Tea with Science

Our setting is eastern Africa, more than 100,000 years ago. Local people kneel around a woman as she lies dying. We can’t see her face, so we don’t know her age. Maybe she’s old, and the two women holding her hands are her daughters, or maybe she’s still a teenager, and her daughters are those two young girls playing in the dirt. Why do we care about this woman? Because - spoiler alert - this woman, Eve, is your long-dead grandma. Her family dynasty extends to every person alive today, and that includes you. Image: Acacia tortilis , b y JMK / CC BY-SA 3.0 / via Wikimedia Commons You see, Mitochondrial Eve, to use her full name, died leaving at least two daughters, who were healthy enough to each have their own daughter. Those daughters had a daughter, who had a daughter, and so on, all the way to your mother, who had you. Her glamorous name comes from where her legacy lies. When an egg is fertilized, the embryo receives most of its genetic data, the nuclear DNA, ...

The winter solstice isn’t named for the “shortest” day, nor is it named for the longest night, nor even the beginning of winter. The name solstice comes from Latin, and it means, “sun stands still”. Winter sun in the Mohonk Preserve / Image: Kate Comisso That seemed plain nonsense to me when I first read it. From our Earthbound perspective, the sun rises every morning in the east, makes its way across the sky during the day and sets every evening in the west. Every day. Even on the winter solstice. When does it ever stand still? The answer lies in the south. Those of us in the USA, Europe, Canada - anywhere north of the Tropic of Cancer - never see the the sun directly overhead, even in the middle of the day. Instead of craning our necks to look straight upwards, we must look up a bit and south a bit. There, in the middle of the day on the summer solstice, we will find the sun higher than it will be at any other moment of the year. Following that, as summer turns to falling le...

If you know one thing about the winter solstice you’ll know it’s the “shortest day”, meaning there is less daylight than on any other day of the year. That’s because of the way the Earth spins on a tilted axis as it makes its yearly journey around the sun. Picture it for yourself while gawking at the scale of the solar system. First, take a Sun. The largest sphere I own is a stability ball, the kind you can balance on, sit on and put your feet on for all manner of unknowable benefits. Mine measures 65 centimetres across. On this scale, your Earth shouldn’t be much bigger than a half-centimetre across: a particularly small grape will do, the grape on the bunch that seems never to have got growing. Put a toothpick through your grape from top to bottom and take your very own solar system outside. Place your Sun somewhere it won’t roll around, and walk your Earth 100 steps away from the Sun. That’s about the distance between two streets in Manhattan. Hold your Earth by the toothpi...