Week 06 · A Disk of Dust and Gas

Thursday

Reading the fact sheet: one flat plane

// How a spinning cloud became a solar system

⏱ about 20 min

Thursday: Reading the Fact Sheet: One Flat Plane

Raven lays the NASA fact sheet on the Sky Deck and points to a row called orbital inclination.

"Inclination means tilt," she says. "How far each orbit tilts from Earth's orbit, in degrees."

Rocket scans the row. "Zero for Earth. Then tiny numbers. Seven is the biggest for a planet. That is almost nothing!"

"What do you notice about the last column?" Raven asks.

"Pluto. Seventeen degrees." Rocket taps it. "That one sticks out of the disk."

Nova hovers over the row. "A flat racecourse with one tilted lane," she says. "Now you know why Pluto is sorted differently."

A table from NASA

The NASA Planetary Fact Sheet lists each orbit's inclination. That is the tilt of the orbit, in degrees, measured from Earth's orbit.

A full circle is 360 degrees. A right angle is 90 degrees. So a tilt of 7 degrees is a very small lean.

Earth shows 0.0 because the other orbits are measured from its own.

The table also lists how many days each world takes to orbit the Sun once.

World	Orbital inclination (degrees)	Orbital period (days)
Mercury	7.0	88.0
Venus	3.4	224.7
Earth	0.0	365.2
Mars	1.8	687.0
Jupiter	1.3	4331
Saturn	2.5	10,747
Uranus	0.8	30,589
Neptune	1.8	59,800
Pluto (dwarf planet)	17.2	90,560

What the rows show

Every one of the eight planets orbits within 7 degrees of Earth's orbit. Most are within 3 degrees. That is a flat disk.

OpenStax Astronomy notes that Pluto's orbit is not in the plane of the planets. The table shows it: 17.2 degrees.

NASA lists Pluto as one of five dwarf planets, along with Ceres, Haumea, Makemake and Eris.

The orbital period column shows another pattern. The farther a world is from the Sun, the longer it takes to go around.

Mercury, closest to the Sun, needs 88 days. Neptune, the farthest planet, needs 59,800 days.

READ THE ROWS

Read the question.
Tap your answer.

Which planet's orbit is tilted the most from Earth's?

Which world in the table does not fit the flat disk pattern?

As distance from the Sun increases, what happens to the orbital period?

All eight planets orbit within ____ degrees of Earth's orbit.

Pluto's orbital inclination is ____ degrees.

Statement	True or false?
Earth's orbit is the reference, so it shows 0.0 degrees.	?
Jupiter takes fewer days to orbit than Mars.	?
Pluto is listed by NASA as a dwarf planet.	?
Most planets are tilted more than 10 degrees from Earth's orbit.	?

WHY THIS EXERCISEThe small tilts are the data behind the phrase one flat plane.

Try it

Hold a dinner plate flat. Tilt it 7 degrees, barely a lean. Now tilt it 17 degrees. Notice the difference.
Write which tilt matches Mercury and which matches Pluto.

Try this

Cover the period column. Using only the order of the planets from the Sun, predict which has the longest year.
Uncover and check. Write whether the pattern held for every row.

Draw the Sun and a flat line through it for the disk. Draw eight orbits almost on the line, and one orbit for Pluto tilted well off it.

Explain, using two numbers from the table, why scientists say the planets orbit in nearly one plane.Why do you think Pluto is sorted differently from the eight planets? Use the table as evidence.Write one question the table raises that it cannot answer.

Sharp reading. Tomorrow you meet the leftovers: the asteroids and comets that never became planets.

Grown-up checked my answers

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