Science 7–10 · Year 7
Kepler's third law from the planetary fact sheet
Earth and space sciences (no ACARA Year 7 to 10 content description covers planetary orbits, which ACARA places in Year 6; coded here to Year 7 science inquiry; NSW Stage 4 focus areas: Observing the Universe and Data science 1, placed in Year 7 with the other Observing the Universe practicals; Observing the Universe has no content point on the relationship between a planet's period and its distance, so the outcome is carried by Data science 1's content on analysing a model to identify trends and by Observing the Universe's Practice of science content on tabulating and graphing)
This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.
The idea
Orbital period and distance are locked together: the square of the period grows as the cube of the distance, so the outer planets crawl while Mercury races.
What you need
- the NASA Planetary Fact Sheet table of distance from the Sun and orbital period for the eight planets
- a spreadsheet or calculator with powers
How to do it
- Convert each distance to astronomical units (divide by 149.6 million km) and each period to years (divide by 365.242).
- Compute a cubed and T squared for each planet and tabulate the ratio T squared over a cubed.
- Plot T against a on log-log axes (or T squared against a cubed on linear axes) and fit a line; read the slope or gradient.
- Predict the period of Pluto from its distance in the same fact sheet (5,906.4 million km, 39.48 AU), and check it and the Neptune prediction against the table.
What you should see
T squared over a cubed comes out close to 1.00 year squared per AU cubed for every planet: the predicted periods are Mercury 87.9 d (table 88.0), Venus 224.7 (224.7), Mars 687.2 (687.0), Jupiter 4,336 (4,331), Saturn 10,817 (10,747), Uranus 30,643 (30,589) and Neptune 60,558 (59,800) days, all within 1.3 percent, and Pluto at 39.48 AU 90,608 days (90,560); the log-log slope is 1.50. The learner knows it worked when every value in the ratio column lies within 3 percent of 1.00 (Neptune, the furthest off, gives 0.975).
What changes
- What you change
- mean distance from the Sun
- What you measure
- orbital period
- What you keep the same
- the same central body (the Sun)
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Outer planets take longer only because their orbits are longer; they also move more slowly.
- All planets orbit at the same speed.
Safety card
Hazards
No hazard is listed.
Controls
No control is listed.
Note
No hazardous chemical and no flame or heating apparatus: a generic classroom risk assessment (CSIS 1.7 or RiskAssess) covers trips, spills, warm lamps and sharp edges.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC4-OTU-01SC4-DA1-01SC4-WS-05SC4-WS-06
- Australian Curriculum v9AC9S7I04AC9S7I05
Sources
The pages the author read to write this activity.