Fill in Your Birthdate

8/14/2019 Fill in Your Birthdate

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Fill in your birthdate below in the space indicated. (Note you

must enter the year as a 4-digit number!)

Click on the "Calculate" button.

Notice that your age on other worlds will automatically fill in.

Notice that Your age is different on the different worlds. Notice

that your age in "days" varies wildly.

Notice when your next birthday on each world will be. The date

given is an "earth date".

You can click on the images of the planets to get more

information about them from Bill Arnett's incredible NinePlanetsweb site.

http://www.exploratorium.edu/ronh/age/index.html

The Days (And Years) Of Our Lives

Looking at the numbers above, you'll immediately notice that

you are different ages on the different planets. This brings upthe question of how we define the time intervals we measure.What is a day? What is a year?

The earth is in motion. Actually, several different motions all atonce. There are two that specifically interest us. First, theearth rotates on it's axis, like a spinning top. Second, theearth revolves around the sun, like a tetherball at the end ofa string going around the center pole.

The top-like rotation of the earth on its axis is how we definethe day. The time it takes the earth to rotate from noon untilthe next noon we define as one day. We further divide thisperiod of time into 24 hours, each of which is divided into 60minutes, each of which is broken into 60 seconds. There areno rules that govern the rotation rates of the planets, it all
http://www.nineplanets.org/http://www.nineplanets.org/http://www.exploratorium.edu/ronh/age/index.htmlhttp://www.nineplanets.org/http://www.nineplanets.org/http://www.exploratorium.edu/ronh/age/index.html


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depends on how much "spin" was in the original material thatwent into forming each one. Giant Jupiter has lots of spin,turning once on its axis every 10 hours, while Venus takes 243days to spin once.

The revolution of the earth around the sun is how we definethe year. A year is the time it takes the earth to make onerevolution - a little over 365 days.

We all learn in grade school that the planets move at differingrates around the sun. While earth takes 365 days to make onecircuit, the closest planet, Mercury, takes only 88 days. Poor,ponderous, and distant Pluto takes a whopping 248 years forone revolution. Below is a table with the rotation rates and

revolution rates of all the planets.

PlanetRotatio

n

Period

Revolution

Period

Mercury58.6days

87.97days

Venus243

days224.7days

Earth0.99

days

365.26

days

Mars1.03days

1.88 years

Jupiter0.41days

11.86years

Saturn0.45days

29.46years

Uranus0.72days

84.01years

Neptune

0.67

days

164.79

years

Pluto6.39days

248.59years

Why the huge differences in periods? We need to go back tothe time of Galileo, except that we're not going to look at his


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work, but rather at the work of one of his contemporaries,Johannes Kepler (1571-1630).

Kepler briefly worked with the great Danish observationalastronomer, Tycho Brahe. Tycho was a great and extremelyaccurate observer, but he did't have the mathematical capacityto analyze all of the data he collected. After Tycho's death in1601, Kepler was able to obtain Tycho's observations. Tycho'sobservations of planetary motion were the most accurate ofthe time (before the invention of the telescope!). Using theseobservations, Kepler discovered that the planets do not movein circles, as 2000 years of "Natural Philosophy" had taught.He discovered that they move in ellipses. A ellipse is a sort ofsquashed circle with a short diameter (the "minor axis") and a

longer diameter (the "major axis"). He found that the Sun waspositioned at one "focus" of the ellipse (there are two "foci",both located on the major axis). He also found that when theplanets were nearer the sun in their orbits, they move fasterthan when they were farther from the sun. Many years later,he discovered that the farther a planet was from the sun, onthe average, the longer it took for that planet to make onecomplete revolution. These three laws, stated mathematicallyby Kepler, are known as "Kepler's Laws of Orbital Motion."

Kepler's Laws are still used today to predict the motions ofplanets, comets, asteroids, stars, galaxies, and spacecraft.

Here you see a planet in a very elliptical orbit.Note how it speeds up when it's near the Sun.

(Requires QuickTime Plugin)

Kepler's third law is the one that interests us the most. Itstates precisely that the period of time a planet takes to goaround the sun squared is proportional to the average distancefrom the sun cubed. Here's the formula:

Let's just solve for the period by taking the square root of bothsides:
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Note that as the distance of the planet from the sun isincreased, the period, or time to make one orbit, will get

longer. Kepler didn't know the reason for these laws, though heknew it had something to do with the Sun and its influence onthe planets. That had to wait 50 years for Isaac Newton todiscover the universal law of gravitation.

The Gravity Of The Situation

Closer planets revolve faster, more distant planets revolveslower. Why? The answer lies in how gravity works. The force

of gravity is a measure of the pull between two bodies. Thisforce depends on a few things. First, it depends on the mass ofthe sun and on the mass of the planet you are considering.The heavier the planet, the stronger the pull. If you double theplanet's mass, gravity pulls twice as hard. On the other hand,the farther the planet is from the sun, the weaker the pullbetween the two. The force gets weaker quite rapidly. If youdouble the distance, the force is one-fourth. If you triple theseparation, the force drops to one-ninth. Ten times thedistance, one-hundredth the force. See the pattern? The force

drops off with the square of the distance. If we put this intoan equation it would look like this:

The two "M's" on top are the sun's mass and the planet'smass. The "r" below is the distance between the two. Themasses are in the numerator because the force gets bigger ifthey get bigger. The distance is in the denominator because

the force gets smaller when the distance gets bigger. Notethat the force never becomes zero no matter how far youtravel. Knowing this law helps you inderstand why the planetsmove faster when they are closer to the sun - they are pulledon with a stronger force and are whipped around faster!


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Its copied from

http://www.exploratorium.edu/ronh/age/index.html
http://www.exploratorium.edu/ronh/age/index.htmlhttp://www.exploratorium.edu/ronh/age/index.html

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