Drake Equation (to estimate the number of extraterrestrial civilizations) tattoo
Now that we have found and seen actual planets on other stars, it is an appropriate time to discuss the Drake equation again.
The Drake equation attempts to estimate the number of extraterrestrial civilizations in our galaxy with which we might come in contact.

This has to be one of the most delightful tattoo that I have come across. It is a pictorial representation of the Drake Equation.

where:
N is the number of civilizations in our galaxy with which communication might be possible;
and
R* is the average rate of star formation in our galaxy
fp is the fraction of those stars that have planets
ne is the average number of planets that can potentially support life per star that has planets
fl is the fraction of the above that actually go on to develop life at some point
fi is the fraction of the above that actually go on to develop intelligent life
fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L is the length of time such civilizations release detectable signals into space.
The tattoo above is a pictorial representation of the above seven factors. It starts out with a picture of galaxy and then each of the element of the equation is shown in a pictorial form.
Of course, our friends at XKCD have an extra factor:






















