If you could hover above the Sun and look back toward Earth, over the course of a year you’d see our home planet bob up and down like a planetary rocking chair. For about half of the year, the north pole would dip toward you; and for the other half, the south pole would dip your way.
But today, neither pole would take the spotlight. That’s because today is the autumnal equinox — the start of autumn in the northern hemisphere, and spring in the southern hemisphere. The Sun shines directly on the equator today, so both poles see the Sun sitting along the horizon all day long.
The bobblehead effect actually is only a matter of perspective. Earth is tilted on its axis, which always points toward the same spot in the sky — at Polaris, the Pole Star.
At the time of the summer solstice in the northern hemisphere, in June, the north pole tilts most directly toward the Sun. But at the winter solstice, in December, the northern hemisphere tilts away from the Sun, giving the spotlight to the southern hemisphere.
So over the next three months, the Sun will move farther south in the sky, giving those of us in the northern hemisphere shorter days and longer nights. It won’t reverse that course until the winter solstice in December — when the north pole once again begins to rock back toward the Sun.
Incidentally, the exact moment of the equinox is 3:44 p.m. Central Daylight Time.
Tomorrow: slipping up on the celestial fox.
Script by Damond Benningfield, Copyright 2012
For more skywatching tips, astronomy news, and much more, read StarDate magazine.