Semimajor axis — Science definition
Half the longest diameter of an ellipse.
Where it is taught
- Predicting Orbits with Mathematical Models — Earth, Moon & Solar System, Earth & Space Science, Grade 9
In use
For an object orbiting the Sun, a useful model is P² = a³ when P is in Earth years and a is the semimajor axis in astronomical units.
Predicting Orbits with Mathematical Models
Questions using semimajor axis
Problem 1
For the same star, orbit B has semimajor axis 4 times A's. What is its period ratio? Explain the reasoning in your own words.
Answer and explanation
Orbit B's period is 8 times orbit A's under this model.
T² scales with a³, so the squared period ratio is 4³=64.
Take the square root: T_B/T_A=8.
Response criteria
- Use the stated conditions.
- Show the evidence or calculation.
- Explain the conclusion and any stated limitation.