Astronomy/Velocityq

Advertisement

Expert: - 7/20/2004


Question
Sir,

Can you explain how velocity is effected in concerns with planets (i.e Mercury to the Sun versus Pluto to the sun)?

Any help would be greatly appreciated.

Very Respectfully,
Nathan Smith  

Answer
Nathan,
Comparing planet to planet as you are, you are dealing
with Kepler's 3rd law where the Period squared is proportional to the distance (from the sun) cubed, where the period is in years and the distance is in AU's (Astronomical Units) -that is, the average Earth-Sun distance = ~ 93 million miles.

Without getting into the actual mathematics of it,
basically Kepler's 3rd law states (or infers) that the farther you are from the sun, the slower your orbital velocity around that body. (Which is, of course, true).
That's why Mercury zips (revolves) around the sun in just about 88 days, Earth one year, Jupiter takes just under 12 years, Saturn 29.5 years, whereas Pluto takes about 248 years.

Of course, to compute the *actual* average velocity, one
would first have to compute the circumference of the
circle (actually ellipse)the planet describes around the sun, then use average velocity = average distance (that circumference number), divided by time.

BTW, if you are dealing with just a single planet in an
elliptical orbit, then you want to use Kepler's 2nd law
which states that 'equal areas are swept out in equal
time intervals'....which really means that the planet is
traveling faster the closer in it is, like perihelion,
as opposed to its slowest orbital velocity at aphelion.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA  

Astronomy

All Answers

Answers by Expert:

Ask Experts

Volunteer

Tom Whiting

Expertise

Astronomy has been my hobby/pasttime for over 50 years.  Currently own 3 telescopes, the largest of which is a 30 inch Newtonian truss Dob that is portable.I taught Astronomy/Meteorology at the University Level for 13 years before retiring in 1995. Being retired and home most of the time, I am able to answer all questions relatively quickly, unless it's a new moon weekend with good observing conditions.  No astrology questions please, or questions about alleged UFO picture identifications.

Experience

Experience: Astronomy has been my hobby and study for over 50 years. We currently now own a 30 inch portable telescope (Updated - Pennsylvania`s largest portable telescope). It can be seen on our website at:http://www.velocity.net/~bwhiting and also attend several regional starparties during the year, and have been on 5 total solar eclipse expeditions.

Organizations: President, Erie County Mobile Observers Group for over 15 years.

Publications: Wrote the "Over Erie Skies" newspaper article in our local newspaper for 11 years (1975-86).

Education: Masters Degree- Taught at the University level for 13 years. Retired 20 years -USAF Pilot - KC-135 with 180 combat missions;  Also Eagle Scout, Philmont staff 2 Yrs, Order of Arrow Lodge Chief, Ham Radio (inactive).

Awards: two discoveries: The mini-coathanger asterism in Ursa Minor (the little dipper) And the mini-ladle- another asterism in the bowl of Ursa Minor. Clients: Currently President of the ECMOG as mentioned above.

Education/Credentials
BS  Metallurgical Engineering Grove City College, PAMaster's Degree, Gannon University, Erie, PA Also retired USAF pilot, 20 years.

©2012 About.com, a part of The New York Times Company. All rights reserved.