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[parent] example of Kepler's second law with Earth's orbit (Example)

Problem Statement

Let's dive into an example problem for Kepler's Second Law, the Law of Equal Areas, which states: A line joining a planet to the Sun sweeps out equal areas in equal intervals of time. This law reflects a planet's faster motion near the Sun (perihelion) and slower motion farther away (aphelion). We'll use Earth's orbit to solve:

Earth orbits the Sun in an ellipse with a semi-major axis $a = 149.6$ million km (1 AU) and eccentricity $e = 0.0167 $. The orbital period is 365.25 days (1 year). Calculate the area swept out by Earth's radius vector (the line from the Sun to Earth) in 10 days when Earth is:

  1. At perihelion (closest to the Sun).
  2. At aphelion (farthest from the Sun).
Then, verify Kepler's Second Law by comparing the areas. Assume the Sun is at one focus of the ellipse.

Solution

Step 1: Understand Kepler's Second Law

Kepler's Second Law implies the areal velocity (area swept per unit time) is constant:
$\displaystyle \frac{dA}{dt} =$   constant$\displaystyle .$ (1)
For an ellipse, this constant relates to the total area and period. Equal time intervals (e.g., 10 days) should yield equal areas, regardless of position.

Step 2: Calculate Orbital Parameters

  • Semi-major axis: million km.
  • Eccentricity: .
  • Focal distance: $c = a e = 149.6 \times 0.0167 \approx 2.496$ million km.
  • Distances:
    • Perihelion: $r_p = a - c = 149.6 - 2.496 = 147.104 $ million km.
    • Aphelion: $r_a = a + c = 149.6 + 2.496 = 152.096 $ million km.
  • Semi-minor axis: $b = a \sqrt{1 - e^2} $.
    $\displaystyle b$ $\displaystyle = 149.6 \times \sqrt{1 - (0.0167)^2}$    
      $\displaystyle \approx 149.579 \,$   million km    

Step 3: Total Orbital Area

The area of the ellipse is:
$\displaystyle A$ $\displaystyle = \pi a b$    
  $\displaystyle = \pi \times 149.6 \times 149.579 \approx 70,283 \,$   million km$\displaystyle ^2.$    
Orbital period $T = 365.25$ days, so the areal velocity is:
$\displaystyle \frac{dA}{dt}$ $\displaystyle = \frac{A}{T} = \frac{70,283}{365.25} \approx 192.41 \,$   million km$\displaystyle ^2/$day    

Step 4: Area Swept in 10 Days

For any 10-day interval:
$\displaystyle \Delta A$ $\displaystyle = \frac{dA}{dt} \times \Delta t$    
  $\displaystyle = 192.41 \times 10 = 1,924.1 \,$   million km    
This should hold at both perihelion and aphelion if Kepler's Second Law is true.

Step 5: Verify with Angular Momentum (Optional Check)

Areal velocity relates to angular momentum $L = m r^2 \dot{\theta} $:
$\displaystyle = \frac{1}{2} r^2 \dot{\theta} = \frac{L}{2m} =$   constant    
  • Perihelion speed: Approximate via vis-viva equation:
    $\displaystyle v = \sqrt{GM \left( \frac{2}{r} - \frac{1}{a} \right)},$    
    where $GM = 1.327 \times 10^{11} \,$   km$^3/$s$^2, \( r_p = 147.104 \,$   million km$\( v_p \approx 30.29 \,$   km/s$. \item \textbf{Aphelion speed}: \( v_a \approx 29.29 \,$   km/s$. $

Step 6: Compare Areas

  • Perihelion: $\Delta A = 1,924.1 \,$   million km$^2 $.
  • Aphelion:    million km.
The areas match, confirming Kepler's Second Law! Earth moves faster at perihelion and slower at aphelion, but the area swept is identical.

Step 7: Real-World Twist

On January 3, 2025 (near perihelion), Earth sweeps 1,924.1 million km in 10 days at $\sim 30.3 \,$   km/s. On July 4, 2025 (near aphelion), the same area is swept at $\sim 29.3 \,$   km/s - slower, yet balanced.

Answer Summary

  • Perihelion area: 1,924.1 million km in 10 days.
  • Aphelion area: 1,924.1 million km in 10 days.
  • Verification: Equal areas in equal times, as Kepler predicted.
Earth's varying speed balances its orbit's geometry - a celestial choreography governed by a simple, profound law!

This example was generated by Grok, an AI developed by xAI, on February 28, 2025.

References

Bibliography

1
Murray, C. D., & Dermott, S. F. (1999). Solar System Dynamics. Cambridge University Press.
2
Seidelmann, P. K. (Ed.). (1992). Explanatory Supplement to the Astronomical Almanac. University Science Books.
3
Lang, K. R. (2011). The Cambridge Guide to the Solar System (2nd ed.). Cambridge University Press.
4
NASA JPL. (2025). "Planetary Fact Sheet." Solar System Dynamics.
5
Kepler, J. (1619). Harmonices Mundi. (Trans. Aiton, E. J., et al., 1997). American Philosophical Society.



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See Also: example of Kepler's first law with Earth's orbit, Kepler's three laws of planetary motion summarized


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Cross-references: speed, angular momentum, position, velocity, radius vector, motion

This is version 5 of example of Kepler's second law with Earth's orbit, born on 2025-03-01, modified 2025-03-01.
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Physics Classification: 45.50.Pk (Celestial mechanics )
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