
Uses data from the models or the unit to support each claim
Claims are made with little or no evidence
05
Quiz: 20 Questions
Interactive, with answers
Instructions
Use the models from the lesson. Distances are published values, rounded; simulation data and invented data are made up for practice. 1 AU is about 150 million km. Your score updates as you answer, and Reset quiz clears everything so you or your students can try again.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
What keeps Earth moving in an orbit around the Sun?
Answer: B
The Sun's gravity pulls Earth toward the Sun while Earth moves sideways, so its path keeps bending into a loop. Choice A gets the direction wrong: gravity pulls, and sunlight does not hold Earth in orbit. Choice C confuses spinning (day and night) with orbiting. Choice D is wrong: without gravity, Earth would leave in a straight line.
Question 2 of 20 · Multiple Choice
Seen from above, a student swings a ball on a string in a circle. The string stands for gravity. When the ball is at the north side of the circle, moving east, the string breaks. Which way does the ball go?
Answer: A
With no pull from the string, nothing bends the ball's path, so it keeps going straight in the direction it was moving: east. Choice B assumes the ball was being pushed outward, but the string only pulled inward. Choice C would need a pull that is no longer there. Choice D assumes the circle continues without the pull.
Question 3 of 20 · Multiple Choice
A class puts the Sun on one goal line and Saturn (9.6 AU) on the other, 100 yards away. Where should Jupiter (5.2 AU) go?
Answer: D
The scale is 100 ÷ 9.6 ≈ 10.4 yards per AU, so Jupiter goes at 5.2 × 10.4 ≈ 54 yards. Choice A uses the Neptune scale from class (3.33 yards per AU). Choice B measures from Saturn's goal line instead of the Sun's (100 - 54 = 46). Choice C uses 1 yard per AU.
Question 4 of 20 · Multiple Choice
What holds the Milky Way's billions of stars together as one galaxy?
Answer: C
The gravity of all the galaxy's stars, gas and dust pulls each star toward the center, so the stars orbit the center instead of flying off. Choice A confuses light with force. Choice B names a different force that is far too weak at these distances. Choice D is contradicted by the evidence that stars orbit the galaxy's center.
Question 5 of 20 · Multiple Choice
One trip of the Sun around the center of the Milky Way takes about 220 million years. About how many trips has it made since it formed, 4.6 billion years ago?
Answer: B
4,600 million ÷ 220 million ≈ 21 trips. Choice A treats 4.6 billion as 460 million. Choice C divides 4,600 by 100 instead of 220. Choice D uses the time for one trip as the number of trips.
Question 6 of 20 · Multiple Choice
In the orbit simulation, the star's mass is doubled, so its pull is stronger. The planet is still launched sideways at 30 km/s from 1 AU. What happens to its path?
Answer: D
A stronger pull bends the path more sharply, so the planet falls inward along an oval that swings in to about 0.34 AU and comes back out to the launch point. Choice A is what happens when the pull is too weak for the speed. Choice B ignores the stronger pull. Choice C would need no sideways speed at all.
Question 7 of 20 · Multiple Choice
Some classrooms model gravity with marbles rolling on a stretched spandex sheet that sags around a heavy ball. What is one weakness of this model?
Answer: A
The marbles roll toward the heavy ball because Earth's gravity pulls them down the sag, so the model borrows Earth's gravity to imitate the Sun's. It also shows space as a flat sheet. Choices B, C and D are things the model shows well, not weaknesses.
Question 8 of 20 · Multiple Choice
Earth's gravity keeps the Moon looping around Earth every 27.3 days, while the Sun's gravity keeps both of them going around the Sun every 365.25 days. About how many loops does the Moon make around Earth in one trip around the Sun?
Answer: D
365.25 ÷ 27.3 ≈ 13.4, so about 13 loops. Choice A uses the number of days in one loop. Choice B uses the number of days in a year. Choice C counts calendar months, which follow the 29.5-day cycle of phases rather than the 27.3-day loop around Earth.
Question 9 of 20 · Multiple Choice
If the Milky Way (100,000 light-years across) were shrunk to a state 1,200 km across, about how far from the Sun would its nearest neighbor star, Proxima Centauri (4.24 light-years), be?
Answer: A
Each light-year becomes 1,200 ÷ 100,000 = 0.012 km = 12 m, so 4.24 light-years becomes 4.24 × 12 ≈ 51 m. Choice B forgets to change kilometers to meters correctly (writing 0.012 km as 1.2 km). Choices C and D move the decimal point one place each way.
Question 10 of 20 · Multiple Choice
Which observation is evidence that gravity controls motion inside a galaxy?
