I know it tends to cut into nighttime sky watching, but humans like daylight. Heck, even I do. Every January there comes a time when we start to notice the days getting longer. This happened to me two evenings ago when I got off work around 6 p.m. under a twilit sky. A month ago that same sky would have been twinkling with stars.
A quick check on sunset and sunrise times reveals that we’ve gained a full 40 minutes of daylight since December 21, the shortest day. The earliest sunset occurred around Dec. 8 (4:20 p.m. here in Duluth, Minn.) and the latest sunrise (7:53 a.m.) around Jan. 3. Those times are now 5 p.m. and 7:42 a.m. The sunrise lags behind sunset due to a combination of factors involving the angle of the sun’s path in winter and Earth’s orbital speed.
40 minutes is nothing to sniff at, but we’re just getting going. Each day, we add an additional 1 to 2 minutes of evening sunshine and 1 minute of morning light. What began as a trickle is rapidly becoming a cascade as we set our sights on March 20, the first day of spring.
Those extra minutes are doled out day by day as the sun climbs ever northward on its yearly path around the sky. Its low position (along with short days) happens in winter, because that’s when the Earth’s northern hemisphere is tipped away from the sun. Summer happens when we’re tipped toward the sun.
The full range of the sun’s north to south movement in the sky is 47 degrees or nearly five fists held vertically against the sky. Why 47? If you divide it in half you get 23.5 degrees, and that’s the angle at which our planet’s axis is tipped. You can see that the difference between the high summer sun and low winter sun is simply a reflection of our planet’s tilt.
Astronomers divide the sky in half with an imaginary circle called the celestial equator. This is really nothing more than a projection of Earth’s actual equator into the sky above. For an observer on the equator, the celestial equator begins at the due-east horizon point, passes directly overhead and ends at the due-west point. For mid-northern latitudes, the celestial equator starts and ends at the same points but passes midway between the southern horizon and the zenith (overhead point). At the north pole the celestial equator runs right along the horizon from due east to due west.
The sun spends 23.5 degrees below the celestial equator from the first day of fall until the first day of spring and 23.5 degrees above the equator from the first day of spring until the first day of fall. At the equinoxes, the sun’s path momentarily crosses the celestial equator as it moves north or south.
If you’re looking for something to thank for providing the precious daylight you’ve been looking forward to for months, give a nod to Earth’s tilt.