So there I was, just twenty-three months old and the King of the World. I had a pair of doting giants trained to meet my every need. One whimper and I instantly became the center of the universe. But trouble was brewing in paradise and I knew it. My mother had entered her second trimester, pregnant with The Interloper: my sister Jan was on the way and my kingdom would soon be ripped in half.    Right on schedule, Mars was returning to where it was on the day I was born. I was embattled and ready to kill somebody if only my body were coordinated enough to pull it off. Welcome to the infamous “Terrible Twos.”   In broad terms, Mars takes twenty-six months to return to a given zodiacal degree – just over two years in other words. Due to several factors, the cycle is somewhat variable. One of those factors is that, in common with the rest of the planets, Mars’ orbit is elliptical – it speeds up when it’s closer to the Sun and slows down as it gets further away. Ditto for Earth, of course – and we’re watching Mars from our own careening planet, so our perspective is always shifting. Because of retrograde motion, sometimes Mars makes not one but three conjunctions to its natal position. All of those wild cards complicate the timing. It’s a mess, but say twenty-six months, give or take two or three months, and your timing of Mars returns will be more or less on target. My own first Mars return, for one example, occurred after only twenty-three months and ten days. The bottom line is that you have to look it up.