Answer: B
Stars all over the galaxy move in loops around its center, so something pulls them toward the center: gravity. Choice A is about temperature, choice C about size and choice D about Earth's air. None of them shows a pull that controls motion.
Question 11 of 20 · Multiple Choice
A space probe coasts straight away from the Sun with its engines off. What happens to its speed as it moves outward?
Answer: A
The Sun's gravity pulls back on the probe the whole time, toward the Sun, so the probe slows down as it moves away. Choice B gets the direction of the pull wrong. Choice C forgets that the Sun's gravity still acts far from the Sun. Choice D would need a huge sudden force.
Question 12 of 20 · Multiple Choice
The Sun has about 99.8% of all the mass in the solar system. What does this fact help explain?
Answer: B
The pull depends on mass, and almost all of the solar system's mass is in the Sun, so its gravity controls the motion of every planet. Choice A is about the pull of each planet on nearby moons. Choice C is about how planets formed, and choice D is about the energy the Sun makes, not its gravity.
Question 13 of 20 · Multiple Choice
The Andromeda galaxy, 2.5 million light-years away, is moving toward the Milky Way at about 110 km/s. Which explanation fits this?
Answer: C
Gravity pulls between any two objects with mass, even whole galaxies millions of light-years apart, so the two galaxies are pulled toward each other. Choice A gets the force wrong: light does not pull galaxies together. Choice B names no outside force. Choice D is contradicted by galaxies that orbit each other, such as the Large Magellanic Cloud and the Milky Way.
Question 14 of 20 · Multiple Choice
Jupiter's moons stay with Jupiter as it travels around the Sun. Why?
Answer: C
The moons are close to Jupiter, so its gravity keeps each one in a loop around it, while the Sun's gravity pulls on Jupiter and its moons together. Choice A is wrong: the Sun pulls on the moons too. Choice B names a push, but gravity pulls. Choice D confuses gravity with Jupiter's atmosphere.
Question 15 of 20 · Short Answer
Use a model to explain why Earth does not fall into the Sun, even though the Sun's gravity pulls on it all the time.
Earth is moving sideways, at about 30 km/s. The Sun's gravity pulls Earth toward the Sun and keeps bending its path, but Earth keeps moving sideways, so it falls around the Sun instead of into it. Run 1 of the simulation shows what happens with no sideways speed: the planet falls straight in. A good answer includes a sketch with an arrow for the pull (toward the Sun) and an arrow for the motion (sideways).
Question 16 of 20 · Short Answer
On the football field model from class (Sun on one goal line, Neptune 100 yards away), 1 AU is about 3.33 yards. The nearest star, Proxima Centauri, is about 268,000 AU from the Sun. How far away would it be in the model, in yards and in miles (1 mile = 1,760 yards)? What does this tell you about the size of the solar system compared with the distance to other stars?
268,000 × 3.33 ≈ 890,000 yards (more exactly 268,000 × 100 ÷ 30.07 ≈ 891,000), and 891,000 ÷ 1,760 ≈ about 510 miles, roughly 800 km. The whole solar system fits on one football field, but the next star would be hundreds of miles away. The Sun's gravity controls the planets on the field; the stars are so far apart that each one's own system is held together by its own gravity.
Question 17 of 20 · Short Answer
In a new simulation run, the planet is launched sideways at 30 km/s from 1 AU, as in run 4, but the star has only half the Sun's mass. Predict what the path will do and explain the role of gravity.
Prediction: the planet never comes back. With half the mass, the star's pull is weaker, and at 30 km/s the planet is moving too fast for that weaker pull to bend its path into a closed loop, so it curves a little and then keeps moving away. (In the simulation, a star of half the Sun's mass can hold a planet at 1 AU only if it moves slower than about 29.8 km/s.) Gravity is still there the whole time: it bends the path and slows the planet down, but it is not strong enough to turn it around.
Question 18 of 20 · Short Answer
A student says, "Gravity only matters near planets. Out in space between stars, there is no gravity." Give two pieces of evidence against this claim.
Sample answer: (1) The Sun, 26,000 light-years from the center of the Milky Way, keeps orbiting the center instead of moving off in a straight line, so the galaxy's gravity reaches it across that huge distance. (2) Small galaxies such as the Large Magellanic Cloud orbit the Milky Way, so gravity acts even between galaxies. Halley's comet turning around at 35 AU, far from any planet, is also good evidence.
Question 19 of 20 · Short Answer
The Sun moves around the galaxy's center at about 220 km/s. A year is about 31.6 million seconds. About how far does the Sun travel in one year? Compare it with the distance from the Sun to Neptune, about 4.5 billion km.
