Your Age on Other Worlds
Want to melt those years away? Travel to an outer planet!
• Fill in your birthdate.
• Use the Planetary Birthday Calculator Table below to come up with your
“ages” on other planets.
MERCURY
Your age is
Mercurian days
Mercurian years
Next Birthday
VENUS
Your age is
Venusian days
Venusian years
Next Birthday
MARS
JUPITER
Your age is
Martian days
Martian years
Your age is
Jovian days
Jovian years
Next Birthday
Next Birthday
EARTH
Your age is
Earth days
Earth years
Next Birthday
SATURN
Your age is
Saturnian days
Saturnian years
Next Birthday
URANUS
NEPTUNE
PLUTO
Your age is
Uranian days
Your age is
Neptunian days
Your age is
Plutonian days
Uranian years
Neptunian years
Plutonian years
Next Birthday
Next Birthday
Next Birthday
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 up the 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 at once. There are two that specifically
interest us. First, the earth rotates on its axis, like a spinning top. Second, the earth revolves around the
sun, like a tetherball at the end of a string going around the center pole.
The top-like rotation of the earth on its axis is how we define the day. The time it takes the earth to rotate
from noon until the next noon we define as one day. We further divide this period of time into 24 hours,
each of which is divided into 60 minutes, each of which is broken into 60 seconds. There are no rules
that govern the rotation rates of the planets, it all depends on how much "spin" was in the original
material that went into forming each one. Giant Jupiter has lots of spin, turning once on its axis every 10
hours, while Venus takes 243 days to spin once.
The revolution of the earth around the sun is how we define the year. A year is the time it takes the earth
to make one revolution - a little over 365 days.
We all learn in grade school that the planets move at differing rates around the sun. While earth takes
365 days to make one circuit, the closest planet, Mercury, takes only 88 days. Poor, ponderous, and
distant Pluto takes a whopping 248 years for one revolution. Below is a table with the rotation rates and
revolution rates of all the planets.
Planetary Birthday Calculator
Planet
Rotation Period Revolution Period
Mercury
58.6 days
87.97 days
Venus
243 days
224.7 days
Earth
0.99 days
365.26 days
Mars
1.03 days
1.88 years
Jupiter
0.41 days
11.86 years
Saturn
0.45 days
29.46 years
Uranus
0.72 days
84.01 years
Neptune
0.67 days
164.79 years
Pluto
6.39 days
248.59 years
What’s Going On?
Why the huge differences in periods? We need to go back to the time of Galileo, except that we're not
going to look at his work, but rather at the work of one of his contemporaries, Johannes Kepler (15711630).
Kepler briefly worked with the great Danish observational astronomer,
Tycho Brahe. Tycho was a great and extremely accurate observer, but he
did't have the mathematical skills to analyze all of the data he collected.
After Tycho's death in 1601, Kepler was obtained Tycho's observations.
Tycho's observations of planetary motion were the most accurate of the
time (before the invention of the telescope!).
Using these observations, Kepler discovered that the planets do not move
in circles, as 2000 years of "Natural Philosophy" had taught. He discovered
that they move in ellipses. A ellipse is a sort of squashed circle with a short diameter
(the "minor axis") and a longer diameter (the "major axis").
Kepler found that when the planets were nearer the sun in their orbits, they move faster than when they
were farther from the sun. Many years later, he discovered that the farther a planet was from the sun, on
the average, the longer it took for that planet to make one complete revolution. Kepler's discoveries are
still used today to predict the motions of planets, comets, asteroids, stars, galaxies, and spacecraft.