
Index
9c. Copernicus to Galileo 10. Kepler's Laws Kepler's Laws (For teachers) 10a. Scale of Solar Sys. 11. Graphs & Ellipses 11a. Ellipses and First Law 12. Second Law 12a. More on 2nd Law 12b. Orbital Motion 12c. Venus transit (1) 12d. Venus transit (2) 12e. Venus transit (3) Newtonian Mechanics 13. Free Fall 14. Vectors 15. Energy 16. Newton's Laws 17. Mass 
The Ratio of Velocities at Perigee and Apogee(Note: this calculation is also found in section #21c)Kepler's 2nd law prescribes the way the velocity V of an object orbiting a single center varies around its orbit:
As noted in section #12, this means that the object's velocity V increases when its distance r decreases, and vice versa. 
At two points of the orbit the directions of r and V are perpendicular to each other: at the point P of closest approach ("perihelion" for a planet, "perigee" for an Earth satellite), and at the most distant point A ("aphelion" or "apogee"). Let their values be r_{1} and V_{1} at P, and r_{2} and V_{2} at A.
Imagine an Earth satellite, located at perigee P (see drawing). In one second it will have advanced a distance V_{1} meters (or whatever units are used). Since that is just a tiny part of the orbit, no great error occurs if, in calculating the area swept by the radius r_{1}, we replace it with a straight line. The swept area is then a long and skinny rightangled triangle with height r_{1} and base V_{1} (skinnier than we could draw here, so the drawing is just to give the labeling). The area A_{1} of such a triangle, by the formula for areas of rightangled triangles, is one half base time height, or Similarly, the area A_{2} covered in one second after passing apogee A equals However, by Kepler's 2nd law A_{1} = A_{2} so
or, multiplying everything by 2 A more useful form of that relation appears if both sides are divided by V_{2}r_{1} : The ratio of velocities equals the inverse of the ratio of distances. The smaller the distance, the faster the motion. If perigee distance is half of the apogee distance, the velocity there is twice as large. (But please rememberthis proportionality only holds with P and A, not with other points along the orbit). Why are Seasons Unequally Spaced?The main dividing points of the year are the two solsticeslongest day in summer, longest night in winterand the two equinoxes, when night and day are equal. These are the starting points of summer, winter, spring and fall, and it is generally assumed that they are equally spaced.But are they? Spring equinox in 2003 was on March 21, while fall equinox was on September 22, 184 days later. In 2004, a leap year, spring equinox occurs March 20, and if you count the days, you will see that to get there only takes 181 days more. The two intervals are not equal. How can that be? Equinox positions are on opposite sides of the Earth's orbit, 180° apart, yet getting through (northern) winter takes Earth 3 days less than getting through summer. Because by Kepler's 2nd law, the Earth moves in its orbit a little faster in winter. As noted at the end of "Seasons of the Year" and also in connection with the Milankovich Theory of ice ages, Earth is closest to the Sunat perihelionaround January 4. By the equations derived here, that's when it moves fastest. Furthermore (see drawing), the half of the ellipse closest to the Sun is also shorter. With Earth the difference is small, since the orbit is quite close to a circle; but it exists, nevertheless, and is the reason for that difference of 3 days.

Optional Nitty Gritty: #12b How Orbits are Calculated
Next Regular Stop: #13 The Way things Fall Timeline Glossary
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated 12 April 2005