The Sun’s Geometry
Enjoy the balance while we can
The waning crescent moon rises around 3am Friday, its upper tip pointing to the red planet Mars, only five degrees away. Both shine within the faint stars of Cancer, home to the Beehive star cluster, which appears as a smudge of light a half-dozen degrees behind moon and planet. By 5am, the moon and Mars are well placed about 30 degrees high in the east.
And then, five minutes past 5:00, our whole world changes.
At 5:05am est, the sun hovers directly above the equator somewhere in Kenya, where it will be high noon. This is the moment of our autumnal equinox, when the sun crosses to the south side of the equator, turning its favor to the Southern Hemisphere, giving them their first day of spring. For us north-siders, it is the beginning of autumn and the decline of our daylight hours.
At equinox, the sun rises due east and sets due west all around the globe, with everyone’s day split equally between day and night.
At the moment of equinox, it is one hour 50 minutes before sunrise for us. But at that same instant, as the sun hovers over the equator in noon-time Africa, it is rising in lower South America and setting in the South Pacific, while it is midnight in Alaska.
While daylight and darkness are balanced for the moment, it doesn’t last. Within a week we’ll have lost 10 minutes of daylight, while come Halloween, darkness will rule by almost 90 minutes.
You can see the geometry behind this in the shrinking distance between the points of sunrise and sunset from now until solstice. As the sun rises later each day, it rises farther to the south of due east, and as it sets earlier, the point where it dips beneath the horizon is farther south of due west each night. Against the sphere of our sky, this southward movement literally decreases the distance the sun travels from one side to the other.