220 km/s × 31,600,000 s ≈ 7 billion km (6.95 billion) each year. That is more than the 4.5 billion km from the Sun to Neptune, about 1.5 times as far. The whole solar system is carried along that fast, and the galaxy's gravity keeps bending its path around the center.
Question 20 of 20 · Short Answer
Design a physical model that shows how gravity keeps the Sun and other stars orbiting the center of the galaxy. Say what each part stands for and name one way your model is not like the real galaxy.
Sample answer: students stand in a large circle and walk around a classmate at the center, each holding a rope tied to the center student. The center student is the galaxy's center, the walkers are stars, and the tension in each rope stands for the pull of gravity toward the center. If a walker lets go of the rope, they walk off in a straight line. Limits: in the real galaxy there is no rope, the pull comes from all the matter in the galaxy and not from one point, and the stars are spread through a disk with billions of members. Accept any model that shows a pull toward the center plus sideways motion and names a real limit.
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does MS-ESS1-2 mean?
MS-ESS1-2 asks middle school students to build and use models that show what gravity does in the solar system and in galaxies: it holds them together and controls how objects orbit. The code stands for Middle School, Earth and Space Science, core idea 1 (Earth's Place in the Universe), performance expectation 2. The practice is modeling, so students use football field models, simulations and proportions, not just facts.
Does MS-ESS1-2 include Kepler's laws?
No. The Assessment Boundary says assessment does not include Kepler's Laws of orbital motion or the apparent retrograde motion of the planets as viewed from Earth. Students can see in a simulation that orbits are ellipses, but they don't calculate orbit times or use orbit laws. Those come in high school with HS-ESS1-4.
What is the football field model in MS-ESS1-2?
It is a physical scale model named in the Clarification Statement: the Sun on one goal line and Neptune on the other, with each planet placed at its scaled distance. With 100 yards for 30.07 AU, each AU is about 3.33 yards, so Earth is about 3.3 yards from the Sun. It shows how spread out the solar system is and how far the Sun's gravity reaches.
How big is the Milky Way compared with the solar system?
The Milky Way is about 100 million times as wide as the solar system out to Neptune. Its disk is about 100,000 light-years across, while Neptune's orbit is about 60 AU, less than one thousandth of a light-year, across. If the galaxy were shrunk to a state 1,200 km wide, the solar system would be about 1 cm across.
What holds a galaxy together?
Gravity holds a galaxy together. The combined pull of all its stars, gas and dust keeps each star orbiting the galaxy's center. The Sun takes roughly 220 million years to go around the Milky Way's center once. Gravity also acts between galaxies: small galaxies, such as the Large Magellanic Cloud, orbit the Milky Way.
What simulations work for MS-ESS1-2?
Any free orbit simulation that lets students switch gravity on and off, change a planet's starting speed and change the star's mass works well. PhET's Gravity and Orbits is one example. Students should change one setting at a time and trace the path, and they should check the simulation against real data, such as Earth's yearly orbit.
How do you use proportions with a school or state for MS-ESS1-2?
Pick a familiar size, divide it by the real size to get a scale factor, then multiply every real distance by that factor. For example, if the Milky Way's 100,000 light-years become a state 1,200 km across, each light-year becomes 12 m, and the Sun sits about 312 km from the center. Students should keep the same scale factor for every object in the model.
What mistakes do students make about gravity and orbits?
Frequent mistakes include thinking there is no gravity in space, thinking an orbit needs a push to keep going, and thinking a planet would fly straight outward if gravity stopped. Others mix scales in one model, or think gravity holds the solar system together but not the galaxy. Asking students to predict what happens when gravity is switched off brings these ideas out.
Is MS-ESS1-2 the same as MS-PS2-4?
No, but they fit together. MS-PS2-4 is about arguing from evidence that gravity attracts and depends on mass. MS-ESS1-2 uses models to show what that attraction does on the largest scales: it controls orbits and holds the solar system and the Milky Way together. Many schools teach them in the same unit.
What comes after MS-ESS1-2?
In middle school it pairs with MS-ESS1-1 (the Earth-sun-moon system) and MS-ESS1-3 (scale properties of solar system objects). In high school, HS-ESS1-4 uses math and computer models to predict orbits, including Kepler's laws and Newton's law of gravitation, which are beyond this standard.
07
Related Standards
5 standards
These standards connect to MS-ESS1-2: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
5-PS2-1Prerequisite
Support an argument that Earth's gravitational force on objects is directed down
Lesson coming soon
5-ESS1-1Prerequisite
Argue that the sun looks brighter than other stars because it is closer to Earth
Lesson coming soon
Alongside
MS-PS2-4Parallel
Argue from evidence that gravity is attractive and depends on the masses involved
NGSS is a registered trademark of WestEd. Neither WestEd nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.
