On January 3rd, 2019, Earth reached the point in its orbit where it's at its closest approach to the Sun: perihelion. Every object orbiting a single mass (like our Sun) makes an ellipse, containing a point of closest approach that's unique to that particular orbit, known as periapsis. For the past 4.5 billion years, Earth has orbited the Sun in an ellipse, just like all the other planets orbiting their stars in all the other mature solar systems throughout the galaxy and Universe. But there's something you may not expect or appreciate that nevertheless occurs: Earth's orbital path doesn't remain...