Investigating Science 11–12 · Year 11
Predicting the return of Halley’s comet with Kepler’s third law
Module 2: Cause and Effect – Inferences and Generalisations
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The idea
A pattern of returns about every 76 years led Halley to infer one comet on one orbit; Kepler’s third law turns the orbit’s size into a period, and the spread in real intervals shows what the simple law omits.
What you need
- computer or tablet with a spreadsheet
- the JPL Small-Body Database record for 1P/Halley (orbital elements) and NASA’s Halley fact page
How to do it
- Record the semi-major axis a and eccentricity e from the JPL record.
- Calculate the period T = a^1.5 (T in years, a in astronomical units) and the perihelion distance q = a(1 − e).
- Compare T with JPL’s listed period and with the intervals between recorded returns.
- Add T to the time of the February 1986 perihelion given in the JPL record to predict the next return, and compare with NASA’s stated year.
- Explain why the intervals differ from one orbit to the next.
What you should see
JPL gives a = 17.93 AU and e = 0.968. Kepler’s third law gives T = 17.93^1.5 = 75.9 years, matching JPL’s listed period of 27 728 days (75.9 years), and q = a(1 − e) = 0.574 AU with these rounded elements, against JPL’s perihelion distance of 0.575 AU, which comes from its unrounded e = 0.96794. Adding 75.9 years to the February 1986 perihelion points to early 2062, while NASA states the return in 2061; NASA also notes that intervals between returns have ranged from 74.42 years (1835 to 1910) to 79.25 years (451 to 530). The two-body law predicts the average; the pulls of Jupiter and Saturn change each orbit.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Comets travel in circles like the planets.
- A comet’s tail streams behind it along its path (it points away from the Sun).
- A law that fits past returns will give the next date exactly.
Safety card
Hazards
No hazard is listed.
Controls
No control is listed.
Note
Screen-based model with no chemicals, heat or apparatus: no practical risk assessment is triggered beyond the school's normal classroom procedures.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-9INS11/12-1INS11/12-2INS11/12-4
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.