In the previous page, you saw how the interstellar gas piles up, due to the Sun's gravitational focusing. We know this in part because of the way our instruments record data about the material they intercept. In this animation, you can see the revolution of the Earth about the Sun, and how parts of its orbit intersect the gas pileup (darker blue portions). The graph at the bottom of the page illustrates how the density of helium (one element of the interstellar medium) varies with the time of the year. In December, the Earth is passing through the most dense part of the gas pile-up, which peaks for us in early December; while in June, when we are "upwind" of this pileup, the helium density we measure is very low.