Letters from an Astrophysicist

by Neil deGrasse Tyson

Most things we measure or quantify in everyday life do not span this breadth. So if you get half-off on your store purchase, or if one item is twice as large as another, or if one object moves three times as fast as the other, or contains half as many items, we psychologically think these represent large differences. In astrophysics, however, these differences are small, knowing that the measured properties of things can range by factors of hundreds, thousands, or even billions. When communicating with each other in astrophysics, we invoke high precision only if some other physical quantity depends upon it. Otherwise, the precision is not only distracting, but in most cases, observationally or theoretically unjustified. When the Sun dies, in about five billion years, it will swell to be so large that it will engulf the inner planets. The “edge” of such bulbous entities are actually poorly defined—where is the edge of the cirrus cloud overhead? Where is the edge of the fog through which you drive? The limit of Earth’s atmosphere has no sharp boundary either, so people pick a value that suits their needs. That’s why, if you look up the size of Earth’s atmosphere in multiple (independent) places, you will likely find very different answers, none of which are wrong. In another example, a question as simple as, “how many planets are there in the solar system?” does not have an unambiguous answer. Six moons, including our Moon, are bigger than Pluto. Not only that, several objec...